1. HSM Applications to Rural Multilane Intersections
Prediction of Crash Frequency and Application of CMFs for Rural Multilane Intersections
- Session #7
Session #7– Prediction of Crash Frequency and Application of CMF’s for Rural Multilane Intersections

2. Predicting Crash Frequency and Application of CMFS for Rural Multilane Intersections
Learning Outcomes:
Describe the models to Predict Crash Frequency for Rural Multilane Intersections
Calculate Predicted Crash Frequency for Rural Multilane Intersections
Describe CMFs for Rural Multilane Intersections
Apply CMFs to Crash Frequency for Rural Multilane Intersections
Learning Outcomes for Session #8 SPF models – 50 minutes

3. 2008 US Total Intersection Crash Characteristics
Crash Type
Total Crashes
Fatal/Injury Crashes
Number %
Non Intersection
2,638,000
45%
722,680
43%
Stop/No control Intersection
984,000
17%
321,520
19%
Signalized Intersection
1,182,000
20%
380,511
23%
Unclassified
1,005,000
240,306
14%
Total
5,801,228
100%
1,637,476
55%
57%
Instructor:
Two key points:
Although there are more fatalities for non-intersection crashes, there are more total crashes at intersections
Signalized intersections are a problem and the frequency of crashes is far beyond their 1 out of every 11 portion.

4. Expressway Two-Way Stop Control
From NCHRP 650
- Crashes for unsignalized divided expressways greatly outnumber crashes for signalized divided hwy intersections

5. Safety Performance of Rural Multilane Expressway Intersections
Instructor: from NCHRP 650
“Maze et al. (13) found that as expressway volumes increase, crashes more commonly occur at intersections (as opposed to between intersections). These results are presented in Figure 4 (Slides 8-4 & 8-5).
In addition, Maze et al. (2) found that intersection crash rates and severity are highly dependent on the volume of traffic entering the intersection from the minor road. As minor road traffic volumes increase, the intersection crash rates increase and the crashes become more severe, as shown in Figure 5.
Furthermore, as entering minor road volumes increase, the distribution of intersection crash types changes as shown in Figure 6, with a higher proportion of right-angle crashes occurring. Because right-angle crashes are more likely to be severe, increasing minor road volume results in increased crash severity, as illustrated in Figure 5. When similar bar charts were developed and stratified by entering expressway volumes, intersection crash rates, severity rates, and crash type distribution did not tend to change with increasing expressway volumes in Iowa (2).
*NCHRP 650

6. Safety Performance of Rural Multilane Expressway Intersections
Instructor:
The Highway Safety Information System (HSIS) data base was used for this study. In examining the existing data for the five States in the system, which has since been expanded to eight States, it was determined that supplemental information on roadway access points and roadside condition would be needed. An efficient way to collect this type of data is through the use of videodisc photologs. Videodisc photologs contain images of the roadway taken at 16-m intervals. These images can be randomly accessed in seconds under the control of a microcomputer. At the time that this study was conducted, videodisc photologs were only available for Minnesota. Consequently, only data from Minnesota were used for statistical modeling analysis. Data for accidents reported from 1985 through 1990 were used for the study.
*NCHRP 650

7. Safety Performance of Rural Multilane Expressway Intersections
Instructor:
The Highway Safety Information System (HSIS) data base was used for this study. In examining the existing data for the five States in the system, which has since been expanded to eight States, it was determined that supplemental information on roadway access points and roadside condition would be needed. An efficient way to collect this type of data is through the use of videodisc photologs. Videodisc photologs contain images of the roadway taken at 16-m intervals. These images can be randomly accessed in seconds under the control of a microcomputer. At the time that this study was conducted, videodisc photologs were only available for Minnesota. Consequently, only data from Minnesota were used for statistical modeling analysis. Data for accidents reported from 1985 through 1990 were used for the study.
*NCHRP 650

8. Safety Performance of Rural Multilane Expressway Intersections
1. 87% of the right-angle crashes were due to the inability of minor road drivers to recognize oncoming expressway traffic and/or select safe gaps in the expressway traffic stream;
2. 78% of the right-angle crashes were “far-side” collisions
[i.e., right-angle crashes involving left-turning or crossing
minor road vehicles that successfully cross the first (near-side) set of expressway lanes, but collide with expressway
traffic in the second (far-side) set of lanes after traversing
through the median (the concept of near and far-side intersections is illustrated in Figure 8)
Instructor:
The Highway Safety Information System (HSIS) data base was used for this study. In examining the existing data for the five States in the system, which has since been expanded to eight States, it was determined that supplemental information on roadway access points and roadside condition would be needed. An efficient way to collect this type of data is through the use of videodisc photologs. Videodisc photologs contain images of the roadway taken at 16-m intervals. These images can be randomly accessed in seconds under the control of a microcomputer. At the time that this study was conducted, videodisc photologs were only available for Minnesota. Consequently, only data from Minnesota were used for statistical modeling analysis. Data for accidents reported from 1985 through 1990 were used for the study.
*NCHRP 650

9. Safety Performance of Rural Multilane Expressway Intersections
3. Intersection recognition (i.e., running of the STOP sign)
by drivers on the minor, stop-controlled approaches was
not a contributing factor in any of the right-angle crashes
at these intersections.
Similarly, Burchett and Maze (14) found that the ratio of
far-side to near-side collisions at 30 TWSC rural expressway intersections with the highest crash severity indices in Iowa was 62% to 38%; however, at 7 of these intersections where horizontal curves were present along the expressway, far-side and near-side collisions were nearly equally distributed at 51% and 49%, respectively. Therefore, horizontal curves on the mainline seem to create a unique hazard for minor road
Instructor:
The Highway Safety Information System (HSIS) data base was used for this study. In examining the existing data for the five States in the system, which has since been expanded to eight States, it was determined that supplemental information on roadway access points and roadside condition would be needed. An efficient way to collect this type of data is through the use of videodisc photologs. Videodisc photologs contain images of the roadway taken at 16-m intervals. These images can be randomly accessed in seconds under the control of a microcomputer. At the time that this study was conducted, videodisc photologs were only available for Minnesota. Consequently, only data from Minnesota were used for statistical modeling analysis. Data for accidents reported from 1985 through 1990 were used for the study.
*NCHRP 650

