Fall 2014 Most of the Material taken from: Roadside Design Guide (2011) published by AAHSTO.
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Transcript of Fall 2014 Most of the Material taken from: Roadside Design Guide (2011) published by AAHSTO.
Roadside Safety Design
Fall 2014
Most of the Material taken from:Roadside Design Guide (2011) published by AAHSTO
History of Roadside Safety
1940s and 50s
1960s
1970s
1970s - now
Most highway design components established here: horizontal alignment, vertical alignment, etc.Roadside safety design started to be discussed here
Roadside safety design incorporated into highway projects
National guidelines and substantial amount of research conducted on roadside safety design
Driver Fatigue Excessive Speed Driving Under the Influence Crash Avoidance Roadway Conditions Vehicle Failure Poor Visibility
Forgiving Highway Concept
1. Remove obstacle 2. Redesign obstacle 3. Relocate obstacle 4. Use breakaway devices 5. Shield obstacle 6. Delineate obstacle
Forgiving Highway Concept
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
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
Crash Testing
Crash CharacteristicsFirst Harmful Event Fixed Object Fatalities
(FARS)
Crash Characteristics
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)
Roadside Topography
Roadside Topography
Clear Zone: Area located between the edge-of-travel way and the location of fixed objects (trees, posts, etc.)
Roadside Topography
Foreslope
1V:4H or flatter: Recoverable
1V:4H – 1V:3H: Non-Recoverable
<1V:3H: Critical (barrier needed see Chapter 5)
Roadside Topography
Backslope
Roadside Topography
Drainage Channel
Roadside Topography
Roadside Topography
Roadside Topography
Traversable slope
Roadside Topography
Traversable slope
Roadside Topography
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.
Roadside Topography
Adjustment for horizontal curves:
Roadside TopographyDrainage 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
Roadside Topography
Gradual design
Sign Supports Traffic Signal Supports Luminary Poles Utility Poles Callboxes Trees Many are now designed breakaway
supports
Roadside Support Structures
SLIP BASED SYSTEMS
SLIP BASED SYSTEMS
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.
Roadside Barriers
ROADSIDE BARRIERSWarrant for embankment
ROADSIDE BARRIERSWarrants for non-transferable terrain and
objects
ROADSIDE BARRIERSExample Design based on Speed, Volume, Slope
Geometry
ROADSIDE BARRIERS
Definition of roadside barriers
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
BARRIER PLACEMENT
ROADSIDE BARRIERS
Shy Line Offset
Shy-line offset: Distance between objects and barriers.
ROADSIDE BARRIERS
Deflection Distance
ROADSIDE BARRIERS
Terrain Effects
ROADSIDE BARRIERS
Terrain Effects
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 gradingAdvanced grading (no less than 1:10)Adjacent grading
ROADSIDE BARRIERS
End Treatments
MEDIAN BARRIERSSuggested Guidelines
MEDIAN BARRIERSSuggested Guidelines: End
Treatments
MEDIAN BARRIERS
Three-Stand Cable
MEDIAN BARRIERS
Box-Beam Barrier
MEDIAN BARRIERS
Strong Post W-Beam
MEDIAN BARRIERSBarrier Placement
ROADSIDE BARRIERSBarrier Placement
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).
ROADSIDE SAFETY ANALYSIS PROGRAM
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)
RSAP -Issues