The following is part of a monograph completed earlier this month. It attempts to explain based on two key research studies of "before and after" injury rates at intersections converted to single lane roundabouts the sharp reduction in injury rates the single lane roundabout--the predominant type--accomplish.
The basic safety by the numbers for roundabouts—car occupants,
walkers and bicyclists
First and foremost one must consider the incontrovertible evidence that roundabouts substantially reduce injury rates for the principle modes: walking, bicycling and car travel. For overall safety, the authoritative U.S. study by the Insurance Institute of Highway Safety (2000) found anything but a roundabout generated serious and fatal injuries at a rate about 800 percent higher than the average of signals and stop control level. (The study language that roundabouts reduce serious and fatal injuries about 90% translates—using arithmetic—one roundabout [injury] to [ten] non-roundabout injuries, so one calculates non-roundabout injuries occur at an 900% greater rate.) The roundabouts surveyed in the before and after analysis were both single and multi-lane—and the statistical measures compared the “after” performance against typical intersections thereby avoiding the “reduction to the mean” issue. Results did to a great extent reflect car travel as the U.S. walking mode share still remains below those of many developed nations, and very far below the bicycle mode share—with American bicyclists primarily younger adult males.
But two studies—one Dutch and the other Swedish—examining single lane
roundabouts clearly show the safety benefits to those who walk and bike reach near the same levels of safety gain as the overall numbers in the U.S. study. And secondary findings of both those studies point to a major breakthrough for bicycle facility design and the pivotal role roundabouts play in that breakthrough…[provision of separate .]paths for both bicyclists and walkers]…
In the U.S. any discussion of the walking and bicycling mode takes place during a time when car occupant fatalities in recent years have decreased, walking fatalities increased, and while bicycle fatalities have decreased the decrease is slower so that overall bicycle fatality numbers represent a slow increase in the overall percentage of U.S. highway fatalities.
Single lane roundabouts will be evaluated—they are roughly about 75% of all
roundabouts built. In Vermont to date 10 of 11 roundabouts are single lane. While studies of two lane roundabout have been produced, recent design practice involvesminimizing roundabout sizes when serving walkers and bicyclists since their safety depends in great part from lower speeds—and smaller roundabouts constrain speeds to a lower level than do larger roundabouts. For example, the Swedish study cited here two-lane data found roughly no change in bicyclist crash rates and possibly a slight increase. However that study reported in 2001 involved roundabouts with about
a 200 foot diameter typically versus the current practice of 150 to 180 foot diameter two-lane roundabouts where serving bicyclists and walkers.
Still, with the knowledge of the sizes of the study roundabouts, it can be concluded that there is a small increment of bicyclist safety at two-lane roundabouts in view of current designs which are smaller and employ on/off ramping or pathing so bicyclists do not have [to] “take the lane”…
Walker and bicyclist safety at one lane roundabouts—anything but a
Roundabout on average generates injuries at rate 200% greater for
bicyclists and 500% greater for walkers
The two best studies to date determining a safety gain for walkers and bicyclists at one lane roundabouts are: (1) “What roundabout design provides the highest level of safety?”, The Swedish National Road and Transport Research Institute (VTI) (2000)
http://www.alaskaroundabouts.com/Nordic2safety.pdf and (2) “The safety of roundabouts in the Netherlands” by Chris Schoon and Jaap van Minnen, Traffic
Engineering and Control, March 1994.
The VTI study of 58 conversions of intersections to single lane roundabouts predicted “after” injuries and fatalities based on standard Swedish formulas, formulas applicable to either signals or sign control as Swedish research determined crash experience for both signals and signs were the same. The Dutch study produced before and after
crash data on the conversion of 181 intersections to roundabouts. Most conversions were from sign control though a small number were signal to roundabout conversions. No such similar studies are possible in the U.S.—even today—for a key reason: Western European nations modal shares of those who walk and bike in urban areas average upwards of third of all trips compared less than a tenth of U.S. urban trips on foot and bicycle. A cross section of Western European walkers and bicyclists represent a cross section of population by age and skills. In the U.S. small proportion of the populace who bicycle tend to be young adults and male. One needs to keep in mind when the Dutch and Swedish study reveal high reductions in either bicycle or walker injuries following converting intersections to roundabouts, those benefitting from the injury reductions are all ages for those who walk, and for bicyclists a cross section of all ages and skills. From another standpoint, the two studies reflect what the United States and other nations with low walking and bicycling levels can expect in terms of safety as walking and bicycling modal shares rise along with the addition of “non-traditional” populations of riders and walkers. The reduction of walker injuries were: (1) Swedish study 78% and (2) Dutch study 89%--an average of 84% reduced incidence of walker injuries, or any intersection other than a single lane roundabout generates, on average, a 740% greater rate of walker injuries. For bicyclists the reductions: (1) Swedish study 75% and (2) Dutch study 30%--an average of 52% reduction, or any intersection other than a single laner roundabout generates, on average, a 108% greater rate of bicyclist injuries.
Because the Dutch study revealed use of a bike lane within the roundabout generated a low percentage improvement in bicyclist safety (about a third of the conversions) that design subsequently was dropped, and other results of that Dutch study indicate that excluding laned roundabouts resulted in a bicyclist injury reduction in line with the Swedish study findings. In turn, the Swedish study—and likewise the Dutch study--found “take the lane” bicycle injuries at roughly 2.5 times the rate of roundabouts with paths which give the bicyclist the choice of taking a path through separate or shared crossings.
So, the Swedish and Dutch studies indicate the true bicyclist injury reduction may reach far closer to the 90%reduction for walkers. One can expect further studies in this regard in the near future. In sum, both the Swedish and Dutch studies clearly show a far higher level of safety single lane roundabout versus an alternative—an average reduction of 84% for walkers and at a minimum and likely higher than the 52% reported for bicyclists with users of all ages and skills. The quick adoption of the roundabout by some states and Canadian provinces as the standard intersection rests perhaps as much on the safety performance for walkers and bicyclists compared to signaled and signed approaches of the past as it does for the unquestioned benefits to car occupants.