traffic engineers and planners are using roundabouts with enthusiasm
for several reasons. Roundabouts increase traveler safety, reduce
travel delay, are economical, are beneficial to the environment,
and improve the appearance of streets and intersections.
Roundabouts are safer than
any other at-grade intersection form because roundabouts have
fewer conflict points, slower speeds, and easier decision making.
Data from Europe, the United Kingdom, Australia, and the United
States of America demonstrate improved safety over all other at-grade
intersection forms in two distinct ways: 1) reductions in the
total number of collisions, and 2) even greater reductions in
injury producing collisions. Collision frequency and severity
will decline for pedestrians, and motor vehicles.
The Insurance Institute for Highway
Safety conducted a study published in the American Journal of
Public Health. The study analyzed actual and expected crashes
at stop and signal controlled intersections in rural and urban
environments. Findings applicable to multilane roundabouts vs.
signals include a reduction in all crashes of 32% and injury causing
crashes of 68%. Injury crash reduction is greater than all crash
reduction due to the elimination of most head-on, left turning
across oncoming traffic, and right angle crashes. Head-on, left
turning, and right angle crashes generate the highest energy and
thus the highest number of injuries compared to rear-end and sideswipe
crashes. Data for single and multi-lane roundabouts from other
countries confirms the USA experience. Reductions in overall crashes
range from 36% to 61%, and injury crash reduction ranges from
25% to 87%. (Roundabouts: An Informational Guide. Federal Highway
Pedestrian safety is also improved
at roundabouts over traffic signals. Pedestrians using roundabouts
are able to cross a much smaller roadway, consider traffic traveling
only one direction at a time, and are exposed to traffic that
is traveling at much slower speeds. Pedestrian crashes at British
intersections occurred at the following rates: 0.33 crashes per
million trips at flared roundabouts, and 0.67 crashes per million
trips at signalized intersections. (Roundabouts: An Informational
Guide. Federal Highway Administration) Clearly, signals are inferior
to roundabouts for pedestrian safety.
Roundabouts typically carry about 30% more vehicles
than similarly sized signalized intersections during peak flow
conditions. Traffic signals can cause delay to side streets and
left turning traffic from the major street. Increased capacity
at roundabouts is due to the continuously flowing nature of “yield
only until a gap is available” vs. stopping at a red light
until my turn comes.
Pedestrian travel distances will
increase by about 20% at a roundabout over a traffic signal. Pedestrian
delay at either type of intersection is difficult to predict and
will depend upon random factors. Assuming no driver stops for
a waiting pedestrian, finding a gap in traffic that is large enough
to cross at a roundabout may take some time. This situation will
only exist during moderate traffic volumes. At high volume periods
where drivers are moving slowly, pedestrians will be able to cross
as autos yield while waiting to approach the yield line. At low
volume periods, acceptable gaps will be available frequently.
At traffic signals, pedestrian wait time to receive a WALK signal
may be up to 120 seconds depending upon at what point in the signal
cycle the pedestrian pushes the pedestrian button. Pedestrians
will be crossing traffic lanes for about 80 seconds.
Roundabouts save money. The
City saves because operations and maintenance expense of roundabouts
is less than that of traffic signals. Signal maintenance and electricity
annual cost is $3,000 to $5,000. The driver saves time through
reduced delay and lower fuel consumption. The community at large
saves because collisions are less frequent and much less severe,
reducing insurance cost, medical cost, and the human cost of injury
and death. Roundabouts also reduce the need for added lanes along
roadways because the capacity of a system is most often determined
by the intersections. Roadways are widened from intersection to
intersection to accommodate the queues generated by traffic signals.
Roundabouts conserve land
since road systems are narrower overall. A roundabout at Hayden
Bridge Way/Parkway will need about 157,000 square feet of land
and the traffic signal will need about 183,000 square feet of
land, or about 17% more.
Fuel consumption and air pollution
are reduced significantly due to lower travel delay, especially
in the off peak travel periods. Some areas of the country within
Air Quality Containment areas are using Federal funds from the
Congestion Mitigation and Air Quality Mitigation account to remove
traffic signals and replace them with roundabouts to reduce both
congestion and improve air quality.
Roundabouts central and splitter
islands provide area for landscaping, sculpture, or other aesthetic
features. They also avoid the clutter of traffic signal controller
boxes, poles and wires, and pavement cuts for detector loops.