Crash Data Analysis

Introduction  Accident Analyses  Additional Information 


The purpose of this brochure is to help explain how crash data is analyzed by the Maryland State Highway Administration (SHA) and what types of reports and summaries are produced. In a typical year, an estimated 18 million traffic accidents occur in the United States. These accidents involve some 27 million vehicles, and result in some 40,000 fatalities, over 5 million injuries, and losses that exceed $150 billion. The MSHA strives to reduce the number and severity of crashes in Maryland to the lowest attainable levels.

Accident Records

All crashes resulting in a vehicle being towed away, personal injury, or fatality are reported. The state, county, or local law enforcement officer who first arrives at the scene of a reportable accident records the crash data.Typically, within 10 days, the accident report is submitted to the Maryland State Police Central Records Division for transfer, into the Maryland Automated Accident Reporting System (MAARS) database files; within 30 days, the data is uploaded to the Maryland State Highway Administration's database and the Maryland State Police database.

Accident Study Terminology

Using accident data to determine the cause(s) of crashes and finding ways of preventing them or reducing their severity is a primary objective of SHA. In order to understand how highway safety professionals accomplish these goals, one needs to first understand a few commonly used terms.

Accident Frequency
Accident frequency is the number of accidents that occur at an intersection or on a section of highway over a period of time. Frequency is often used to assess trends of accidents at a single location. These trends are studied for three, five, or ten year periods, so that the average annual experience can be assessed, smoothing out the high and low fluctuations that occur yearly.

Accident frequency alone is not sufficient to compare the relative hazard of one location to that of another; the traffic volume exposure and the highway section length must be taken into consideration to have a valid comparison. For instance, if two intersections have the same accident frequency, the location having less traffic exposure, and hence less opportunity for accidents, is considered to be more hazardous.

Traffic Volume Exposure
Traffic volume exposure is the amount of traffic traveling along a section of road or entering an intersection. This is commonly expressed as average daily traffic (ADT).

Accident Rates and Percentages
For a section of highway, the rate is calculated as accidents per 100 million vehicle miles of travel (Acc/100MVM). For an intersection, the rate is calculated as accidents per million vehicles entering (Acc/MVE).

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