Citation

Mountain L, Fawaz B, Sineng L. Traffic Eng. Control 1992; 33(2): 85-87.

Affiliation

Univ of Liverpool, Liverpool, England.

Copyright

(Copyright © 1992, Hemming Group)

DOI

unavailable

PMID

unavailable

Abstract

The assessment of the effectiveness of engineering measures in reducing accidents at a site requires a comparison of the observed accident frequency at the site in the period after treatment with the expected accident frequency had no treatment taken place. The most obvious method of assessment is to assume that, if no treatment had taken place, the ecpected accident frequency in the after period would have been the same as the observed accident frequency in the before period. A straightforward before-and-after comparison of this type does not, however, distinguish changes in accident frequency due to treatment from changes due to trends over time and regression-to-mean effects. To eliminate the effect of any trend it is common to use control sites of areas. Although control sites can be used to allow for regression-to-mean effects it then is essential that the control site is selected using the same selection criteria as the treated site. The use of controls has a number of disadvantages, and thus various alternative methods have been proposed for dealing with the regression-to-mean effect. This paper attempts to summarize some of theses methods in a simplified way and compares the estimates obtained using data for a group of intersections supplied by Cheshire County Council.