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