In general, two lane roundabouts pose greater crash risk for vehicles, as discussed previously.

The Trinity/Central roundabout could have higher accident rates than San Ildefonso, as a result of the following design and installation differences:

• More traffic at Central
• Much more complicated geometry and lane structure
• Higher speed design
• Paths with low curvature will lead to even higher transit speeds, especially for aggressive drivers

Considerable USA experience with roundabouts shows that, under some circumstances, a single-lane roundabout can reduce the severity of collisions, compared with other intersection configurations. We can say a few things about this.

• Los Alamos County has a very good safety record. Traffic fatalities at our intersections are extremely rare. (Have there been any in the past 30 years?)
• Injury accidents are infrequent. Furthermore, severity data is difficult to extract because the LAPD tracking system is somewhat clumsy in this regard. Therefore, one must bear in mind that comparisons among intersections are crude.
• For data from 2008-2009, injury accidents have been analyzed.
• With the above caveats, we can say that San Ildefonso has about as many injury accidents as other, much busier intersections in Los Alamos County.

Insurance Institute for Highway Safety has sponsored research on two 2-lane roundabouts in WA. A summary of their conclusions is available in a 2014 article titled "Two-lane roundabouts bring benefits but also some confusion" here. For intrepid readers, the entire research paper is available here as an Adobe pdf file. This and other reports on double-lane roundabouts are available on the IIHS site (when viewing material on this site, make sure you do a search for "double lane"-- much of the material applies only to single-lane roundabouts). Here are a few of the IIHS conclusions from the 2014 report:

• "Many of the well-documented benefits of roundabouts for intersection safety and efficiency extend to the two-lane variety, new Institute research shows. However, the study also sounds a note of caution about driver confusion when navigating these wider circles."
• "The two roundabouts in the recent study are located about 5 miles north of Bellingham on Guide Meridian Road. One intersection, at Pole Road, previously had a traffic light, while the other, at Wiser Lake Road, had stop signs on the two minor approaches."
• "The injury crash rate is arguably the most important measure and the one roundabouts would be expected to affect most. Although the decreases weren't statistically significant, possibly because the number of injury crashes at these intersections was small to begin with, they are consistent with benefits measured for less complicated single-lane roundabouts."
• On the other hand, the rates of crashes with only property damage rose after the roundabouts were built. Researchers concluded that noninjury crashes were 6 times as high at the Wiser Lake intersection as they would have been without the roundabout conversions. At the Pole Road intersection, they were 13 percent higher than they would have been.
• The surveys revealed that even after a year many drivers continued to find the revamped intersections confusing. Nearly half of respondents said it wasn't clear from the signs and pavement markings which lane has the right of way when exiting or that they shouldn't drive next to large trucks in the roundabouts. More than a third said it wasn't clear what speed to drive.

NOTE: These IIHS reports concentrated on the effects of roundabouts on motor vehicles.

Building the planned roundabout on Trinity at Central is likely to degrade that intersection's excellent safety record.