Citation

Billheimer JW. Transp. Res. Rec. 1978; 663: 1-7.

Copyright

(Copyright © 1978, Transportation Research Board, National Research Council, National Academy of Sciences USA, Publisher SAGE Publishing)

DOI

unavailable

PMID

unavailable

Abstract

The Santa Monica Freeway Diamond Lanes, a pair of concurrent-flow preferential freeway lanes for buses and carpools linking the City of Santa Monica, California with the Los Angeles Central Business District (CBD), opened on March 16, 1976, and operated amid much controversy for 21 weeks until the U.S. District Court halted the project. One of the most disturbing aspects of the project was the high incidence of freeway accidents, which increased by a factor of 2.5 times pre-project levels when the barrier-free preferential lanes were operating. This paper tabulates accident levels before, during and after the project; postulates and analyses a number of hypotheses regarding potential accident causes; compares the Santa Monica Freeway accident history with that of other preferential lane projects; and identifies the most likely causes of the increased accident levels. This analysis is part of a broader study of the Diamond Lane Project sponsored by the Urban Mass Transportation Administration's Service & Methods Demonstration program. Factors contributing to the increased accident rate included the distracting effect of increased enforcement activities and the congestion resulting from the removal of freeway lanes from general use. However, it appears that the most significant factor was the pronounced speed differential between the free-flowing traffic in the sparsely-occupied preferential lane and the stop-and-go traffic in congested adjacent lanes, coupled with the frequent lane changes made by vehicles entering and leaving the freeway. The experiment in Santa Monica raises serious questions about the use of barrier-free preferential lanes.