San Ildefonso

San Ildefonso roundabout: delays, queues & safety

The photo at left, taken from one of my traffic videos, shows a moment during the morning commute. Large vehicles move more slowly in the circle, increasing queue formation.

The composite photo at right shows two cars entering the queue in the upper right. In the lower left photo, 39 seconds later, the two cars are in the roundabout circle, completing their left turn onto Diamond Drive.

I've extracted some rough queue delay and stop statistics from the traffic video of 4/20/11.

For the S. San Ildefonso cars coming from North Mesa, aproximately 45 out of 119 cars, or 38%, came to a complete stop. I estimate that there were many other cars that slowed down to ~10 mph, and some cars stopped more than once.

I've timed the transit from arriving at the queue to exiting the circle after a left turn for a few drivers who passed through S. San Ildefonso queues. Some of the less fortunate cars took 26, 55, 24, 32, and 39 seconds to return to open travel.

For 26 cars coming from North Mesa Rd., 15 came to a complete stop (58%). Those arriving at bad times stopped for periods of 28, 33, 19, and 24 seconds. Average stopping delay over all 26 cars was 6 sec.

This roundabout is adequate for the existing traffic conditions. However, the San Ildefonso/Diamond performance would degrade significantly with a 20-30% increase in traffic volume.

The San Ildefonso roundabout has "local" or "unofficial" rules-of-the-road. With experience, drivers entering the roundabout from San Ildefonso (North Mesa approach) have learned that it is wise to yield to traffic about to enter from Westbound Diamond Drive, since that traffic is moving at higher speed and, at some times of day, is much greater in volume. Some residents (perhaps remembering how much better the roundabout is than the old 4-way-stop configuration) get along OK with this, others are annoyed, and drivers who are new to the roundabout must learn this-- often by trial and error. As a driver who enters from Diamond drive frequently, I regret interfering with North Mesa traffic, but can do little about it as an individual. Somebody has to assume the right-of-way or case-by-case confusion reigns.

A second (official) rule-of-the-road that confuses users unfamiliar with roundabouts is that entering cars do not have to stop if there is no traffic in the roundabout.. Sometimes, when a driver follows another vehicle approaching the roundabout and (properly) glances left to spot traffic in the roundabout, he looks back to the front and finds that the car ahead has stopped. This can be a nasty surprise and a cause of rear-end collisions.

Safety data is inconclusive. While this roundabout's safety record is good, it had the third highest number of accidents of any Los Alamos intersection during 2008-2009. However, further information is needed to determine whether these accidents occurred in the intersection or simply nearby.