Traffic Tech #194: Urban Interstate Crashes Have Typical Patterns

URBAN INTERSTATE CRASHES HAVE TYPICAL PATTERNS

In August 1993, a series of major crashes on the Washington Capital Beltway focused Federal, State, and local attention on the need to further improve safety on this 64-mile interstate facility. The National Highway Traffic Safety Administration (NHTSA) sponsored six studies to document how, why, who, and where crashes occur on the Beltway (see Traffic Techs No. 124 and 175). Preusser Research Group, Inc. of Trumbull, Connecticut extended an earlier study to examine the patterns of crashes during a four year period - 1993 through 1996, categorizing driver behavioral errors that so often cause crashes.

In-Depth Study of Beltway Crashes

The narrative portion of each of 9,403 Virginia and Maryland police crash reports for Beltway crashes were analyzed and coded into a database for the four years between 1993 and 1996. While the number of crashes increased each year from 2,173 (1993), 2,274 (1994), 2,442 (1995) to 2,514 in 1996, the crashes per vehicle miles traveled remained relatively constant because of increased travel on the Beltway. The figure above shows that the number of serious crashes, those involving injury or fatalities, have changed on the four major segments of the Beltway. Maryland's portion is about twice as long as Virginia's. In Maryland, the number of serious crashes decreased on the I-495 segment from 1993 to 1996, but increased on the I-95 segment while the opposite occurred on Virginia's I-495 and I-95 segments. The report includes graphs showing changes through the years by mile marker. The number of fatal crashes decreased; there were 17 in 1993, 15 in 1994, 14 in 1995, and 9 in 1996, and alcohol-related crashes remain infrequent.

Crash Patterns Remain Consistent

Three types of crashes accounted for 78 percent of all Beltway crashes. Many of the crashes related to congestion during peak travel hours, and these remained relatively consistent over time.

Stop-Slowing (3,381 crashes; 36%) In most cases, the crash report had enough information to assign one of several sub-groups of stop-slowing. Typically, congested-related (2,756) crashes show that a lead vehicle slowed or stopped because of congestion, and a following vehicle in the same travel lane did not slow down fast enough to avoid it. In lead-vehicle swerve (113), a vehicle changed lanes in front of a second vehicle and then immediately slowed down and in follow vehicle swerve (118), a vehicle changed lanes coming in behind a vehicle that had already slowed or stopped. These were two maneuvers where a swerving vehicle attempted to avoid a stop-slowing crash in one lane only to become involved in a crash in an adjacent lane.

Ran Off Road (2,282; 24.3%) This type is not usually congested related and typically involves a single vehicle, often late at night. A vehicle leaves the road and strikes some object or overturns at the road-side, the shoulder, or another point off the main lanes.

Sideswipe-Cutoff (1,672; 17.8%) This crash type differs from lead and follow vehicle swerve crashes in that the collision occurred during the lane change and not immediately thereafter. Congestion was often a factor, as lane change maneuvers were made in response to slower moving traffic ahead. In some cases, it was made while attempting to exit the Beltway and in a few cases, by an inattentive driver. Most commonly, this crash type involves a car sideswiping another car (768), followed by a tractor-trailer sideswiping a car (399) and, less commonly, a car sideswiping a tractor-trailer (208).

Drivers

The number of passenger vehicle drivers in crashes increased 14 percent from 1993 to 1996, and was distributed across all age groups. Drivers between the ages of 21-30 (18.2 percent) accounted for 33.3 percent of crashes across the years. Drivers between the ages of 31-40 grew from 22.7 percent in 1993 to 25.7 percent in 1996. Drivers under the age of 21 had more ran off road (20.9 percent) crashes than any other age group, closely followed by the 21-30 age group. Across all years, 55.7 percent of passenger vehicle drivers who have crashed on the Beltway lived at least ten miles outside of the Beltway; 17 percent lived in states other than Maryland, Virginia, or the District of Columbia.

Trucks in Crashes

More than 19,158 vehicles were involved in the 9,000 Beltway crashes during this four year period. Nearly 8 percent of these vehicles were tractor-trailers, 6 percent were straight trucks, and 86 percent were passenger and other light vehicles. There are differences in the types of crashes for different types of vehicles. Tractor-trailers were much more likely to be involved in a sideswipe-cutoff crash; straight trucks and passenger vehicles were more often in a stop-slowing crash.

HOW TO ORDER

For a copy of Analysis of the Capital Beltway Crash Problem 1993-1996 (25 pages plus appendices) write to the Office of Research and Traffic Records, NHTSA, NTS-31, 400 Seventh Street, S.W., Washington, DC 29050, send a fax to (202) 366-7096. Linda Cosgrove, Ph.D., was the contract manager for this project.

http://www.nhtsa.dot.gov/people

U.S. Department
of Transportation
National Highway
Traffic Safety
Administration
400 Seventh Street, S.W. NTS-31
Washington, DC 20590

Traffic Tech is a publication to disseminate
information about traffic safety programs,
including evaluations, innovative programs,
and new publications. Feel free to copy it as you wish.
If you would like to receive a copy contact:
Linda Cosgrove, Ph.D., Editor, Evaluation Staff
Traffic Safety Programs
(202) 366-2759, fax (202) 366-7096
mailto:lcosgrove@nhtsa.dot.gov