Roundabouts

Roundabouts: Variations in design lead to large differences in behavior

All roundabouts share the same pros and cons.

No! Two-lane roundabouts are quite different and can be relatively dangerous.

Other design factors matter, too.

The MIG Study has proposed one-lane roundabouts for NM502. However, two-lane roundabouts are an obvious alternative if capacity limitations of one-lane roundabouts are a problem, and there will undoubtedly be future discussions of using two-lane roundabouts. The figure below illustrates the yield rules for 2-lane roundabouts.

There are actually worse difficulties than this, some of which rear their ugly heads if you (misguidedly) allow two lanes into the roundabout. The diagram below shows the traffic turns allowed in a typical two-lane roundabout, according to rules of the road in New Mexico and Colorado.

Whether a given merge or lane-change conflict occurs depends only on the random arrival times of vehicles.

If a glance at the diagram above does not intimidate you, I suggest that you trace some paths for typical movements and look carefully at the sources of conflicting traffic. Or, if you are a pedestrian, imagine a crossing, and consider that the cars coming from these various directions are trying to avoid hitting each other—how much time do you think they have to notice and avoid you? Bicyclists and motorcyclists, what do you think?

Single-lane roundabout safety is discussed on the previous page. Only a little imagination is needed to see that two-lane roundabouts pose accident dangers not present in single-lane roundabouts. The first significant difference is that average speeds are higher, 25 mph for a typical two-lane design. Experience with actual two-lane roundabouts confirms these greater risks. Collected safety data for two-lane roundabouts is difficult to find. There are, however, some case studies, such as those of Venice, FL and the Albany, NY area (below). These show accident rates for two-lane roundabouts that can be much higher than those of comparable signalized intersections.

Finally, note that there are subtle installation-dependent safety factors. Traffic density, approach characteristics, lane widths, and local driver habits and abilities lead to variable performance in different cases. Some problems can be mitigated by minor adjustments to the roundabout, while others lead to major intersection changes.

The Venice, Florida 2-lane roundabout, built in 2012, is dangerous and does not serve its purpose well for many users, as indicated by the following news reports.

MySunCoast (WWSB) article: "Venice roundabout has residents afraid to enter", Bobeth Yates, 9/29/14

"It was installed in 2012 to help relieve traffic congestion and beautify the gateway into Venice, but navigating the traffic feature has become a major concern for many area residents."
" 'She was in the right-hand lane and as she was getting ready to turn [when] a man in the left-hand lane came over and hit her,' says Jeff Mangrum, talking about his wife’s mother. He says his mother-in-law is among the hundreds who have been in car accidents while driving though the roundabout."
" 'Having the roundabouts probably has decreased the wait time at the four-way intersection, but going through it constantly and seeing the fender bender accidents that are a regular occurrence in there kind of makes me wonder if it was worth losing the wait time,' says area resident Jennifer Behrens."
The comments on this article indicate a variety of viewpoints, some positive, some negative.

Sarasota Herald-Tribune article: "Venice roundabout: Worst intersection in Sarasota County?", Christi Womack, 8/15/15 . This article has a good video of the roundabout in operation.

"Five years ago, Sarasota County installed the $3.2 million roundabout to make the busy intersection safer and more efficient."
"From the start, critics questioned the risks of installing a two-lane roundabout feeding from Interstate 75 into a city with one of the oldest populations in the nation."
"Crash rates quadrupled, making the roundabout the No. 1 intersection to avoid each year since it opened. Traffic counts decreased by 45 percent as tens of thousands of drivers like Hunt began finding alternate routes."
"When there was a traffic light at Jacaranda and Venice Avenue, there were 11 accidents in 2008 and 2009. After the roundabout opened in 2011, crash numbers began to spike. Flash-forward to the past three years: 52 crashes in 2012, 57 in 2013 and 50 in 2014."
"That exceeds by nearly 30 percent the county's worst traffic-light regulated intersection, at Price and Sumter boulevards in North Port. There were 23 crashes in 2013 and 21 crashes in 2014 with rates of 3.28 and 3.0, respectively. The Venice roundabout rate was 4.74 in 2013 and 4.16 in 2014."

A personal report: My wife and I drove through this roundabout several times while visiting Florida in January, 2016, before we had read these articles. In spite of a lot of experience driving the Los Alamos San Ildefonso roundabout, we found the Venice, FL experience quite intimidating. Fortunately, we did not increase the number of accidents.

A 2011 Albany Times-Union article discusses the poor accident record for a string of roundabouts in the Albany, NY area (first mentioned on the previous page) and reveals significant safety problems for vehicles. Here are some passages that I consider very telling.

Crashes increased at 15 of the 20 roundabouts built where a previous intersection existed, with fender benders even more likely to rise, sometimes dramatically, at two-lane rotaries. And while crashes were reduced at all the single-lane roundabouts built by the state, they rose at almost all those built by counties and towns.
Aggressive drivers are speeding through rotaries and failing to yield the right of way, said Mark Kennedy, director of traffic and safety for the state Department of Transportation in the Capital Region.
"We have found that single-lane roundabouts definitely reduce accidents," he said, referring to the state rotaries. "With multi-lane roundabouts, some are better, some are worse."
"The accidents tend to be experienced drivers between 25 and 50 with a local zip code," Kennedy said. "It tells us that people familiar with the roundabouts and where they want to go, they are driving too fast. They are also choosing to be in the wrong lane."
To fully reduce crashes, drivers need to be more careful, said Carol Breen, a DOT spokeswoman.

So, the roundabout builders and operators blame the safety problems on the drivers! Perhaps the drivers didn't get the memo about roundabouts being safer? Sad story.