Criteria and Guidelines for Three-Lane Road Design and Operation

Principal Investigator(s):

Gary Davis, Professor, Civil, Environmental and Geo-Engineering


  • John Hourdos, Former Research Associate Professor, Civil, Environmental and Geo-Engineering

Project summary:

Road diets, which convert four-lane undivided highways to three-lane cross sections, are an innovative solution to address mobility and safety concerns under budgetary constraints. Such conversions have become increasingly popular with non-motorized transportation advocates since, seemingly with only just paint, new bike facilities can be added on the existing road width. Past research has primarily focused on evaluating the safety of road diets. There has been very little research on the operational effects of such redesigns and limited guidance on the differences of geometric design choices. The most recent guidelines, covered in the Federal Highway Administration (FHWA) Road Diet Information Guide, stresses the need for more research on operational effects and highlights several operational characteristics as important but lacking proper guidelines or means to include in a cost/benefit analysis. As pointed out by the RNS-506 champion, this lack of guidance and quantifiable pros and cons, other than crash reduction, prevents agencies from avoiding conversions on roadways with annual average daily traffic (AADT) greater than ~15,000, although evidence has been offered of successful operation of three-lane roads with AADT upwards to 25,000. From a preliminary scan of the available information, it is also possible to identify the empirical way limiting factors like pedestrian level of service (LOS), freight LOS, high volume, and un-signalized intersections are currently approached, resulting in a plethora of geometric variations for crossings, access management, lane widths, transit stops, and more. This project seeks to: 1) codify existing guidance on three-lane road conversions through a review of existing research and practice; and 2)conduct research needed to close identified and important gaps in existing knowledge. The final products will be a HCM-type model relating traffic demand and geometric/control design features to predicted LOS-based performance and a HSM-type model relating the same to predicted crash experience.

Project details:

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