They sure seem more efficient.

They certainly are. The new-ish traffic circle at Northshore and Concord has totally removed congestion at that intersection. Before the roundabout, southbound traffic on Concord Road would back up all the way to Turkey Creek Road, which is almost exactly one mile. Westbound traffic on Northshore would back up almost the same distance. Since the roundabout, traffic never backs up for more than two or three cars, even during rush hour.

The difference is amazing.

~Russ

An Oak Ridge company is testing cameras on traffic lights to control traffic flow more smartly. The purpose of the cameras is to replace the magnetic strips in the road that detect cars at the lights.

Doug McCaughan
(link...)

There are three distinct different things: Rotaries, Roundabouts and Traffic Circles. The easiest of these to explain are traffic circles. They're aestheticly pleasing means of joining roadways where a standard stop sign would typically be used. Now due to their small radius, they still for all intensive purposes operate as a stop sign controlled intersection, but often times with a landscaped area broken up in the middle.

Now roundabouts are designed with the idea of keeping traffic moving around the circle, but at lower speeds, to prevent idling. They have a typical travel lane with a driveable apron to the inside that allows for 18 wheelers to traverse them. In a roundabout you're supposed to travel somewhere in the magnitude of 18-20 mph around them. They're very efficient to a point, but once you get to a highly congested state they break down. Also they take a lot of land. FHWA's minimum width is around 100' from pavement edge to pavement edge. This means when you add in curbing and sidewalks it's close to a minimum of 120'. In areas where the land is available and traffic projections don't go into a highly congested state, they're more effective. But as a retrofit they're difficult to use due to the large amounts of land area needed.

Rotaries are an even large version of a roundabout, that is designed for much higher speeds. These are typically used in high congestion areas but require a lot of land and a lot of planning.

Now all these are often confused with each other by the general public, but are starting to receive more thought in implementation.

Regarding the changes made to the Knoxville road,

the road has some lines painted across it, and some ruts have been carved into the road to audibly warn you to slowdown. These are fairly intangible but work! You cannot help but slow down.

After having looked into slowing people down in our neighborhood and working with the City of Alcoa, I never came across the "rut" solution. Most of the reports (trafficcalming.org) have the humps, tables, traffic circles, narrowing, and more, but no ruts.

Have you seen any statistics or measurements on the effectiveness of ruts?

Well, we haven't been spending on infrastructure, that's for sure. Although, the vehicle detectors that are increasingly common do help a lot. In 1973, you might have to wait for 2 minutes at a light when no one else wanted to pass through the intersection.

Liberty and justice for all.

My home

"Look kids, Big Ben,Parliment!"

Interesting question. This came up recently in Blount County. The answer, at least in this case, is surprising:

Public hearing set on congested intersection in Louisville

With a traffic light, TDOT would install turn lanes in all four directions. The problem with the light is that TDOT would have to cut a nearby hill down four to six feet to get the needed sight distance a traffic light requires. That would require taking part of the Shell Station entrance and the driveways of two or three houses, which would mean the houses would have to be acquired because they are so close to the road, the mayor explained.

With a roundabout, similar to the one at Five Point on West Broadway in Maryville, the intersection could be moved sufficiently so no families would be displaced.

The roundabout would be moved slightly toward Louisville (across Mentor Road from the Shell Station) into the large vacant field owned by Fort Sanders Regional Medical Center and across Louisville Road from the gas station on some property owned by the Scott McClellans, Anderson said.