10. Safety Performance of Rural Multilane Expressway Intersections
Instructor:
The Highway Safety Information System (HSIS) data base was used for this study. In examining the existing data for the five States in the system, which has since been expanded to eight States, it was determined that supplemental information on roadway access points and roadside condition would be needed. An efficient way to collect this type of data is through the use of videodisc photologs. Videodisc photologs contain images of the roadway taken at 16-m intervals. These images can be randomly accessed in seconds under the control of a microcomputer. At the time that this study was conducted, videodisc photologs were only available for Minnesota. Consequently, only data from Minnesota were used for statistical modeling analysis. Data for accidents reported from 1985 through 1990 were used for the study.
*NCHRP 650

11. Defining Rural Multilane Highways
Methodology applies to four-lane undivided and divided rural highways.
“Rural”:
Defined per AASHTO (2004) Guidelines
Places outside the boundaries of urban places where the population is less than 5,000 inhabitants.
Any highway located outside the city limits of an urban agglomeration above 5,000 inhabitants is considered rural.
The boundary delimitating rural and urban areas can at times be difficult to determine, especially since most multilane rural highways are located on the outskirts of urban agglomerations.
Instructor: Rural is away from urban/suburban of 5,000 population or more; it is a functional determination.
These procedures may be used for any multilane road in which the general design features and land use setting are rural rather than urban or suburban in nature. In other words, if the road is designed according to the rural road design standards in the AASHTO (2004) “Green Book,” and development along the road is relatively sparse, these procedures apply.

12. Defining Multilane Highways
Multilane Facilities:
Have four through lanes.
May be divided with a rigid or flexible barrier, paved or landscaped median
Should not have access and egress limited by grade-separated interchanges (i.e., not freeways).
May have occasional grade-separated interchanges, but these should not be the primary form of access and egress
Instructor:
Go through the definitions for Multilane and Rural carefully in this slide and the following slides so that the participants have a solid understanding of how the Highway Safety Manual is being applied to multilane highways, rural or urban/suburban.

13. Limitations of Methodology
Methodology incorporates the effects on safety of many -but not all- geometric and traffic control features.
Only includes geometric design elements:
whose relationship to safety are well understood
Associated data is available for
The Statistical Model:
treats the effects of individual geometric design element and traffic control features as independent of each other
Ignores any potential interactions between them.
Instructor:
The limitations are critical information to the user of the highway safety manual and crucial to successful application of this new technology.

14. Predicting Crash Frequency of Rural Multilane Highways
Separate Safety Performance Functions (SPFs) for:
Homogeneous highway segments
Intersections
Sum of Individual Intersection Calculations
Key Message: The Analysis divides the highway into homogeneous analysis sections.
Additional Info: Analysis sections include both (1) homogeneous highway segments, and (2) individual intersections. Each analysis section is homogenous with respect to geometry and traffic conditions.
Homogeneous highway segments have uniform horizontal, vertical, cross section, traffic characteristics, and roadside geometry. At any location where there is a change in geometry (e.g., changing from a horizontal curve to a tangent or a change in shoulder width) or a change in traffic volume, a new highway segment begins.
For suburban/urban, the HSM does not apply CMF for horizontal curves
Each intersection is also defined as a separate, homogenous analysis section.
Question/Interactivity: Ask participants to study the roadway plans for IHSDM Pike and to identify the first few homogenous highway segments. Show how the first tangent and horizontal curve would be different homogeneous segments. (refer to sheet 2 of the plan/profile)
Reference:

15. Definition of Segments and Intersections
From Final HSM, Section ROADWAY SEGMENTS AND INTERSECTIONS
Roadway segments begin at the center of an intersection and end at either the center of the next intersection, or where there is a change from one homogeneous roadway segment to another homogenous segment. The roadway segment model estimates the frequency of roadway-segment-related crashes which occur in Region B in. When a roadway segment begins or ends at an intersection, the length of the roadway segment is measured from the center of the intersection.
The Chapter 11 predictive method addresses stop controlled (three- and four-leg) and signalized (four-leg) intersections. The intersection models estimate the predicted average frequency of crashes that occur within the limits of an intersection (Region A of Figure 11-2) and intersection-related crashes that occur on the intersection legs (Region B in Figure 11-2).
The definition of an intersection crash tends to vary between agencies (5). Some agencies define an intersection crash as one which occurs within the intersection crosswalk limits or physical intersection area. Other agencies consider all crashes within a specified distance, such as 250 ft, from the center of an intersection to be intersection crashes (5). However, not all crashes occurring within 250 ft of an intersection can be considered intersection crashes because some of these may have occurred regardless of the existence of an intersection. Consideration should be given to these differences in definitions when evaluating conditions and seeking solutions.
A - All crashes that occur within this region are classified as intersection crashes
B – Crashes in this region may be segment or intersection related, depending on the characteristics of the crash

16. Definition of Intersections:
“the general area where two or more roadways join or cross, including the roadway and roadside facilities for traffic movements within the area.”
Intersections may be:
signalized,
stop controlled, and
roundabouts
Instructor: from Chapter 11 of the Highway Safety Manual

17. Definition of Intersections:
An at-grade intersection is defined by both:
its physical and
functional areas”
Instructor: from Chapter 14 of the Highway Safety Manual

18. Definition of an Intersection
The functional area “extends both upstream and downstream from the physical intersection area and includes any auxiliary lanes and their associated channelization.”
the functional area on each approach to an intersection consists of three basic elements:
Decision distance;
Maneuver distance; and,
Queue-storage distance.
Instructor: from Chapter 11 of the Highway Safety Manual

19. Functional Area of an Intersection
From Final HSM, Chapter 14
14.3. DEFINITION OF AN INTERSECTION
The functional area “extends both upstream and downstream from the physical intersection area and includes any auxiliary lanes and their associated channelization.”(1) As illustrated in Exhibit 14-2, the functional area on each approach to an intersection consists of three basic elements:(1)
􀂃 Decision distance;
􀂃 Maneuver distance; and,
􀂃 Queue-storage distance.
Decision Distance
Maneuver Distance
Queue-Storage Distance

