1. Lec 33, Ch.5, pp : Accident reduction capabilities and effectiveness of safety design features (Objectives)
Learn what’s involved in safety engineering studies
Learn how to compute accident reduction capabilities of countermeasures
Learn how to estimate the effectiveness of safety design features (Reduction of the number of accidents)

2. What we discuss in class today…
Components of engineering studies
Condition diagram and collision diagrams
Accident reduction capabilities of countermeasures
Accident reduction factors – definitions
Accident reduction factors relating to improvements to roadway cross section

3. Component of the Highway Safety Improvement Program (HSIP) by FHWA
Need estimates of the effectiveness of safety design features

4. Conducting engineering studies (after hazardous locations have been identified)
Steps:
In-depth study of the accident data obtained for the study site
Conduct a field review of the study site
List possible accident (contributory) causes
Determine specific safety deficiencies at the site
Develop general countermeasures
Conduct an economic analysis (cost-effectiveness, rather than cost-benefit)
Recommend a list of countermeasure actions

5. Site analysis – Draw a condition diagram
Purposes:
To identify contributing causes
To develop site specific improvements
Two types of info:
Accident data
Environment & physical condition data
The first thing you do is visit the site and prepare a condition diagram of the site.

6. Site analysis (cont) – Prepare a collision diagrams

7. Site analysis (cont) – Questions to ask
Group accidents by type and answer the following 3 questions, which will lead you to possible countermeasures. See Table 5.3.
What driver actions led to the occurrence of such an accident?
What conditions existing at the location could contribute to drivers’ taking such actions
What changes can be made to reduce the chance of such actions occurring in the future?
Rear-end collisions:
Driver: Sudden stop & Tailgating
Environment: Too many accesses and interactions with vehicles in/out of the accesses (drive ways), bad sight distance, short/long yellow interval, inappropriate location of stop lines (against driver expectancy), etc.

8. Crash reduction capabilities
Used to estimate the expected reduction in crashes that will occur during a given period as a result of implementing a proposed countermeasure.
CR = crash reduction (CR) factors are used to indicate potential crash reduction capabilities.
N = expected number of crashes if countermeasure is not implemented and if the traffic volume remains the same.
Example 5-5:
CR = 0.3, ADT before = 7850, ADT after = 9000, No. of specific types of crash occurring per year = 12, 14, 13 for the same 3 years where ADT average values were computed.
Avg no. of crashes/year = ( )/3=13
Crashes prevented = 13 x 0.3 x (9000/7850) = say, 4 accidents

9. Procedure to determine Crash reduction factor (CR)
When multiple countermeasures are selected…
CR = overall crash reduction factor for multiple mutually exclusive improvements at a single site
CRi = crash reduction factor for a specific countermeasure i
m = number of countermeasures at the site
Example 5-6
CR1 = 0.40, CR2 = 0.28, and CR3 = 0.2. Determine the overall CR factor. Note that countermeasures are ordered in the descending order of their accident reduction factor values.
CR = (1 – 0.4)* (1 – 0.4)(1 – 0.28)*0.2
= 0.66

10. Effectiveness of safety design features (eventually we want to estimate the number of crashes that can be prevented (CP).)
In this chapter, we will see how (1) access control, (2) alignment, (3) cross sections, (4) intersections, and (5) pedestrian and bicyclist facilities might affect the overall safety of roadways. Among these cross section related factors are used as an example to compute CP values.
Access Control: Defined as “some combination of at-grade intersections, business and private driveways, and median crossovers”
More access control  Less accidents
e.g. interstates
Streets

11. Access control (cont)
Some methods to reduce crashes by controlling access:
Remove access points (remove median openings)
Provide frontage roads for business access
Provide special turning lanes (TWLTL or LT bays)
Warn motorists of changing conditions along the roadway using proper traffic control devices
More access, higher crash rates
(Note that the access control section of the chapter does not give CR values.)

12. Alignment (This topic was discussed in Ch. 16
Alignment (This topic was discussed in Ch. 16. Review that chapter to find out what affected vertical and horizontal alignment design.)
Vertical alignment  Most important factors include sight distance (especially crest vertical curves) and the vertical curve length.
Improvements to safety of horizontal curves include:
Improve the combination of V- and H-curves
Assure adequate pavement surface drainage
Provide increased skid resistance
Use a less sharp H-curve
Widen lanes and shoulders
Add spiral transition curves
Increase the amount of superelevation < max allowed
Increase the clear roadside recovery distance
(Note that the alignment section of the chapter does not give CR values.)

13. Cross sections (this section gives CR values)
Clearance
(CR values)

14. Cross sections (cont) (CR values) for shoulders (Combined effects)
(AR values)

15. Cross sections (cont) (CR values)
Table 5.11 is slightly different from other tables. It does not give CR values. It gives % or cross-section related crashes (RC values) including run-off-road, head-on, and opposite- and same-direction sideswipe.
Table 5.11 Ratio of Cross Section Related Crashes to Total Crashes on Two-Lane Rural Roads

16. Auxiliary lanes can reduce crashes (because they provide safer passing opportunities.
F = fatal accidents
I = injury accidents
(CR values)

17. Example 5-7 Given: A two-lane two-way highway in mountainous terrain
53 crashes per year (3 year average)
Currently 10-ft wide lane, 2-ft unpaved shoulder
ADT = 4000 vpd
Improvement options:
Widen 10-ft lane to 12-ft lane (2 ft increase)
Widen unpaved 2-ft shoulder to paved 6-ft shoulder
A combination of the two options
Find the expected number of accidents reduced:
RC = 53 x 0.61 = 32 related crashes (Tab 5.11)
Crashes prevented (CP) by lane widening = 32*0.23 = 7 accidents/yr (Tab 5.8)
CP by shoulder widening = 32*0.29 = 9 accidents/yr (Tab 5.9)
CP by the combination = 32*0.46 = 15 accidents/yr (Tab 5.10)