1. Design Consistency and Positive Guidance
Fall 2018
See the list of references in the syllabus for the material used in these slides.

2. Basic Principles Goals: “Safe” and efficient movement of people.
Designers seek to provide the safest and most efficient designs that may be practically be provided.
Several tools and techniques are available.
One technique is to analyze the consistency in design.
Designers are concerned with and providing consistent designs in order to:
Reduce failures
Conflicts
Need to understand working knowledge of these expectancies.

3. Basic Principles Concepts and Definitions:
Can be stated as to represent a set of possibilities regarding a given situation.
Known determinant of reaction time, signal detection, and vigilance.
Ellis (1972):
Driver expectancy relates to the observable features of the driving environment which:
(1) Increase a driver’s readiness to perform a driving task in a particular manner, and
(2) Cause the driver to continue the task until it is completed or interrupted.

4. Basic Principles Concepts and Definitions (cont’d):
Alexander and Lunenfeld (1990):
Expectancy relates to a driver’s readiness to respond to situations, events and information in predictable and successful ways.
Criteria:
Consistency checklist (subjective judgment, empirically based, or a combination of both).
Speed consistency measures (promote uniform vehicular speeds and reduce speed variability).
Driver workload (consistent level of effort).

5. Driver Performance Driver Information Processing
Mental workload
Definition: Time rate of information processing
Demand: Time Rate at which the information must be processed in order to operate a vehicle safely; it varies as a function of roadway, roadside development, and environmental conditions
Supply: maximum rate at which a driver can process information; part of the process is used for the driving task
Relationship between workload and driver performance (see graph)
There is an optimum workload demand
Driver performance is degraded at demand levels both below and above optimum

7. Driver Expectancy Examples of Design Inconsistencies: Left-hand exits
Lane drops
Freeway splits
Construction joints that do not follow lane markings
Narrow brides
Dips

8. Design Consistency Pavement Width Radius of curve
Crashes decrease with wider width until about 13 ft where it increases again.
Same kind of relationship with shoulder width.
Divided highways usually safer than undivided.
Radius of curve
Shorter radius, higher crash risk.
Unbalanced curves seem to be associated with higher crash risk.
Very short radius, higher crash risk (also a function of superlevation)

9. Design Consistency Degree of Curve/Curve Radii Ratio Spirals Grades
Similar relationship as before
Greater changes in operating speeds (design consistency).
Spirals
Safety effects mixed. Some studies say that using spirals reduces crash risk, while others report no relationships.
Grades
Below 2% is the safest.
Higher than 6% higher crash risk (speed variance) (although the original study examined flat segments)
Downgrade more dangerous than upgrade sections.

10. Design Consistency Sight Distance Traffic volume Design Speed
Better sight distance, lower risk (relationship is difficult to establish).
Higher risk for sight distance below 100 m.
Traffic volume
U-relationship between safety and traffic volume.
Very complex, since it is also a function of the speed- density-flow relationship.
Design Speed
Higher design speed usually associated with higher design standards (crash rates are lower given the exposure).

11. Design Consistency
Safety Criterion I

12. Design Consistency
Safety Criterion II

13. Design Consistency NCHRP Report 502
Propose Expert System for examining Design Consistency.
Function in the Interactive Highway Safety Design Model (IHSDM)
Warning levels, i.e.,
– Level 2 warning—lower severity and
– Level 1 warning—higher severity; and
• Potential remedial treatments.
Use Crash Modification Factors (CMFs), Changes in workload and/or speed

14. Design Consistency

15. Design Consistency

16. Design Consistency Lane Width CMF Warning Levels for Lane Width
Level 1: 10% increase in crashes
Level 2: 5% increase in crashes

17. Design Consistency Flow: 2,000 veh/day (ADT) Same shoulder width
10-ft lanes
12-ft lanes

18. Design Consistency 12 ft: CMF = 1.0
Anticipated increase in crashes of 30%. The design is inconsistent.

19. Positive Guidance

20. Positive Guidance

21. Positive Guidance

22. Positive Guidance

23. Positive Guidance

24. Positive Guidance

25. Positive Guidance

26. Positive Guidance Several Documents about PG in Work Zones
Complex road sections
Higher risk both for Road Users and Workers
Positive guidance principles ensure that information needed by travelers to negotiate safety through a work zone is provided:
At locations where it is needed;
In a format that is easily understood and is unambiguous; and,
At a rate that motorists can safely process and use.

27. Positive Guidance

28. Positive Guidance Good positive guidance: Effective, credible signing
Proper use of arrow panels
Use of warning flags and lights on signs
High quality work zone pavement markings
Reduction of channelizing device spacing
Use of longitudinal channelizing barricades.

29. Positive Guidance Important questions:
Have all driver and pedestrian information needs been met to allow them to navigate safely through the work zone?
Is the information provided to drivers and pedestrians standardized (or consistent) throughout the work zone so that it meets their expectations?
Has critical information been spread out through the approach and work are so that drivers and pedestrians are not over-loaded at any given time?
Are all hazards in the work identified so that they are visible, and therefore avoidable, for drivers and pedestrians?

30. Positive Guidance
Signing

31. Positive Guidance
Signs too close to each other

32. Signs not attracting enough attention
Positive Guidance
Signs not attracting enough attention

33. Positive Guidance

34. Arrow signs too far from edge of traveled-way
Positive Guidance
Arrow signs too far from edge of traveled-way

35. Positive Guidance

36. Incomplete Removal of Pavement Marking
Positive Guidance
Incomplete Removal of Pavement Marking

37. Positive Guidance

38. Devices not providing enough visibility
Positive Guidance
Devices not providing enough visibility

39. Positive Guidance