roundabout ~' ~ design guidelines - Institute of Transportation ...

1.0 INTRODUCTION
Well designed roundabouts have proven to be safe and efficient forms of inteisection control
in the countries that have adopted modern guidelines. These countries include Great Britain,
Australia, France, Germany, Spain, Norway, The Netherlands, among other countries.
These guidelines are intended to be temporary and updated as ofteq as is necessary. Design
guidelines from other countries form the basis of this document. Over time, these guidelines
will be analyzed as to their applicability to the driving conditions in Maryland. Input into these
guidelines from users is encouraged.
Roundabouts operate by gap acceptance, in that approaching drivers must give way to
circulating traffic in the roundabout. The proven safety performance of most roundabouts is due
to the low relative speeds of all vehicles and the relative simplicity of decision making required
of drivers.
Conditions will be encountered wherein the procedures highlighted in these guidelines cannot
be fully implemented. It is expected that the designer make modifications as necessary while
ensuring the major concepts of safety and design.
The aims of these guidelines are as follqws:
a. to give guidance on where roundabouts may be used
b. to describe the performance and operation of roundabouts
C. to give guidance on design standards for roundabouts so that high standard and
uniform design will be encouraged.
The designer of a modem roundabout should be fully aware of the difference between a
roundabout and a traffic circle. Basically, there are three main differences; Yield-at-entry,
Deflection, and Flare. These are illustrated in Figure 1.1. .‘.
FIGURE 1.1 YieId at entry, Deflection, and Flare
Yield-at-entry allows vehicles in the roundabout to continue through the roundabout to their
appointed exit and. eliminates gridiock which occurs when entering vehicles are afforded the
right-of-way. Yield-at-entry also enables the traffic engineer to design smaller roundabouts.
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