Do Roundabouts Reduce Traffic Volume? 2

[ccedva]

Hello All,

A developer is putting in a collector road that will run roughly parallel to an unimproved arterial but will cut about one-half mile off a commuter route. The adjacent neighborhood where the collector will connect is concerned about cut-through traffic using this new route.

The developer has proposed using three large roundabouts spaced evenly over the one-mile segment of new road. The developer is arguing that these three roundabouts will discourage any cut-through traffic from using this new road.

Does anyone know of any research related to the deterrent effect of roundabouts on through traffic? There seems to be evidence that roundabouts cut accident rates at busy intersections but very little has been written about the effects of roundabouts on traffic volume.

Thanks,

Woody Carr
Chesterfield County Virginia

 

[ACtrafficengr]

If properly designed, roundabouts are very efficient. I doubt they will cause much diversion of traffic. If the parallel route has signals on it, you may actually get more traffic through the roundabouts!

The roundabouts may have some "traffic calming" effect, though.

How large is large? Research has shown that safety decreases as the roundabout's diameter increases. It shouldn't be larger than it has to be to handle the traffic that will use it.

 

[dragonyian]

Try to see topic "Roundabout" there are some discussion about the roundabout. It doesnt say too much about traffic volumn, but from the book suggested over there, u might find something useful.

Regards,
Dragon

 

[ccedva]

Thanks for the responses. I appreciate the help.

Woody

 

[ontwerp]

I am new to this site but have intrest in traffic engineering. I have a question, why do traffic at a controlled intersection or round-about stop with the nose on the intersection or round-about? Jack Janssen.

 

[trsidn]

I agree with LTAPJim above. Well done roundabouts will draw people off signalized routes.

I don't understand ontwerp's question. If you are asking why people stop so far into an intersection, I would imagine it's to be in the best position to get through it when the light changes. Or to minimize the amount of time lost while stopping at a sign. People largely look at traffic controls as a major inconvenience.

Tom

 

[teeman]

If travel time through the neighborhood is less than the travel time along the arterial, then some traffic will divert to the new route. A series of rounabouts is unlikely to create enough of a disincentive for cut-through traffic.

 

[Tran1]

Roundabouts can be used for traffic calming by reducing speeds but will not serve as a traffic deterant. Also, roundabouts do not actually lower accident rates, but rather severity of accidents. The geometry prevents cars from crossing perpendicular to each other, thereby "eliminating" direct side impact accidents. The lower travel speeds help to reduce accident severity as well. The main benefit of roundabouts is improved flow. There is no reduction in traffic volumes - just a reduction in delay times, therefore reduction in queue length and the "impression" that there is less traffic. The best traffic deterant for cut through traffic is speed bumps or speed humps. People ignore stop and yield signs, but there is no way around a speed hump. They also help keep speeds down. Roundabouts also require a bit more area than traditional intersections - which developers tend to dislike. If the road would otherwise be a straight shot through the development, speed humps would be the best deterant (in my opinion).

 

[CityGov]

According to the US Insurance Institute for Highway Safety, roundabouts have resulted in 40% fewer crashes overall and 80% fewer injury crashes. Keep in mind, for statistical purposes, that the US has a relatively low number of roundabouts. Other sources have stated that at very high flows roundabouts tend to operate at a similar rate as signalized intersections with respect to frequency of crashes. However, the severity is still much lower.

Perhaps I'm being picky about semantics, but speed BUMPS should not be used on public roadways. Courts of law have ruled against the use of speed bumps on the basis of their physical charactersitics and potential for damage to vehicles driving reasonably and prudently. Speed HUMPS are a ligitimate device.

 

[BUD007]

Tran1 wrote:

"The best traffic deterant for cut through traffic is speed bumps or speed humps. People ignore stop and yield signs, but there is no way around a speed hump. They also help keep speeds down."

Apparently your parents are not equipping their children with four wheel drive trucks as they are in our area.
The shoulder, ditch, and adjoining yards are all fair game for bypassing the speed bumps.

 

[trafficPro]

Take a careful look at the Municipal Ordinance. You may not be able to do what you want to do. Many communities have ordinances passed years ago, at the "suggestion" of fire departments, that preclude building any bump or hump in the roadway. Educating the fire dept and the Council is often an uphill job. Chicanes are usually easier to get built.
Other measures such as breaking up long tangents, narrowing intersection approaches, curvilinear design, can be built in if the development is not yet finalized.

 

[iankirkellis]

Roundabouts are used extensively in the UK to great effect, the normal hierarchy of delay is

priority junction (give way)
signalised junction (traffic lights)
roundabout
signalised roundabout

However there are grey areas between the uses depending upon the size and geomtery.

The upshot is will delay on the atrerial route be greater than the signalised route, if it isnt you will expect a shift to people using the new areterial route. It is purely a function of delay. You will need to run the calcs. We use Arcady produced by the TRL in the UK, to analyse delay and ratio of flow to capacity.