20. Predicting Crash Frequency for Rural Multilane At-Grade Intersections
Procedure for safety prediction for At- Grade Intersections:
1st Apply SPF Model for base conditions,
2nd Apply CMFs and calibration factor
Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin))
Npredicted int = Nspf int x(CMF1i x CMF2i x …. CMFni)Ci
Instructor: from Chapter 11 of the Highway Safety Manual
Safety Performance Functions for Intersections
The predictive model for estimating predicted average crash frequency at particular rural multilane intersection was presented in Equation The effect of traffic volume (AADT) on accident frequency is incorporated through the SPF, while the effects of geometric design and traffic control features are incorporated through the CMFs. The SPFs for rural multilane highway intersection are presented in this section. Three and four-leg STOP controlled and four-leg signalized rural multilane highway intersections are defined in Section 11.3.
SPFs have been developed for three types of intersections on rural multilane highways. These models can be used for intersections located on both divided and undivided rural four-lane highways. The three types of intersections are:
􀂃 Three-leg intersections with minor road stop control (3ST)
􀂃 Four-leg intersections with minor road stop control (4ST)
􀂃 Four-leg signalized intersections (4SG)
The SPFs for four-leg signalized intersections (4SG) on rural multilane highways have no specific base conditions and, therefore, can only be applied for generalized predictions. No CMFs are provided for 4SG intersections and predictions of average crash frequency cannot be made for intersections with specific geometric design and traffic control features.
Models for three-leg signalized intersections on rural multilane roads are not available.
The SPFs for three- and four-leg stop-controlled intersections (3ST and 4ST) on rural multilane highways are applicable to the following base conditions:
-- Intersection skew angle 0°
-- Intersection left-turn lanes 0, except on stop-controlled approaches
-- Intersection right-turn lanes 0, except on stop-controlled approaches
-- Lighting None

21. Predicting Crash Frequency for Rural Multilane At-Grade Intersections
SPF Models and Adjustment Factors addresses three types of Intersections:
Three-leg intersections with STOP control on the minor road approach (3ST)
Four-leg intersections with STOP control on the minor-road approaches (4ST)
Four-leg signalized intersection (4SG)
Instructor: from Chapter 11 of the Highway Safety Manual
Safety performance functions for intersections
The predictive model for estimating predicted average crash frequency at particular rural multilane intersection was presented in Equation The effect of traffic volume (AADT) on crash frequency is incorporated through the SPF, while the effects of geometric design and traffic control features are incorporated through the CMFs. The SPFs for rural multilane highway intersection are presented in this section. Three- and four-leg stop-controlled intersections and four-leg signalized rural multilane highway intersections are defined in Section 11.3.
SPFs have been developed for three types of intersections on rural multilane highways. These models can be used for intersections located on both divided and undivided rural four-lane highways. The three types of intersections are:
■■ ■Three-leg intersections with minor-road stop control (3ST)
■■ ■Four-leg intersections with minor-road stop control (4ST)
■■ ■Four-leg signalized intersections (4SG)
The SPFs for four-leg signalized intersections (4SG) on rural multilane highways have no specific base conditions and, therefore, can only be applied for generalized predictions. No CMFs are provided for 4SG intersections and predictions of average crash frequency cannot be made for intersections with specific geometric design and traffic control features.
Models for three-leg signalized intersections on rural multilane roads are not available.
The SPFs for three- and four-leg stop-controlled intersections (3ST and 4ST) on rural multilane highways are applicable to the following base conditions:
■■ ■Intersection skew angle 0°
■■ ■Intersection left-turn lanes 0, except on stop-controlled approaches
■■ ■Intersection right-turn lanes 0, except on stop-controlled approaches
■■ ■Lighting None
Used for both divided and undivided rural four-lane highways

22. Predicting Crash Frequency for Rural Multilane At-Grade Intersections
SPF Model for Rural Multilane Intersections (Applies to BOTH Divided and Undivided):
Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin))
Where:
Nspf int = expected number of intersection-related crashes per year for base conditions
AADTmaj = average daily traffic volume for the major road (vpd)
AADTmin = average daily traffic volume for the minor road (vpd)
a, b, and c = regression coefficients from Table 11-7
Instructor: from Chapter 11 of the Highway Safety Manual
Safety performance functions for intersections
The predictive model for estimating predicted average crash frequency at particular rural multilane intersection was
presented in Equation The effect of traffic volume (AADT) on crash frequency is incorporated through the
SPF, while the effects of geometric design and traffic control features are incorporated through the CMFs. The SPFs
for rural multilane highway intersection are presented in this section. Three- and four-leg stop-controlled intersections and four-leg signalized rural multilane highway intersections are defined in Section 11.3.
SPFs have been developed for three types of intersections on rural multilane highways. These models can be used for intersections located on both divided and undivided rural four-lane highways. The three types of intersections are:
■■ ■Three-leg intersections with minor-road stop control (3ST)
■■ ■Four-leg intersections with minor-road stop control (4ST)
■■ ■Four-leg signalized intersections (4SG)
The SPFs for four-leg signalized intersections (4SG) on rural multilane highways have no specific base conditions
and, therefore, can only be applied for generalized predictions. No CMFs are provided for 4SG intersections and
predictions of average crash frequency cannot be made for intersections with specific geometric design and traffic
control features.
Models for three-leg signalized intersections on rural multilane roads are not available.
The SPFs for three- and four-leg stop-controlled intersections (3ST and 4ST) on rural multilane highways are
applicable to the following base conditions:
■■ ■Intersection skew angle 0°
■■ ■Intersection left-turn lanes 0, except on stop-controlled approaches
■■ ■Intersection right-turn lanes 0, except on stop-controlled approaches
■■ ■Lighting None

