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

oundabout, except in the unusual situations where “metering” traffic signals are required.
3.8 Environmental Issues
Roundabouts can offer considerable scope for environmental enhancement and are
sometimes favored over other forms of intersection treatment in environmentally sensitive
areas.
The central island can be landscaped and planted provided:
l the treatment does not block any of the sight triangles (refer Section 4)
0 any landscaping will yield to out-of-control vehicles and not be a hazard;
l
the treatment does not constitute an unnecessary distraction to drivers.
Planting can be used to discourage pedestrians from crossing at undesirable locations.
Compared to traffic signals, roundabouts may operate with reduced queue lengths and
shorter average delays. This results in:
- less air and noise pollution;
- lower fuel consumption;
- less parking restrictions;
- better access to private driveways.
In addition, the use of a roundabout eliminates potential traffic safety and disruption
problems associated with the malfunction of traffic signals.
2.
Roundabouts can be used on local streets to discourage high traffic speeds and the
intrusion by very large vehicles. Provisions for emergency and service vehicles need to be
considered in the design of these roundabouts.
3.9 Means of Improving the Performance of Roundabouts
3.9.1 Continuous (Slip) Lanes
Where there is a heavy right turn traffic movement, this may be either separated from
the operation of the roundabout by providing a separate right turn slip lane or by providing
an auxiliary lane for this traffic. In the latter case, the right turn entry conditions can be
improved by positioning the splitter island past the entry lanes thereby shielding the right
turn entry movement.
Where a separate right turn slip lane is used, right turning traffic can be excluded from
the capacity and delay calculations. To be fully effective, the layout must ensure that the
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