United States Department of Transportation
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Analysis and methods of improvement of safety at high-speed rural intersections [technical summary].
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2012-01-01
[PDF-600.09 KB]
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Edition:Technical summary.
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Abstract:INTRODUCTION: Since 2006, INDOT has been preparing an annual fivepercent
report that identifies intersections and segments
on Indiana state roads that require attention due to the
excessive number and severity of crashes. Many of the
identified intersections are two-way, stop-controlled intersections
located on high-speed, multi-lane, rural roads.
Some contributing design and human factors have been
identified, while other factors still await investigation.
Multivariate ordered probit models have been developed
to help identify additional factors of the frequency and severity
of crashes. These models can estimate how much
different factors increase the frequency of crashes at
several levels of injury severity (fatal/incapacitating, nonincapacitating/
possible, and property-damage-only). They
have a unique ability to account for unobserved but common
conditions that affect all of the crash severity levels.
Recommendations for safety countermeasures are made
based on both of these research results and our study of
published reports of other authors.
FINDINGS: The statistical analysis was performed on 553 existing
intersections in Indiana and 72 existing intersections in
Michigan using crash data reported during a four-year
period. The identified safety factors include the following:
presence of horizontal curves within the intersection vicinity,
traffic volume on the major road, land use, population
of the area surrounding the intersection, the minor road
functional class (traffic volume on minor road unknown),
nearby at-grade railroad crossings, intersection conspicuity
to drivers on the major road, acceleration lanes for
both left and right turns, median width, intersection angle,
and number of intersection legs. These results are in line
with other research results as documented in the literature
review.
Based on the results of this and other studies, recommendations
are made to improve safety at new intersections
as well as at existing intersections. For new intersections,
construction of medians wider than 80 feet is suggested.
Where this is not possible and a narrower median needs
to be constructed, adding a parallel acceleration lane for
vehicles turning left from the minor road is suggested. Intersections
should be placed at a sufficient distance from
horizontal curves and from at-grade railroad crossings.
Solutions with indirect left-turn lanes (Michigan U-turns,
J-turns) are recommended.
At existing intersections experiencing excessive numbers
of crashes involving vehicles from the minor road, median
closure should be considered or a median opening should
be restricted to certain maneuvers. Median acceleration
lanes can be added in order to allow a two-stage maneuver
for left turns from the minor road. Enhanced guide
and warning signage can be used to improve intersection
conspicuity; adding road illumination can especially help
at night. The practice of adding left- and right-turn bays
should be continued as this is a proven intersection safety
improvement practice. Applying these countermeasures
may help improve safety and avoid the construction of
expensive grade separations.
Finally, advanced intersection collision avoidance systems,
such as road-side dynamic signs warning drivers
on the minor road about a short gap on the major road,
should be the subject of pilot studies in Indiana. Experiments
in other states have indicated that these systems
help drivers choose safe gaps.
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