Road Injury Prevention Litigation Journal
Road Injury Prevention & Litigation Journal
Copyright © 1998 by TranSafety, Inc.
April 1, 1998
TranSafety, Inc.
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Recommendations for Shoulder-Drop Delineations at Bridges

Slower drivers and drivers with emergencies sometimes travel on or near the road shoulder. This practice may cause a problem when there is inadequate notice about shoulder gaps--such as a "shoulder drop," a discontinuation of the paved shoulder across a bridge span.

Recent controlled field tests studied driver behavior when a bridge shoulder drop was marked with an edgeline taper/transverse marking delineation pattern. Drivers encountering this pattern left the shoulder farther from the upcoming bridge than when they only saw object markers or post-mounted delineators at the bridge.

The shoulder-drop delineation study, sponsored by the Texas Department of Transportation (TxDOT) and conducted by the Texas Transportation Institute, evaluated alternative shoulder-drop bridge delineation methods. This study resulted in a report, "Delineation at Bridges with Paved Shoulder Drops," by Gerald L. Ullman and Val J. Pezoldt. The report appeared as Transportation Research Board Paper No. 980721, prepared for the January 1998 meeting of the Transportation Research Board in Washington, D.C.

BACKGROUND

Reviewing previous dropoff studies, the authors found several warning devices suggested for reducing the number of bridge crashes. They did not find specific data about which improvement or combination of improvements was most effective, or under which conditions. In field testing, many site-specific factors affect driving behavior. Moreover, narrow bridges and culverts are often on rural highways, where low vehicle volumes make it difficult for meaningful statistical comparisons.

The researchers used a model to divide the approach to a road hazard into five information-handling zones:

Recognizing these zones helped researchers assess the visibility of a hazard and identify where to locate hazard warning information. The diagram below shows these zones.

FIGURE 1
Information Handling Zones Upstream of a Bridge

The middle three zones--approach, non-recovery, and hazard are the most critical for hazard delineation system design. Drivers must change their speed, path, and (or) direction by the time they reach the non-recovery zone--170 to 260 meters from the bridge for speeds of 88 to 113 km/h.

The researchers considered alternative bridge delineation systems at a simulated narrow bridge site. They chose to use a controlled field study approach for several reasons:

STUDY METHODOLOGY

Researchers created a closed driving course. Test subjects drove at night on dry pavement in a vehicle that logged continuous speed, distance, and lateral position data. Each subject tested each of the delineation treatments.

Because roadway delineations deteriorate over time, the test simulated a worn condition with a mist of paint to produce a "near minimum" acceptable level of visibility. Sections of W-beam guardrail, bolted to the pavement, created a bridge mock-up to represent rural highway conditions.

Study Procedures

Study subjects received a description of their driving task but no information about what the researchers were testing. The test vehicle contained this test equipment:

The last two items simulated the appearance of other cars on the road and other distractions. Test drivers practiced driving the vehicle before the test began. As much as possible, they drove the test course at 90 km/h.

Delineation Treatments

The research studied five treatments chosen to test a range of delineation materials, their placement within the visual field, and their location on the approach to the bridge.

The diagrams below show these delineation treatments.

FIGURE 2
Schematic Layout and Delineation Treatments Tested in Studies






Subject Characteristics

The study examined two age groups: drivers younger than 25 and drivers 55 or older. Drivers tested each delineation treatment twice. Researchers recognized that having the drivers pass the same bridge site more than once may have had some effect on the driver response data.

Measures-of-Performance

The test vehicle provided the following information, with recording beginning 300 meters from the bridge:

Performance measures included (the numbers in brackets refer to Figure 3 below):

FIGURE 3
Measures Taken During Shoulder Drop Delineation Studies

Analysis of these measures used two-factor Analysis-of-Variance (ANOVA) statistical techniques, except for the variability in lateral positions at the beginning of the bridge, which used the Hartley Test of equal variances.

Results

Test results showed that older drivers tended to drive slightly more to the left than younger drivers, regardless of delineation. Older drivers also applied more deceleration force, regardless of the delineation treatment.

Delineation treatment only affected the distance from the bridge at which drivers of all ages began to leave the shoulder. Data clearly showed that drivers left the shoulder farther away from the bridge at Treatment 5 than at Treatments 1 and 2. Treatments 3 and 4 were less definitive. Researchers expected these results.

Test drivers ranked all the treatments as making their driving task "very easy" or "easy" as far as knowing where they were supposed to drive and as making "very clear" or "clear" what they were supposed to do.

The researchers concluded:

Roadway safety does not automatically improve if additional delineation is added at a point or along a roadway segment. It is even possible that excessive delineation can lead to operational problems in some instances. The goal of the transportation agency is to utilize an effective system of delineation techniques for a given condition at the least cost.

Copyright © 1998 by TranSafety, Inc.



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