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I hope to solve several issues raised by the column on traffic circles on Dec. 21 by a retired civil engineer.
One is giving the “skinny’” on roundabouts, which are different from “traffic circles” and “rotaries.”
Another is a reminder that in New Mexico, people claiming or representing to be civil engineers, retired or not, are supposed to be currently registered in the state, by state law. …
Here is my attempt at distilling roundabouts into 500 words from a two-volume seminar book used in educational professional development hours training.
Modern roundabouts, as with most interchange designs, reduce or eliminate conflict points of a conventional intersection in favor of merges.
There are 16 crossing conflict points in a conventional intersection and zero in a roundabout.
There are eight ‘diverging’ conflicts and eight ‘merging’ conflicts for a conventional intersection and four and four respectively for a roundabout.
The concept of roundabouts is to “deflect” all legs on the approach, bringing all vehicles to about the same speed as the circulator, yield, as needed, at the leg entering the roundabout and release the driver as streamlined and with the least obstacle possible to get them out of the roundabout and on their way.
Here is where not to install roundabouts:
1. In a coordinated (synchronized) signal system with platooned traffic
2. Where right of way is not available
3. Where there is inadequate stopping sight distance
4. On steep grades
5. Where the addition of turn bays would take care of the problem
6. Intersections that are too close to a signal in example No. 1
7. Where they would be too close to a railroad crossing
8. Where there is very little side street traffic and heavy mainline traffic.
An example of a non-roundabout solution is a current project to install advance detectors and alternative signal control systems to the Alameda Boulevard traffic signals to test “adaptive” corridor timing in the region.
It could be described as “intelligent synchronization” that gives the side roads and turn bays only what they need instead of pre-set “green time” if triggered.
Civil engineers specializing in traffic engineering seek things that work for the right places, whatever the alternative.
Here is where to consider roundabouts:
1. Intersections with historically high crash rates
2. Roads with a historical problem of excessive speeds
3. Intersections with more than four legs or with difficult approach angles
4. Roads with a high percentage of turns and U-Turns
5. Where there is high traffic volumes at peak hours
6. Intersections where widening one leg is cost prohibitive
7. At the bottom of an off-ramp.
This makes Bridge Boulevard with its current irregularly spaced non-grid signals and five-point intersection a good candidate.
As to statistics and crash rates, adding anything to an intersection trades one set of accidents for another.
Signals increase rear-enders and reduce T-bones. Roundabouts increase merging accidents and completely eliminate T-bones.
Roundabouts can be up to 50 percent more expensive, especially when retrofit, however require no need for expensive fiber networking, T-1 phone lines, telemetry, power and specialized ongoing maintenance.
Signalized intersections, when turn bays, pedestrian facilities and curbing are installed, can be 20 times more expensive, especially if a mini-roundabout within the existing right of way can be placed. The cost depends on the individual circumstances and varies widely.
Rio Rancho has an ordinance requiring roundabouts to be evaluated as an option if an official traffic signal warranting condition exists at a previously unsignalized intersection.
It has been illegal for many years to install traffic signals or stop signs to attempt to “calm” or slow down mainline traffic as was suggested as an economical alternative.
One doesn’t have to ask a civil engineer, they can get that information from any accident attorney.
