Date of Award

Fall 2014

Degree Type


Degree Name

Master of Science (MS)


Civil Engineering

First Advisor

Andrew P. Tarko

Committee Member 1

Kumares C. Sinha

Committee Member 2

Darcy M. Bullock


The objective of this thesis was to investigate the factors that affect capacity-related driver behavior on modern roundabouts built on high-speed roads. The capacity of roundabouts is strongly affected by the behavior of drivers as represented by critical headway (critical gap) and follow-up headway (follow-up time). The effects of heavy vehicles (single-unit truck, bus, and semi-trailer) and area type (rural or urban) on roundabout capacity were investigated by comparing the critical headways for roundabouts located on high-speed and low-speed roads. The effects of nighttime conditions (in the presence of street lighting) were also considered. Data were collected using the Purdue Mobile Traffic Lab at four roundabouts built on state roads located in Indiana. The data were used to estimate a Probit model of the critical headways and their factors, as well as the follow-up headways. The findings revealed that drivers of heavy vehicles accepted critical headways that were 1.1 seconds longer than those of the passenger car drivers; on roundabouts built on high-speed roads in rural areas, drivers accepted critical headways that were 0.6 seconds longer than on roundabouts on low-speed roads in urban areas; and in nighttime conditions, drivers accepted critical headways that were 0.6 seconds longer than in daylight conditions. In addition, it was determined that the gap-acceptance parameters for a single-lane roundabout on a low-speed state road were less than those of the National Cooperative Highway Research Program (NCHRP) Report 572 average estimated values - which are currently incorporated into Highway Capacity Manual (HCM) 2010, resulting on average in 30% higher capacity for Indiana conditions. In contrast, the estimated critical headway was larger for dual-lane roundabouts on high-speed state roads, resulting in 15% reduced capacity (for medium to high circulatory traffic volumes) for Indiana conditions. The findings of this thesis are intended to improve capacity estimation for the roundabouts planned on Indiana state roads. The HCM 2010 capacity equations were updated with the new estimated gap-acceptance parameters for Indiana. The findings contribute to better understanding of the roundabout capacity factors.