23. Base Conditions for Rural Multilane Intersections
Stop-Controlled Intersections:
Intersection Skew Angle: odegrees
Presence of Left-Turn Lanes: none
Presence of Right-Turn Lanes: none
Lighting: none
Key Message:These are the nominal geometry or base conditions represented by the base models.
Additional Info:
The SPFs for three- and four-leg stop-controlled intersections (3ST and 4ST) on rural multilane highways are
applicable to the following base conditions:
■■ ■Intersection skew angle 0°
■■ ■Intersection left-turn lanes 0, except on stop-controlled approaches
■■ ■Intersection right-turn lanes 0, except on stop-controlled approaches
■■ ■Lighting None

24. Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin))
Predicting Crash Frequency for Rural Multilane Stop-Controlled Intersections
Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin))
Instructor: from Chapter 11 of the Highway Safety Manual
Circled CMFs are for Total Crashes
Table 11-7 presents the recommended values of the coefficients a, b, and c used in applying Equation 11-13a for stop-controlled intersections, along with the overdispersion parameter and the base conditions. Alternate sets of models are presented for two sets of base conditions. The first set was developed by Washington et al. (7) from Michigan, Georgia, and California intersections. The second set was developed in HSM research conducted by Lord et al. (5). It is recommended that the user choose the set that is most appropriate to the base conditions for the particular application at hand.

25. Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin))
Predicting Crash Frequency for Rural Multilane Signalized Intersections
Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin))
Instructor: from Chapter 11 of the Highway Safety Manual
Table 11-8 presents the values of the coefficients a, b, c, and d used in applying Equations and for fourleg signalized intersections along with the overdispersion parameter. Coefficients a, b, and c are provided for total crashes and are applied to the SPF shown in Equation Coefficients a and d are provided for injury crashes and are applied to the SPF shown in Equation SPFs for three-leg signalized intersections on rural multilane roads are not currently available.
If feasible, separate calibration of the models in Tables 11-7 and 11-8 for application to intersections on divided and undivided roadway segments is preferable. Calibration procedures are presented in Appendix A to Part C.

26. Safety Prediction for a Rural Multilane Intersection: EXAMPLE
Four-Leg Stop-Controlled Intersection:
10,000 AADT and 2,500 AADT
From Table 11-7: a = , b=0.848, c=0.448
Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin))
N = exp( ln(10,000)) ln(2,500)
= exp( )
= exp(1.3075)
Instructor: the predicted number of crashes for a RURAL Multilane intersection. Answer is 3.70 crashes per year
= 3.7 crashes per year (base conditions)

27. Severity of Rural Multilane Intersections:
For example, for Rural 4-approach Signalized intersection with AADTs of 37,000 and 16,100:
Predicted Total Crashes = crashes/yr
Predicted Injury + Fatal Crashes = 13.0 crashes/yr
Predicted Severity Index = 13.0/ = 32.9%
Instructor:
Use the a, b, c coefficents for injury crashes to predict the crash frequency of injury crashes (which includes fatals)
Then use the a, b, and c coefficients for total crashes to predict the crash frequency for total crashes
Then divide the predicted injury crashes by predicted total crashes to obtain the Predicted Severity Index and compare to the actual
4. Severity Index from this table from page 37 of NCHRP 486 is 32% SI for Roadway Segments and 40% for intersections

28. Predicting Crash Frequency for Rural Multilane At-Grade Intersections
Procedure for safety prediction for At- Grade Intersections:
1st Apply SPF Model for base conditions,
2nd Apply CMFs and calibration factor
Nspf int = exp(a + b ln(AADTmaj) + c ln(AADTmin))
NEXT:
Npredicted int = Nspf int x(CMF1i x CMF2i x …. CMFni)Ci
Instructor: from Chapter 11 of the Highway Safety Manual

29. CMF’s for Rural Multilane Intersections
Instructor: This slide highlights the geometric features that have been studied and determined to influence the substantive safety of rural intersections that are stop controlled

30. Effect of Angle or Skew Skew Angle studies show adverse effect of skew
Skews increase exposure time to crashes; increase difficulty of driver view at stopped approach
@ 90 degrees
Instructor:
Instructor should point out that research on older drivers shows they tend to have difficulty in moving their head to look to the side; hence skewed intersections represent particular difficulties to them.
Intersection angles that are close to 90o are considered safer than severely acute and obtuse angles. Modern design guidelines tend to limit intersection angles to 70o (110o) or better. The findings of the literature review on this topic are summarized as follows:
· Staplin et al. (2001), in the Highway Design Handbook for Older Drivers and Pedestrians state that “decreasing the angle on the intersection makes detection of and judgments about potential conflicting vehicles on crossing roadways much more difficult”. They indicate that skewed intersections pose particular problems for older drivers due to the decline in head and neck mobility which usually accompanies advancing age.
ITE (1999) states that “crossing roadways should intersect at 90 degrees if possible, and not less than 75 degrees." It further states that: "Intersections with severe skew angles (e.g., 60 degrees or less) often experience operational or safety problems. Reconstruction of such locations or institution of more positive traffic control such as signalization is often necessary.“
Regarding intersection design issues on two-lane rural highways, ITE (1999) states that: "Skew angles in excess of 75 degrees often create special problems at stop-controlled rural intersections. The angle complicates the vision triangle for the stopped vehicle; increases the time to cross the through road; and results in a larger, more potentially confusing intersection.“CMF1i—Intersection Skew Angle
The SPF base condition for intersection skew angle is 0 degrees of skew (i.e., an intersection angle of 90 degrees). Reducing the skew angle of three- or four-leg stop-controlled intersections on rural multilane highways reduces total intersection crashes, as shown below. The skew angle is the deviation from an intersection angle of 90 degrees. Skew carries a positive or negative sign that indicates whether the minor road intersects the major road at an acute or obtuse angle, respectively.
SKEW = Intersection Angle (degrees) as difference (absolute value) between 90 degrees and actual intersection angle

