Traffic Flow Evaluation of Pavement Inset Lights for Use During Fog: Final Report

Report No: 78-R25

Published in 1977

About the report:

Reduced visibility resulting from fog presents a very hazardous condition on the highway because of the inability of motorists to readily observe pavement markings and signs and the movement of traffic. Afton Mountain, which is traversed by I-64, often is the site of such reduced visibility because of the low cloud covering the mountain top during periods of rain. An awareness of the fog problem on Afton Mountain led to a decision to install a lighting system consisting of pavement inset lights to aid motorists during periods of fog on a 5.8-mile (9.4-km) section of I-64 encompassing the top of the mountain. The purpose of this research was to investigate the characteristics of the traffic flow with the system of pavement inset lights during fog through the collection and analysis of data before and after the lights were installed. The traffic flow parameters evaluated were vehicle speeds, headways, queues, and lateral placement. A before and after accident analysis was made also. Overall, the lighting system led to higher nighttime speeds, an increase in speed differentials for various cases during both daytime and nighttime, and a decrease in nighttime headways and queuing. These changes in traffic flow characteristics may be construed as producing an increase in the potential for accidents; however, they are thought by the author to be a result of the inset lighting system providing improved delineation for the guidance of motorists. This improvement in guidance, especially during fogs at night, should provide safer driving conditions than hitherto existed.

Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.


  • Frank D. Shepard

Last updated: January 27, 2024

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