August 14, 2002
Roundabouts have been shown to reduce intersection injury
crashes by 76% compared to signals, and to reduce fatalities by 90% (Retting
et al, 2001). Results of these American studies, published by the
Insurance Institute for Highway Safety, and by the American Journal of
Public Health, are identical to findings from decades of overseas
research. Put another way, comparable crossroad intersections have FOUR
TIMES AS MANY INJURIES - including severe brain and spinal cord injuries -
and about TEN TIMES AS MANY FATALITIES as a roundabout. Where roundabouts
have replaced signals, repeated studies have shown they reduce pedestrian
accidents by 30-50% (Lalani 1975, Daley 1981, O'Brien 1985).
In lay terms, roundabouts can keep people out of hospitals,
wheelchairs, and graveyards, and that's a common reason they're built.
Unfortunately, these safety facts were not emphasized in Dr.
Richard Long's report to the Access Board: a report which was used to
develop the Access Board's current design proposals. The proposed new
unfunded federal mandate would require complex signals and barrier systems
at every roundabout crosswalk, regardless of how easily the
crosswalk operates or whether a blind person ever uses it. At $100,000 or
more per intersection, that's an expensive proposal, and since it
has serious ramifications on public safety, it bears close examination.
Like more than 90% of US crosswalks, most roundabout
crosswalks are not signalized. Because a pedestrian refuge is provided
mid-crossing (shortening the crossing distance), and because vehicles
operate at unusually low speeds (typically 15-20 MPH), the overwhelming
majority of roundabout crosswalks are extremely easy to use, and like the
vast majority of all crosswalks in the US, they simply don't need signals.
In such locations, even if signals were provided, pedestrians wouldn't use
Traffic engineers have known for decades that, if unwarranted
and infrequently used, signals can confuse drivers, become
ineffective, and INCREASE accidents, causing more injuries to pedestrians
and vehicle occupants alike. That's bad for public safety, but too few lay
people understand it. People tend to think that signals always improve
safety, but signals can increase speeding, distract drivers' eyes away from
traffic and pedestrians, and create a false sense of security for
pedestrians. Signals do not put a concrete wall between vehicles and
pedestrians: pedestrians are struck at traffic signals with sickening
In locations where pedestrian and traffic volumes warrant
them, crosswalks should have signals, and many roundabouts in the United
Kingdom and Europe have pedestrian signals. They're common in London,
Birmingham, and other cities. Examples of appropriate locations for
signalized crosswalks include high-volume urban roundabout crosswalks, and
locations where pedestrians (both blind and sighted) are most frequent.
These factors are easily quantifiable. At rural intersections or low-volume
locations, and locations where pedestrians are infrequent, signals are
not used because they confuse drivers and unnecessarily increase highway
construction, maintenance, and operating costs.
To assure that traffic signals are only
installed where prudent, "warrants" have been developed for traffic signal
installation in the United States. In the United Kingdom - a country
with decades of experience with roundabouts in a wide variety of locations -
the warrant for a signalized pedestrian crosswalk at a roundabout is where
PV squared is greater than 1*10^8 (in words, where the number of
pedestrians, times the number of vehicles per hour squared, exceeds a value
of 100 million). Use of appropriate signal warrants assures that signals
are provided only where needed, and not where they are unnecessary
and potentially harmful.
The ramifications of an ill-considered intersection design
policy can negatively impact the general public in unintended ways. For
example, if ALL roundabout crosswalks were required to have signals, about
$100,000 would be added to the cost of each roundabout, making them
unnecessarily expensive in comparison to other intersection alternatives.
As a result, far fewer roundabouts would be built, and many more of
the common alternative - a signalized crossroad - would be built
instead. As stated previously, studies show these have FOUR TIMES as many
injuries, and TEN TIMES as many fatalities as a roundabout. The United
States currently has about 15,000 deaths and about 1 million injuries at
intersections every year, and installation of well-designed roundabouts
might prevent countless human tragedies. Meanwhile, signals at
unwarranted locations may help no one at all, and could in fact be harmful.
A single-user approach to traffic engineering would be
a mistake. All users of an intersection must be taken into account, and the
appropriate solution needs to be provided that will provide the greatest
benefit to the public in each specific situation. No one wants more
people injured or killed because we impose an ill-considered intersection
The Access Board proposal to require signals at all
roundabout crosswalks needs to be reconsidered. Signals should be installed
where they are warranted, and where there is no better alternative. In
specific locations where users have special needs, the needs should be
evaluated and provisions should be made in the design.
The opinions expressed above are those of the author, and
do not represent an official policy statement of the State of Michigan or
the Michigan Department of Transportation.
Edmund Waddell, Senior Transportation Planner
MDOT Project Planning Division