31. Rural Multilane Intersection CMF for Intersection Skew Angle
3- legged Intersections (Stop-Control) on Minor Approach:
CMF1i = (0.016 x Skew) (11-18)
( x Skew)
4- legged Intersections (Stop –Control) on Minor Approach:
CMF1i = (0.053 x Skew) (11-19)
( x Skew)
CMF1i = CMF for the effect of intersection skew on total crashes
SKEW = Intersection Angle (degrees) as difference (absolute value) between 90 degrees and actual intersection angle
Instructor: From HSM Chapt 11
Intersection Skew Angle (CMF1i)
The base condition for intersection skew angle is 0 degrees of skew (i.e., an intersection angle of 90 degrees). Reducing the skew angle of three- or four-leg stop controlled intersections on rural multilane highways reduces total intersection accidents, as shown below.
Where,
CMF1i= accident modification factor for the effect of intersection skew on total accidents; and
SKEW = intersection skew angle (in degrees); the absolute value of the difference between 90 degrees and the actual intersection angle
From HSM Glossary: skew angle, intersection = the deviation from an intersection angle of 90 degrees. Carries a positive or negative sign that indicates whether the minor road intersects the major road at an acute or obtuse angle, respectively.
Older Driver Handbook, 2001, Intersections, Recommendation A. (2). “In the design of new facilities or redesign of existing facilities where right-of-way is restricted, intersecting roadways should meet at an angle of not less than 75 degrees.” (3).“At skewed intersections where the approach leg to the left intersects the driver’s approach leg at an angle of less than 75 degrees, the prohibition of right turn on Red (RTOR) is recommended”. – not in current MUTCD nor in Rev#2.
Strategy 17.1 B16—Realign Intersection Approaches to Reduce or Eliminate Intersection Skew
When roadways intersect at skewed angles, the intersections may experience one or more of the following problems:
• Vehicles may have a longer distance to traverse while crossing or turning onto the intersecting roadway, resulting in an increased time of exposure to the cross-street traffic.
• Older drivers may find it more difficult to turn their head, neck, or upper body for an adequate line of sight down an acute-angle approach.
• The driver’s sight angle for convenient observation of opposing traffic and pedestrian crossings is decreased.
• Drivers may have more difficulty aligning their vehicles as they enter the cross street to make a right or left turn.
• Drivers making right turns around an acute-angle radius may encroach on lanes intended for oncoming traffic from the right.
• The larger intersection area may confuse drivers or cause them to deviate from the intended path.
• Through-roadway drivers making left turns across an obtuse angle may attempt to maintain a higher than normal turning speed and cut across the oncoming traffic lane on the intersecting street.
Harwood et al, found that intersection skew angle was NOT found to be related to crash frequency at signalized intersections.

32. CMF1i = 1 + [(0.016 x Skew)/(0.98 + 0.16 x Skew)]
Safety Prediction for Intersection Skew Angle at a Rural Multilane Intersection: EXAMPLE
3-Leg Stop-Controlled Intersection:
Skew Angle = 35 degrees
CMF1i = 1 + [(0.016 x Skew)/( x Skew)]
CMF1i = 1 + [(0.016)(35)/( (0.16)(35)
= 1 + (0.56/6.58) =
Instructor: Example calculation of the CMF for a 3-approach skew of 35 degrees. The answer is or 8.5% more crashes
= 1.085

33. Left Turn Lanes for Multilane Highways
Left turn lanes remove stopped traffic from through lanes
mitigate rear-end conflict
enable selection of safe gap
Instructor: The advantages of left turn lanes in the rural environment are primarily safety versus capacity. The design of left turn lanes in the rural environment should be based on the deceleration requirements. This differs from the higher volume urban environment in which queuing and storage generally drive design of left turn lanes.
Location of illustrative photo is divided expressway in Missouri.
Add in warrants of turn lanes in the rural environment from NCHRP 457 BonnesonCMF2i—Intersection Left-Turn Lanes
The SPF base condition for intersection left-turn lanes is the absence of left-turn lanes on all of the intersection approaches.
The CMFs for presence of left-turn lanes are presented in Table for total crashes and injury crashes.
These CMFs apply only on uncontrolled major-road approaches to stop-controlled intersections. The CMFs for installation of left-turn lanes on multiple approaches to an intersection are equal to the corresponding CMF for installation of a left-turn lane on one approach raised to a power equal to the number of approaches with left-turn lanes (i.e., the CMFs are multiplicative, and Equation 3-7 can be used). There is no indication of any effect of providing a left-turn lane on an approach controlled by a stop sign, so the presence of a left-turn lane on a stop-controlled approach is not considered in applying Table The CMFs for installation of left-turn lanes are based on research by Harwood et al. (4) and are consistent with the CMFs presented in Chapter 14, Intersections. A CMF of 1.00 is used when no left-turn lanes are present.
“Capacity” is generally not the issue
*NCHRP 500, Strategy 17.1 B1 – Provide Left-Turn Lanes

34. CMF2i for Left-Turn Lanes at Rural Multilane Intersections:
Instructor: From HSM Chapter 11
Intersection Left-Turn Lanes (CMF2i)
CMF2i—Intersection Left-Turn Lanes
The SPF base condition for intersection left-turn lanes is the absence of left-turn lanes on all of the intersection approaches.
The CMFs for presence of left-turn lanes are presented in Table for total crashes and injury crashes.
These CMFs apply only on uncontrolled major-road approaches to stop-controlled intersections. The CMFs for installation of left-turn lanes on multiple approaches to an intersection are equal to the corresponding CMF for installation of a left-turn lane on one approach raised to a power equal to the number of approaches with left-turn lanes (i.e., the CMFs are multiplicative, and Equation 3-7 can be used). There is no indication of any effect of providing a left-turn lane on an approach controlled by a stop sign, so the presence of a left-turn lane on a stop-controlled approach is not considered in applying Table The CMFs for installation of left-turn lanes are based on research by Harwood et al. (4) and are consistent with the CMFs presented in Chapter 14, Intersections. A CMF of 1.00 is used when no left-turn lanes are present.

