Trajectory Deflection Influence on the Performance of Roundabouts

Trajectory Deflection Influence on the Performance of Roundabouts


A Bastos Silva, A Seco, Universidade de Coimbra, PT


This paper identifies empirical relations between driver behavior reflected by speed and trajectory adopted by drivers and the geometric parameters associated with movement deflection on a roundabout.


It is widely accepted that the use of a well designed roundabout usually results in significant changes to driver behavior and in a sizable reduction of approaching and crossing of speed profiles. These facts broadly explain the considerable reduction on accident levels of this type of intersection when compared with other solutions.
However the influence of some geometric aspects on drivers? behavior, speeds and trajectories is not well known. These issues become especially relevant in multiple lane roundabouts since they offer the driver a greater freedom of movements which can cause inappropriate behaviors.
On the other hand it is commonly accepted that the performance level of a roundabout depends largely on its design and particularly in its potential ability to physically restraint driver behavior inducing him to naturally adopt correct behaviors without contradicting his natural expectancies. Recent investigation shows that design solutions with low levels of deflection tend to have low performance levels with very heterogeneous speed profiles. Because they are uncomfortable high deflection designs induce the driver to invade the adjacent lane while searching for more direct and comfortable trajectories.
The present paper presents some results included on the PHd thesis and identifies empirical relations between drivers? behavior and the geometric characteristics associated with movement deflection therefore contributing to the understanding of the relationship between these parameters and the speeds and trajectories adopted by drivers.
The main findings and conclusions are based on international experience and on observations carried out on a set of twenty circuits that had a reasonable variation in their geometric design parameters but within an acceptable range according to specialized technical documents. The data collection sessions were supported by an instrumented vehicle which had equipments to measure driver?s actions on the vehicle?s commands its dynamic response. A set of fourteen young and homogeneous drivers were used for behavior observation.
The presented results show the speed and the trajectories analysis traced for each driver while negotiating and crossing the roundabout and its correlation with some of the deflection design parameters. Finally some design criteria?s are presented on identifying suitable value ranges for deflection that induce correct driver behavior.


Association for European Transport