Eleni Christofa
University of Massachusetts - Amherst
Civil Engineering
Michael A. Knodler, Jr., Ph.D
University of Massachusetts - Amherst
Civil Engineering




Impact of Deflection Angle on Roundabout Driver Behavior

Roundabouts have been gaining popularity as an alternative to signalized intersections, due to their potential for more efficient and safe traffic operations. While several state and federally funded projects have studied the safety benefits of roundabouts, there are no studies that have investigated the correlation of roundabout entry deflection angles with driver behavior to assess safety at roundabouts. The objective of the proposed research is twofold:

1. To investigate the correlation between deflection angle, vehicles' entry speed, gap acceptance, and acceleration while in the roundabout.

2. To investigate the relationship between deflection angle and safety-related performance measures (e.g., expected number of crashes?) with the use of traffic microsimulation software.

The proposed research investigates the correlation between roundabout entry deflection angles and safety based on observation of driver behavior both in the field and in a microsimulation environment. This is in accordance with the SAFER-SIM theme of using simulation techniques to address safety issues. In addition to microsimulation, the proposed research utilizes new technologies to track vehicles and investigate driver behavior in the field for various deflection angles and types of roundabouts. This is consistent with the thematic thrust areas of roadway design and traffic operations.

The proposed study consists of two parts. First, the research team will perform field studies to investigate driver behavior on roundabouts. Several subjects will be recruited to drive through two roundabouts that differ significantly in their geometric and operational characteristics. In particular, an isolated single-lane roundabout and two single lane roundabouts connected by a tangent (i.e., a double roundabout) will be used as the test sites for the field study. Information on the driver behavior will be obtained with the use of on-board devices that provide data on speed, location, and acceleration of subject vehicles. In particular, the i2D (intelligent to Drive) on-board unit that collects such information on a second by second basis is available to the research team and therefore, it will be used for the purposes of this study. In addition, video cameras will be utilized to collect gap acceptance data during the course of the field experiments. This way, the relationship between entry speed, deflection angle, and gap acceptance will be investigated.

Second, field data that have been collected for these two roundabouts will be used for developing and calibrating simulation models using the microsimulation software VISSIM. VISSIM has been chosen due to its ability to use surrogate metrics in order to provide safety performance measures. This will allow for a safety evaluation of various geometric configurations and in particular, various deflection angles for the aforementioned roundabout models.

The outcomes of this research are guidelines that will inform deflection angle design at roundabout entrances. These guidelines are expected to provide insights for improved roundabout entrance designs that can lead to lower vehicle speeds and consequently safer traffic operations at roundabouts. These outcomes are expected to be of interest to local government agencies, such as departments of transportation that are considering building new roundabouts in their jurisdictions or re-designing existing ones to improve traffic operations and safety. Future research could build on the proposed study to investigate combinations of geometric parameters that influence driver behavior and develop more detailed guidelines for roundabout design.