Heavy Truck Gap Acceptance and Critical Gap Length Evaluation at Congested Roundabouts

Heavy trucks are believed to experience increased delays when entering congested roundabouts due to their increased size and more restrictive operating characteristics as compared to passenger cars. The present study evaluated 164 h of video data to transcribe 2,626 heavy truck movements at single-lane roundabouts across Oregon and Washington. Gap acceptance and gap rejection timings were evaluated to quantify heavy truck accessibility when entering congested roundabouts. Further, a variety of heavy truck classifications were observed and evaluated individually, with the most common being the WB-40, WB-50, WB-62, WB-67, WB-67D, and WB-92D. Within each of these classifications, gap acceptance behavior differed. Average rejected gap lengths were 3.84 s, while average accepted gap lengths were 8.69 s, indicating heavy trucks often accepted gaps that were over two times larger than the prior gaps. Critical gap length assessment was carried out following Raff’s method on an individual-classification basis to compare critical gaps across heavy truck classifications. Critical gap lengths for heavy trucks ranged from 5.4 to 6.4 s, approximately two to three times larger than that of passenger car critical gap values of 2.2 and 2.6 s documented in prior research. Additionally, heavy truck critical gap requirements were found to increase proportionately with the heavy truck size. The WB-40 was the smallest heavy truck assessed and was found to have a critical gap of 5.4 s. Conversely, the WB-92D was the largest heavy truck and was associated with a critical gap of 6.4 s. These findings indicate that roundabout locations featuring high heavy truck volumes may need additional considerations due to the operational restrictions of these larger vehicles, and that the assessment of roundabout implementation should include analysis of the vehicle classifications using these facilities to most optimally design for the current and future demand.