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Residential street design contributes significantly to
a sense of community and the level of safety and comfort
experienced by its residents. Neighborhood streets are
often seen by residents as an extension of their living space.
Most people prefer streets they can walk along and cross
safely. Traffic typically moves faster on wide streets,
as drivers often wrongfully perceive that there are few
dangers. Narrowing a street with pavement markings,
on-street parking, curb bulbs, or center medians can help
improve safety by reducing vehicular speed, making streets
easier to enter from driveways and side streets, and reducing
the time and distance that pedestrians are exposed to traffic
when crossing. Typical speed reductions are 3 MPH, or
7%. Narrowing can lessen the impact of motor vehicles on
a neighborhood.
Curb bulbs are a form of roadway narrowing at
intersections that reduce the street width from curb to curb.
They shorten crossing distances for pedestrians and draw
attention to pedestrians via raised peninsulas. They
also tighten the curb radii at the corners, reducing the
speeds of turning vehicles. They can be used to create
protected on-street parking bays. Curb bulbs come in a
wide variety of sizes and typically result in speed decreases
of 7%. They are easily combined with a raised
intersection or raised mid-block crosswalk. Curb bulbs
placed opposite one another mid-block create a "choker," which
can result in more significant speed reduction.
Raised, elongated islands placed on the centerline of
streets are commonly called medians. Medians come in
many sizes and can be designed to follow the curvature of a
street. Medians are often landscaped to provide a visual
amenity. Placed at the entrance to a neighborhood,
medians can become "gateway islands." Fitted with a gap
to allow pedestrians to walk through at a crosswalk, they are
often called "pedestrian refuges." Medians can be used
to narrow lanes, or to provide physical separation from
traffic. While they provide an average 7% decrease in
travel speeds, their speed-reducing effect is somewhat limited
where there is an absence of any vertical or horizontal
deflection. Medians can visually enhance a roadway when
landscaped. They may also reduce parking and driveway
access, but can be a preferred traffic calming measure
for emergency response.
Distinctly different than the "speed bumps" found in
many private parking lots, speed humps, speed tables, speed
cushions, and raised crosswalks are raised areas of pavement
placed across a road, engineered to safely moderate the speed
of traffic. Typically ranging between 3 and 4 inches in
height and 12 to 22 feet long (in the direction of travel),
they are often placed in series or intermixed with other
traffic calming measures to provide continuous speed control
along a corridor. A popular and effective means of
slowing vehicles, these devices typically reduce average speed
by 4 to 7 MPH, and reduce the frequency and severity of
collisions by 11 to 45%. There can be a slight increase
of noise in the immediate area of their installation.
One variation - the speed cushion is designed with gaps that
allow emergency vehicles to straddle the device, thereby
reducing or eliminating the 1 to 9 seconds of delay generally
associated with speed humps. Raised crosswalks,
constructed like a speed table, are marked with high visibility
crosswalk markings, and can be found at mid-block locations
where speed control is an issue. They not only slow
traffic, but also make pedestrians, especially small children,
more noticeable to drivers.
Traffic circles are raised islands, placed in
intersections, around which traffic circulates. Traffic
circles are good for calming intersections, especially within
neighborhoods, where large vehicle traffic is not a major
concern, but speeds, volumes, and safety are problems.
Traffic circles have the advantage of calming two streets at
once. They may require the elimination of some on-street
parking. Entry may be stop controlled. Average
speed reductions of 4 MPH are typical as are collision
reductions of 11%. This device functions best at
intersections that have four legs. Some traffic circles
provide an opportunity for landscaping, typically maintained
by the neighborhood, which can soften the look and feel of the
street.
The public often confuses roundabouts with traffic
circles. Modern roundabouts are circular intersections
where traffic flows counterclockwise around a center island.
Roundabouts can create traffic calming speed reduction while
providing for significant traffic flow. Entering
vehicles negotiate a curve sharp enough to slow speeds to
about 15 to 20 MPH. Within the roundabout and as
vehicles exit, slow speeds are maintained by the deflection of
traffic around the center island and the relatively tight
radius of the roundabout and exit lanes. Slow speeds aid
in the smooth movement of vehicles into, around, and out of a
roundabout.
Roundabout design caters to the preferences of older
drivers - low speed and few decisions. Pedestrians only
have to consider one lane of traffic at a time.
Roundabout designs discourage erratic or undesirable driver
behavior and slow drivers to speeds more compatible with
bicycle speeds. Emergency vehicle drivers are not faced
with the prospect of through-vehicles running the intersection
and hitting them at high speed. Roundabouts have 75%
fewer conflict points than a standard intersection.
Where roundabouts have been installed, motor vehicle crashes
have declined by about 40%, and those involving injuries have
been reduced by 80%. Crash reductions are accompanied by
significant improvements in traffic flow, thus reducing
vehicle delays, fuel consumption, and air pollution.
Studies in Europe indicate that, on average, converting
conventional intersections to roundabouts can reduce
pedestrian crashes by about 75%. Roundabouts provide a
significant opportunity for landscaping and community
centerpieces.
This page was last reviewed on October 24, 2007.
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