35. CMF3i for Right-Turn Lanes at Rural Multilane Intersections:
Instructor: From HSM Chapter 11
CMF3i—Intersection Right-Turn Lanes
The SPF base condition for intersection right-turn lanes is the absence of right-turn lanes on the intersection approaches.
The CMFs for the presence of right-turn lanes are based on research by Harwood et al. (4) and are consistent
with the CMFs in Chapter 14. These CMFs apply to installation of right-turn lanes on any approach to a
signalized intersection, but only on uncontrolled major-road approaches to stop-controlled intersections. The CMFs
for installation of right-turn lanes on multiple approaches to an intersection are equal to the corresponding CMF for
installation of a right-turn lane on one approach raised to a power equal to the number of approaches with right-turn
lanes (i.e., the CMFs are multiplicative, and Equation 3-7 can be used). There is no indication of any safety effect
for providing a right-turn lane on an approach controlled by a stop sign, so the presence of a right-turn lane on a
stop-controlled approach is not considered in applying Table The CMFs for presence of right-turn lanes are
presented in Table for total crashes and injury crashes. A CMF value of 1.00 is used when no right-turn lanes
are present. This CMF applies only to right-turn lanes that are identified by marking or signing. The CMF is not applicable to long tapers, flares, or paved shoulders that may be used informally by right-turn traffic.

36. CMF4i for Lighting at Rural Multilane Intersections:
CMF4i = Pni
Where:
CMF4i = CMF for the effect of lighting on total crashes
Pni = proportion of total crashes for unlighted intersections that occur at night
Instructor: From HSM Chapter 11
CMF4i—Lighting
The SPF base condition for lighting is the absence of intersection lighting. The CMF for lighted intersections is
adapted from the work of Elvik and Vaa (1), as:
CMF4i = 1.0 – 0.38 × pni (11-22)
Where:
CMF4i = crash modification factor for the effect of lighting on total crashes; and
pni = proportion of total crashes for unlighted intersections that occur at night.
This CMF applies to total intersections crashes (not including vehicle-pedestrian and vehicle-bicycle collisions).
Table presents default values for the nighttime crash proportion, pni. HSM users are encouraged to replace the
estimates in Table with locally derived values.

37. CMF4i for Lighting at Rural Multilane Intersections:
Replace default values with local values if available
Instructor: CMF4i – Lighting
CMF4i—Lighting
The SPF base condition for lighting is the absence of intersection lighting. The CMF for lighted intersections is
adapted from the work of Elvik and Vaa (1), as:
CMF4i = 1.0 – 0.38 × pni (11-22)
Where:
CMF4i = crash modification factor for the effect of lighting on total crashes; and
pni = proportion of total crashes for unlighted intersections that occur at night.
This CMF applies to total intersections crashes (not including vehicle-pedestrian and vehicle-bicycle collisions).
Table presents default values for the nighttime crash proportion, pni. HSM users are encouraged to replace the estimates in Table with locally derived values.
*Note the lack of a value for 4SG signalized intersection

38. CMF4i for Lighting at a Rural 2-Way Stop Multilane Intersection: EXAMPLE
CMF4i = Pni
Instructor: Example calculation for the CMF for lighting of a rural 2-way stop controlled multilane intersection. Answer is crashes per year
CMF4i = x 0.273
= 1 –
= 0.896

39. Safety Prediction for a Rural Multilane Intersection: Example
4-Leg Rural Unsignalized Intersection:
10,000 AADT and 2,500 AADT,
35 Deg Skew,
left-turn lane
Right-turn lane on major road,
lighting
Instructor:
Building on the previous exercises ask the workshop participants to calculate the predicted number of crashes for this RURAL Multilane intersection.
NPredicted int = Nspf int (CMF1i x CMF2i x …. CMFni) Ci

40. Safety Prediction for a Rural Multilane Intersection: Example
4-Leg Rural Unsignalized Intersection:
10,000 AADT and 2,500 AADT,
35 Deg Skew, left-turn lanes + right-turn lanes on major road, lighting
Nspf int = 3.70
CMF2i(lt-trn) = ?
0.520
CMF1i(skew) = ?
1.092
CMF4i(lighting)= ?
0.896
CMF3i(rt-trn) = ?
0.740
Instructor:
Building on the previous exercises ask the workshop participants to calculate the predicted number of crashes for this RURAL Multilane intersection.
NPredicted int = Nspf int (CMF1i x CMF2i x …. CMFni) Ci
= 3.70 x (1.092 x 0.52 x 0.74 x 0.896) =
1.39

41. CMF’s for Rural Multilane Intersections
There are no CMF’s in the 1st edition of the HSM for Signalized Rural Multilane Intersections included in Chapter 11 of Part C
Instructor:
There are no CMF’s in the 1st edition of the HSM for Signalized Rural Multilane Intersections included in Chapter 11 of Part C

42. Rural Multilane Intersections
Additional CMF’s:
Increase median opening width
Convert minor road Stop control to All-Way Stop
Convert Stop Control to Signal Control
Instructor: from HSM Chapter 14, there are several additional CMF applications that can be used to predict the crash frequency on Urban/Suburban roads. These will be presented in the new few slides.
Note to Instructor: It is not essential, or the intention, to cover each slide in depth. At the discretion of the instructor(s) (i.e., if time is short) some or all of these slides can be hidden. If this is done, refer the participants to their manual or workbook for CMF values for the conditions above.

43. CMFs for Increasing Median Opening Width
Increase Intersection Median Width
This section presents the crash effects related to median width. Medians are intended to perform several functions.
Some of the main functions are:
■■ To separate opposing traffic;
■■ To allow space for the storage of left-turning, U-turning vehicles;
■■ Minimize headlight glare; and
■■ Provide width for future lanes (1,25)
At an intersection, the following definitions of the median apply.
■■ Median width is the total width between the edges of opposing through lanes, including the left shoulder and the left-turn lanes, if any (18).
■■ Median opening length is the total length of break in the median provided for cross street and turning traffic (18). The design of a median opening is generally based on traffic volumes, urban/rural area characteristics, and type of turning vehicles (1).
■■ Median roadway is the paved area in the center of the divided highway at an intersection defined by the median width and the median opening length (18).
■■ Median area is the median roadway plus the major-road left-turn lanes, if any (18).
■■ The median width, length, roadway, and area are illustrated in Figure 14‑9.

