Abstract
This paper presents a procedure for analysing safety and operational improvements made possible by converting traffic circles to modern roundabouts. An Italian case study is presented for alternative layouts under various traffic demand scenarios. In the application of the procedure, the average waiting times and queue lengths at entries are computed with an analytical capacity model, using default values for gap parameters. Then, the roundabout is dynamically simulated. The simulation results in a revised set of gap parameters that are in turn used as inputs to a second trial of the capacity model, and in turn fed back into the simulation. The two steps are repeated until the parameters reach a pre-selected convergence criterion, so that gap parameter values for both the static capacity and dynamic microsimulation models are in equilibrium. Therefore, the applied procedure can conduct both static and dynamic roundabout design, usually applied separately. One can start with default values in guidelines and couple them with limited field data, improving both the expected results and cost-effectiveness of solutions. Next, safety is estimated using dynamic simulation software and a compatible conflict counting model to acquire surrogate measures of safety. Level-of-serviceand surrogate safety indicators for the existing and redesigned roundabouts are then compared. The procedure is first demonstrated on an old “ultra-large” roundabout. The procedure is tested on this roundabout using the Highway Capacity Manual 2016 (HCM2016), AimsunTM, and Surrogate Safety Assessment Model (SSAM) software. A redesign is shown to be far superior in efficiency and safety. Finally, two cases are described where large first generation roundaboutswere upgraded to modern standards.
Highlights
The modern roundabout was first introduced widely in the United Kingdom in the 1960s
Modern roundabout design is becoming the standard in many urban areas around the world, the public may confuse them with older style traffic circles and rotaries
The following procedure has been tested on an old traffic circle, today operating as ultra-large roundabout, at the junction of Viale Boccaccio, Viale Nazario Sauro and Via di Levante, in Livorno, Italy (Figure 1)
Summary
The modern roundabout was first introduced widely in the United Kingdom in the 1960s. The design is intended for vehicle speeds not lower than 40 km/h (25 mph) and requires a central island radius of at least 23 m (75 ft) so that entering vehicles can merge and interweave with those on the circulating roadway. The California Department of Transportation converted a traffic circle in Long Beach to a modern roundabout in 1993 This conversion was the first of its kind in the US and involved modifications to all entries. The modern roundabout is a more desirable solution for roadway intersections This raises a question: what should be done with older, less safe, traffic circles, rotaries, and first generation roundabouts?. Converting and upgrading traffic circles and rotaries to modern roundabouts is highly recommended to improve safety and functional performances of intersections. It uses SSAM software to obtain surrogate safety indicators of proposed redesigns
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