First Advisor

Avinash Unnikrishnan

Date of Award


Document Type


Degree Name

Master of Science (M.S.) in Civil & Environmental Engineering


Civil and Environmental Engineering




Roads -- Interchanges and intersections -- Design, Traffic circles, Left-turn lanes, Transportation corridors -- Safety measures, Traffic circles -- California -- San Diego -- Design and construction -- Case studies, Traffic calming




Road diets are a popular tool for corridor improvement. Road diets are normally defined as the conversion of a four-lane undivided road to a three-lane undivided road made up of two through lanes separated by a center two-way-left-turn-lane (known as a TWLTL). This new configuration has similar vehicular capacity, greater safety and frees up right-of-way for other uses.

This work examines the possibility of going one step, or one lane, further. What is the possibility of reducing five- or four-lane roads to two-lanes? If this is possible, what are the corridor characteristics within which this process would be feasible?

The transformation of five- or four-lane roads to two-lane roads becomes possible through the replacement of signalized and stop-controlled intersections with roundabouts. Roundabouts allow all left-hand turns along a street segment to be replaced with a U-turn at the roundabout followed by a right turn. With roundabouts, the TWLTL can be removed thereby creating a 2-lane road.

Two major questions arise when determining the feasibility of this conversion. First, under what traffic conditions is this conversion possible? Second, does sufficient right-of-way exist for the conversion of standard intersections to roundabouts?

This paper will examine corridors which have undergone this transformation to estimate the envelope within which this conversion is possible. It will provide estimates of the size of roundabouts for desired capacities and a traffic volume ceiling for the corridor. This work is limited to road diets resulting in corridors with two traffic lanes and single lane roundabouts.

The final result of this work is a pamphlet similar to the 2014 FHWA Road Diet Informational Guide. The intent is to create a guide that accomplishes two goals. The first is to popularize a powerful strategy for roadway renovation. The second is to provide a tool that allows agencies to evaluate whether a chosen corridor is a possible candidate for this conversion.



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A research project report submitted in partial fulfillment of the requirement for the degree of Master of Science in Civil and Environmental Engineering.

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