44. CMFs for Increasing Median Opening Width
Increase Intersection Median Width
This section presents the crash effects related to median width. Medians are intended to perform several functions.
Some of the main functions are:
■■ To separate opposing traffic;
■■ To allow space for the storage of left-turning, U-turning vehicles;
■■ Minimize headlight glare; and
■■ Provide width for future lanes (1,25)
At an intersection, the following definitions of the median apply.
■■ Median width is the total width between the edges of opposing through lanes, including the left shoulder and the
left-turn lanes, if any (18).
■■ Median opening length is the total length of break in the median provided for cross street and turning traffic (18).
The design of a median opening is generally based on traffic volumes, urban/rural area characteristics, and type of
turning vehicles (1).
■■ Median roadway is the paved area in the center of the divided highway at an intersection defined by the median
width and the median opening length (18).
■■ Median area is the median roadway plus the major-road left-turn lanes, if any (18).
■■ The median width, length, roadway, and area are illustrated in Figure 14‑9.
■■ Urban, suburban, and rural four-leg unsignalized intersections,
■■ urban and suburban three-leg unsignalized intersections, and
■■ urban and suburban four-leg signalized intersections
Table 14‑17 summarizes the crash effects of increasing intersection median width by 3-ft increments at intersections
where existing medians are between 14 and 80 ft wide (18).
The base condition for the CMFs summarized in Table 14‑17 (i.e., the condition in which the CMF = 1.00) is a
14-ft-wide to 80-ft-wide median.

45. CMFs for Increasing Median Opening Width
■■ Urban, suburban, and rural four-leg unsignalized intersections,
■■ urban and suburban three-leg unsignalized intersections, and
■■ urban and suburban four-leg signalized intersections
Table 14‑17 summarizes the crash effects of increasing intersection median width by 3-ft increments at intersections where existing medians are between 14 and 80 ft wide (18).
The base condition for the CMFs summarized in Table 14‑17 (i.e., the condition in which the CMF = 1.00) is a 14-ft-wide to 80-ft-wide median.

46. CMFs for Converting Minor-Road Stop Control to All-Way Stop Control:
Instructor: From HSM Chapter 14
Convert Minor-Road Stop Control into All-Way Stop Control
The Manual on Uniform Traffic Control Devices (MUTCD) contains warrants to determine when it is appropriate to convert an intersection with minor-road stop control into an all-way stop control. The effects on crash frequency described below assume that MUTCD warrants for converting a minor-road stop-controlled intersection to an all-way stop-control intersection are met.
Urban and rural minor-road stop-controlled intersections
Table 14-5 provides specific information regarding the crash effects of converting urban intersections with minor road stop control to all-way stop control when established MUTCD warrants are met. The effect on pedestrian crashes is also shown in Table 14-5.
The base condition for the CMFs below (i.e., the condition in which the CMF = 1.00) is an intersection with minor road stop control that meets MUTCD warrants to become an all-way, stop-controlled intersection.

47. CMFs for Converting Stop Control to Traffic Signal Control:
Instructor: From HSM Chapter 14
Convert Stop Control to Signal Control
Prior to installing a traffic signal, an engineering study of traffic conditions, pedestrian characteristics, and physical characteristics of the location is typically performed to determine whether installing a traffic signal is warranted at a particular location as outlined in the MUTCD. The satisfaction of a traffic signal warrant or warrants does not in itself require installing a traffic signal.
Urban and rural minor-road stop-controlled intersections
Table 14-7 summarizes the CMFs related to converting a stop-controlled intersection to a signalized intersection. The CMF presented for urban intersections applies only to intersections with a major road speed limit at least 40 mph. The base condition for the CMFs summarized in Table 14-7 (i.e., the condition in which the CMF = 1.00) is a minor road, stop-controlled intersection in an urban or rural area.

48. Rural Multilane Intersections
Additional CMF’s beyond the HSM 1st Edition:
Positive Offset Left Turn Lanes
Access Density
“J” Turns
Instructor:
NCHRP 650 is recently published in July of 2010
July 2010

49. Rural Multilane Intersections
Additional CMF’s beyond the HSM 1st Edition:
Positive Offset Left Turn Lanes
NCHRP 650: Before and After studies of crashes identified up to a 100% reduction in left turn crashes; one study found an increase in rear-end crashes; typical crash reduction in left turn crashes is 70%.
Instructor:
NCHRP 650 is recently published in July of 2010

50. Offset Left-Turn Lane Geometry
Instructor:
Strategy 17.1 B3—Provide Offset Left-Turn Lanes at Intersections (T)
General Description
A potential problem in installing left-turn lanes at intersections is that vehicles in opposing turn lanes on the major road may block drivers’ views of approaching traffic. This can lead to collisions between vehicles turning left from the major road and through vehicles on the opposing major-road approach. To reduce the potential for crashes of this type, the left-turn lanes can be offset by moving them laterally so that vehicles in opposing lanes no longer obstruct the opposing driver. Two treatments for offsetting turn lanes are parallel and tapered offset left-turn lanes. These treatments have been evaluated in research (Harwood et al., 1995) and are addressed in the AASHTO Policy on Geometric Design of Highways and Streets (AASHTO, 2001).
While offset left-turn lanes have been used most extensively at signalized intersections, they are suitable for use at unsignalized intersections as well.
EXHIBIT V-7
Strategy Attributes for Providing Offset Left-Turn Lanes at Intersections (T)
Technical Attributes
Target The strategy of providing offset left-turn lanes at unsignalized intersections is targeted to reduce the frequency of collisions between vehicles turning left and opposing through vehicles, as well as rear-end crashes between through vehicles on the opposing approach. The strategy is generally applicable to intersections on divided highways with medians wide enough to provide the appropriate offset.
Research has verified that offset left-turn lanes operate safely (Harwood et al., 1995), but there are no reliable estimates of their safety effectiveness. Safety effectiveness is likely to depend upon the traffic volumes of the conflicting turning and through movements and the amount of offset between the left-turn lanes at the intersection.

