Roadside Safety Design

1. Roadside Safety Design Fall 2013 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. 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 Topography

24. Roadside Topography

25. Roadside Topography The issue is related to the fact that the channel is located within the recovery area. If the channel would have been outside, then it would not have had an issue.

26. Roadside Topography Drainage Channel Design

27. Roadside Topography

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

29. SLIP BASED SYSTEMS

30. SLIP BASED SYSTEMS

31. 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.

32. ROADSIDE BARRIERS Warrant for embankment

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

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

35. ROADSIDE BARRIERS Definition of roadside barriers

36. 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

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

38. ROADSIDE BARRIERS Deflection Distance

39. ROADSIDE BARRIERS Terrain Effects

40. ROADSIDE BARRIERS Terrain Effects

41. 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

42. ROADSIDE BARRIERS End Treatments

43. MEDIAN BARRIERS Suggested Guidelines

44. MEDIAN BARRIERS Suggested Guidelines: End Treatments

45. MEDIAN BARRIERS Three-Stand Cable

46. MEDIAN BARRIERS Box-Beam Barrier

47. MEDIAN BARRIERS Strong Post W-Beam

48. MEDIAN BARRIERS Barrier Placement

49. ROADSIDE BARRIERS Barrier Placement

50. 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).