Study Analyzes Crashes on the Capital Beltway

NHTSA · 1996 · ROSA P / United States. Department of Transportation. National Highway Traffic Safety Administration

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Summary

Motivated by a series of major crashes on the Washington Capital Beltway in August 1993, the National Highway Traffic Safety Administration (NHTSA) sponsored studies to improve safety on this 64-mile interstate facility. This report details one such study conducted by Preusser Research Group, Inc., which aimed to understand crash causation by analyzing driver behavioral errors rather than solely physical characteristics. The study examined crash patterns over a two-year period, 1993 and 1994, contributing to a broader effort that resulted in 53 safety initiatives by December 1993. The methodology involved analyzing the narrative portions of 4,447 Virginia and Maryland police accident reports, which were coded into a database. The analysis categorized crashes by type, driver demographics, vehicle type, and location relative to interchanges. Additionally, a separate database sorted crashes by date, time, mile marker, and loop direction to identify temporal and spatial relationships between incidents. The findings revealed that three specific crash types accounted for 78 percent of all Beltway crashes, many linked to congestion during peak hours. "Stop-Slowing" crashes, where a vehicle is rear-ended after slowing for traffic, constituted 36 percent of incidents. "Ran Off Road" crashes, typically involving single vehicles late at night, accounted for 23 percent. "Sideswipe-Cutoff" crashes, often resulting from lane changes in congestion, made up 18 percent. Demographic analysis showed that 57 percent of drivers were aged 21 to 40, with most being local residents rather than out-of-towners. Vehicle analysis indicated that while tractor-trailers comprised nearly 10 percent of vehicles involved, they were disproportionately likely to be involved in sideswipe-cutoff crashes (46 percent of their incidents), whereas stop-slowing was the most common crash type for cars. Location analysis focused on 39 interchanges, categorizing crashes as Main Line, Ramp Related, or On Ramp. Main Line crashes were the most frequent at all interchanges except the Springfield interchange, where On Ramp crashes predominated. The study also identified a "crashes causing crashes" phenomenon, finding that up to 10 percent of crashes were followed by subsequent incidents within a two-hour window. These secondary crashes typically occurred within five minutes and one-tenth of a mile of the primary crash, with effects extending up to one mile upstream for the first hour and a smaller effect downstream. These results provided specific evidence to support ongoing safety improvements and future NHTSA studies.

Key finding

Three crash types accounted for 78 percent of Capital Beltway crashes: stop-slowing rear-ends at 36 percent, run-off-road at 23 percent, and sideswipe-cutoff at 18 percent.

Methodology

other

Sample size: 4447

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

Ranked by relevance to this paper. Hover a topic for its definition.

• crash typology
• incidence prevalence
• secondary collisions
• pre crash contributing factors
• motorcycle crash typology
• naturalistic crash near crash

Information type

What kind of knowledge this paper contributes, grouped by family — independent of topic (what it is about) and method (how it was studied).

• Empirical Findings: crash risk outcomes, observational prevalence