51. Offset Left-Turn Lane Geometry
Instructor:
Strategy 17.1 B3—Provide Offset Left-Turn Lanes at Intersections (T)
General Description
A potential problem in installing left-turn lanes at intersections is that vehicles in opposing turn lanes on the major road may block drivers’ views of approaching traffic. This can lead to collisions between vehicles turning left from the major road and through vehicles on the opposing major-road approach. To reduce the potential for crashes of this type, the left-turn lanes can be offset by moving them laterally so that vehicles in opposing lanes no longer obstruct the opposing driver. Two treatments for offsetting turn lanes are parallel and tapered offset left-turn lanes. These treatments have been evaluated in research (Harwood et al., 1995) and are addressed in the AASHTO Policy on Geometric Design of Highways and Streets (AASHTO, 2001).
While offset left-turn lanes have been used most extensively at signalized intersections, they are suitable for use at unsignalized intersections as well.
EXHIBIT V-7
Strategy Attributes for Providing Offset Left-Turn Lanes at Intersections (T)
Technical Attributes
Target The strategy of providing offset left-turn lanes at unsignalized intersections is targeted to reduce the frequency of collisions between vehicles turning left and opposing through vehicles, as well as rear-end crashes between through vehicles on the opposing approach. The strategy is generally applicable to intersections on divided highways with medians wide enough to provide the appropriate offset.
Research has verified that offset left-turn lanes operate safely (Harwood et al., 1995), but there are no reliable estimates of their safety effectiveness. Safety effectiveness is likely to depend upon the traffic volumes of the conflicting turning and through movements and the amount of offset between the left-turn lanes at the intersection.

52. Positive Offset Left Turn Lanes
Angled positive offset
Parallel positive offset
Florida DOT –
very wide offsets
Instructor: more examples of offset left turn lanes
Ohio DOT

53. Positive Offset Left Turn Lanes
Dec 2007 CTRE study of 2 intersections finds are 35% reduction in total crashes; 45% decrease in crash rate; -77.5% reduction in targeted crashes
NC found 37% crash reduction for conversions of existing negative offset left turn lanes and 10% for new
Nebraska did study in 2004 of “widening” of existing left turn bays to create positive offset for 6 intersections with 2 control intersections found that:
Offsetting of opposing left-turn lanes by widening left-turn lane lines is effective in reducing accidents, The reduction in the expected accident frequency was about 0.285% however, the reduction appears to be city-specific, Offsetting of opposing left-turn lanes by widening left-turn lane lines reduces accident injury severity.
SC 329, Florence, South Carolina

54. Rural Multilane Intersections
Additional CMF’s beyond the HSM 1st Edition:
Access Density
“The effects on crash frequency of access management at or near intersections are not known to a sufficient degree to present quantitative information in this edition of the HSM. Trends regarding the potential crash effects or changes in user behavior are discussed in Appendix A.”
From Chapter 14 of Highway Safety Manual

55. CMF for Access Control for Rural Intersections
* From TTI synthesis
Unsignalized Intersections:
CMFnd = e0.056 * (dn - 3)
Signalized Intersections:
CMFnd = e0.046 * (dn - 3)
Where:
dn = Number of driveways on both the major and minor road approaches within 250 feet of the intersection
- Default value = 3 driveways
Instructor: From Chapter 6 of TTI Synthesis, coefficients (0.046 and 0.056) are from Table 6-12, page 6-20

56. CMF for Access Control for Rural Intersections
Prediction of CMF for an Unsignalized Intersection - Example:
For 4 driveways on US route and
3 driveways on County Route
CMFnd = e0.056 (dn - 3)
= e0.056 (7 - 3)
= e0.056 (4)
Instructor: From Chapter 6 of TTI Synthesis page 6-20; example calculation
The number of driveways (access) has a large safety effect.
= 1.251

57. J-Turns for Expressway Intersections
Instructor: from NCHRP 650 and the HSM
Replace Direct Left-Turns with Right-Turn/U-turn Combination
Replacing direct left-turns with right-turn/u-turn combination is applied to minor streets and driveways intersecting with divided arterials. A directional median is typically used to eliminate left-turns off of the minor street. Closing the side-street left-turn using directional median openings effectively forms a T-intersection with a closed median, eliminating direct left-turns at unsignalized intersections and driveways onto divided arterials. Drivers must turn right and then perform a U-turn on the divided arterial at a downstream location to access the desired side street or access point (32). Figure 14‑10 illustrates a conceptual example of closing a side street left-turn and serving the left-turn movement through a right-turn and U-turn movement.
*NCHRP 650

58. CMFs for Providing Indirect Left-Turns
Instructor: from NCHRP 650 and the HSM
Replace Direct Left-Turns with Right-Turn/U-turn Combination
Replacing direct left-turns with right-turn/u-turn combination is applied to minor streets and driveways intersecting with divided arterials. A directional median is typically used to eliminate left-turns off of the minor street. Closing the side-street left-turn using directional median openings effectively forms a T-intersection with a closed median, eliminating direct left-turns at unsignalized intersections and driveways onto divided arterials. Drivers must turn right and then perform a U-turn on the divided arterial at a downstream location to access the desired side street or access point (32). Figure 14‑10 illustrates a conceptual example of closing a side street left-turn and serving the left-turn movement through a right-turn and U-turn movement.

59. J-Turns for Expressway Intersections
Minor road traffic wishing to cross or turn left directly at the intersection are forced to turn right, make a downstream U-turn, and return back to the intersection to complete their desired maneuver.
This conflict-point management strategy thereby eliminates 20 crossing path conflict points present at a typical TWSC rural expressway intersection and replaces them with less risky conflict points associated with right-turns, U-turns, and weaving maneuvers.
Instructor: from NCHRP 650
*NCHRP 650

60. J-Turns for Expressway Intersections
Instructor: from NCHRP 650
*NCHRP 650

61. Predicting Crash Frequency and Application of CMFS for Rural Multilane Intersections
Learning Outcomes:
Described the models to Predict Crash Frequency for Rural Multilane Intersections
Described CMFs for Rural Multilane Intersections
Calculated Predicted Crash Frequency for Rural Multilane Intersections
Applied CMF’s to Rural Multilane Intersections
Learning Outcomes for Session #8 Prediction of Crash Frequency for Rural Multilane Intersections and application of CMF’s

62. Questions and Discussion:
Predicting Crash Frequency and Application of CMFS for Rural Multilane Intersections
Questions and Discussion: