1. Roadside Safety Design
Fall 2017
Most of the Material taken from:
Roadside Design Guide (2011) published by AAHSTO

2. History of Roadside Safety
1940s and 50s
Most highway design components established here: horizontal alignment, vertical alignment, etc.
1960s
Roadside safety design started to be discussed here
1970s
Roadside safety design incorporated into highway projects
1970s - now
National guidelines and substantial amount of research conducted on roadside safety design

3. Forgiving Highway Concept
Driver Fatigue
Excessive Speed
Driving Under the Influence
Crash Avoidance
Roadway Conditions
Vehicle Failure
Poor Visibility

4. Forgiving Highway Concept
1. Remove obstacle
2. Redesign obstacle
3. Relocate obstacle
4. Use breakaway devices
5. Shield obstacle
6. Delineate obstacle

5. Crash Testing Cannot duplicate every roadside condition
Provides an acceptable level of performance under normalized conditions
NCHRP Report 350 provides recommendations for testing and evaluating the performance of roadside devices:
A basis for comparison of impact performance merits of candidate safety features
Guidance for safety manufacturers
A basis for the formulation of safety feature performance specifications

6. Crash Testing Guidelines:
Vehicle mass, speed, approach angle, and point on the safety feature to hit
Test vehicle: passenger cars, ¾-ton pickup, tractor- trailer
Impact speed: 20 to 60 mph
Angle 0 to 25 degrees

7. Crash Characteristics
First Harmful Event Fixed Object Fatalities (FARS)

8. Crash Characteristics

9. Roadside Topography
Clear-zone concept: provide a traversable and unobstructed roadside area adjacent to the traveled way
Dependent on design speed and vehicular traffic
Roadside design affected by horizontal alignment
Geometry:
Foreslope
Backslope
Transversable slope (used in median)

10. Roadside Topography
Clear Zone: Area located between the edge-of-travel way and the location of fixed objects (trees, posts, etc.)

11. Roadside Topography Foreslope 1V:4H or flatter: Recoverable
1V:4H – 1V:3H: Non-Recoverable
<1V:3H: Critical (barrier needed see Chapter 5)

12. Roadside Topography
Backslope

13. Roadside Topography
Drainage Channel

14. Roadside Topography

15. Roadside Topography

16. Roadside Topography
Traversable slope

17. Roadside Topography
Traversable slope

18. Roadside Topography

19. Roadside Topography Adjustment for horizontal curves:
Table 3.2 provides adjustment factors as a function of radius and design speed.
These values are only needed if the site has been experienced safety problems (e.g., run-off-the-road crashes)
See overhead.

20. Roadside Topography
Adjustment for horizontal curves:

21. Drainage Channel Design
Roadside Topography
Drainage Channel Design
Acceptable only for the following characteristics:
Restrictive ROW
Resurfacing, Restoration or rehabilitation (3R) projects
Rugged terrain
Low Volume Roads
If not, you need a barrier

22. Roadside Topography
Gradual design

23. Roadside Support Structures
Sign Supports
Traffic Signal Supports
Luminary Poles
Utility Poles
Callboxes
Trees
Many are now designed breakaway supports

24. SLIP BASED SYSTEMS

25. SLIP BASED SYSTEMS

26. Roadside Barriers
Used to shield and protect obstacles that cannot be removed (person-made or natural)
A barrier becomes itself an obstacle
Hence, the way the barriers are designed will significantly affect the risk of injuries when they are hit by a vehicle (i.e., this is why they are tested).
There exist several types of barriers:
Rigid: Jersey, Single slope, etc.
Semi-Rigid: Box-beam, Steel-Backed Timber
Flexible: W-beam, Three-stand cable
The type of barriers selected is governed by the initial costs, repair/maintenance costs, risk of injuries, etc.

27. Warrant for embankment
ROADSIDE BARRIERS
Warrant for embankment

28. Warrants for non-transferable terrain and objects
ROADSIDE BARRIERS
Warrants for non-transferable terrain and objects

29. Example Design based on Speed, Volume, Slope Geometry
ROADSIDE BARRIERS
Example Design based on Speed, Volume, Slope Geometry

30. Definition of roadside barriers

31. BARRIER PLACEMENT Lateral Offset (def’d as “shy line offset”)
Depends on speed
Shy line offset not so critical for long runs of railing (as long as the barrier was introduced at or beyond the shy line offset)
Terrain Effects
Best results will occur when all four wheels are on the ground and the suspension is not compressed nor extended
Flare Rate
The rate between the end of the barrier and bridge railing
Can affect how the vehicle will be redirected into traffic
Length of need
This the length needed to shield an object

32. ROADSIDE BARRIERS Shy Line Offset
Shy-line offset: Distance between objects and barriers.

33. ROADSIDE BARRIERS
Deflection Distance

34. ROADSIDE BARRIERS
Terrain Effects

35. ROADSIDE BARRIERS
Terrain Effects

36. ROADSIDE BARRIERS End Treatments
Dependent on the type of barrier: w-beam, cable, concrete, metal (rigid)
Energy versus non-energy-absorbing
Flared versus tangent
Site grading
Advanced grading (no less than 1:10)
Adjacent grading

37. ROADSIDE BARRIERS
End Treatments

38. MEDIAN BARRIERS
Suggested Guidelines

39. Suggested Guidelines: End Treatments
MEDIAN BARRIERS
Suggested Guidelines: End Treatments

40. MEDIAN BARRIERS
Three-Stand Cable

41. MEDIAN BARRIERS
Box-Beam Barrier

42. MEDIAN BARRIERS
Strong Post W-Beam

43. MEDIAN BARRIERS
Barrier Placement

44. ROADSIDE BARRIERS
Barrier Placement

45. ROADSIDE SAFETY ANALYSIS PROGRAM
Program used to perform a cost-benefit analysis of roadside conditions
Four modules:
Encroachment
Crash Prediction
Severity Prediction
Benefit-Cost
Currently being completely revised (Hence, won’t spend a lot time on this).

46. RSAP -Issues
Only good for roadside devices (no rollover, crossover collisions, etc.)
Long computation times (simulation)
Multiple solutions
Encroachment algorithm developed 30 years ago
Vehicle path linear
Lateral encroachment distributions (simplistic model)
Crash Severity (highly variable)