Performance Evaluation of Turbo and Double-Lane Roundabouts

This paper aims to compare and evaluate turbo (TB) and double-lane (DL) roundabouts. The two types of roundabouts have many similarities and some differences. Thus, engineers are facing difficulty deciding which design is more effective. This paper evaluates both roundabouts regarding safety, performance, pollution, directional split, level of service, and capacity. The TB roundabout has fewer conflict points than the DL roundabout. The results show that the TB roundabout provides more capacity when the right-turn movement from the minor entries is high. Otherwise, DL and TB roundabouts perform similarly. The TB roundabout is the best design for reducing local pollution levels. However, if the aim is to reduce CO2 and NOx specifically, the DL roundabout is better. Furthermore, there are various parameters to evaluate the effectiveness of DL and TB roundabout designs, such as stopping sight distance, intersection sight distance, and the fastest path. The approaching speed for DL and TB roundabouts is 25 mph (40 km/h) and 20 mph (32 km/h), respectively. The minimum sight distance to approach the crosswalk and stop safely for DL and TB roundabouts is 152 ft (46 m) and 112 ft (34 m), respectively, indicating that the TB roundabout is a safer design. Nonetheless, the minimum entering sight line distance is larger in TB than DL for intersection sight distance, 197.2 ft (60 m) and 151.7 ft (46 m), respectively.