Do Roundabouts Relieve/Alleviate Traffic? Why/Why Not? (freeway, difference)
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Roundabouts (or circles) can work in low traffic areas were the meeting streets have the same low speed limit like 25 mph. They are a fail in high density areas with speeds above 35 mph. People cannot accelerate fast enough to enter higher speed traffic flow from a complete stop. They also fail if there is a big difference between the intersecting roads.
Here in NJ, in the 1960s, there were circles where 50 mph 2 lane separated highways intersected with secondary roads. It was a complete disaster including deaths. Periodically, drunk drivers would drive straight thru the circle. There were 5 of these crazy things within 5 miles from where I grew up. By the 1980s, most had been replaced with overpasses or the absurd traffic light/jug handle combo.
Roundabouts are replacing a lot of our intersections.
Don't let the local misapplication of the idea shape your opinion. New England has historically used scaled up the roundabout, renamed it ("rotary") and slapped it on high volume roads which then grew way beyond the capacity of the intersection which is why they're being replaced. The two you've noted (Route 2/Concord and Drum Hill/Chelmsford) are quite literally highway interchanges (there are a bunch of others) and need highway interchange infrastructure (Sagamore Bridge Flyover is a successful conversion). The people in this thread have repeatedly highlighted that the best practical application is in low/moderate volume intersections. You're not talking about the same thing that most people are. This... is not the same thing as this. One works well at moderating traffic flow, the other is a nightmare. There is no one size fits all and just because it doesn't work as a highway interchange doesn't mean it doesn't work for anything.
Thing is, by the time the volume of traffic gets low enough that you don't get big backups at the rotary, all the other common layouts would work OK too. After all, once there are a signficant number of cars going around in the rotary, incoming ones still have to stop and wait. Now your intersection is no different than a four-way stop sign.
Thing is, by the time the volume of traffic gets low enough that you don't get big backups at the rotary, all the other common layouts would work OK too. After all, once there are a signficant number of cars going around in the rotary, incoming ones still have to stop and wait. Now your intersection is no different than a four-way stop sign.
It's a balance between safety and travel time at an intersection. A four-way stop sign is the safest intersection out there. But it leads to higher delays for every approach and everybody has to stop/accelerate so it's not the best. The roundabout option is safer than signalizing that intersection but also reduces the amount of time you have to wait (if at all) at an intersection. The traffic engineer's job is to figure out what the difference between those factors and whether it's worth the benefit to put in that roundabout.
It's not uncommon to see two-way stop intersections where the overall volumes are very low and the capacity of the intersection is more than adequate but the side street delays are in excess of 50-75 seconds/vehicle (while a roundabout could reduce that delay down)
Well, I know that rotaries are the current fad, so I'm equally certain I can't dissuade any of their adherents.
I would suggest that the current faddish popularity is as much based on the desire to have something new and cool and foreign ( because they always handle urban planning issues so much better outside the US, at least that's a basic tenet of belief) as it is on actual experience from actual intersections. All I know is that wherever they've been in place a long time, whatever the level of traffic, actual local residents want them gone.
Well, I know that rotaries are the current fad, so I'm equally certain I can't dissuade any of their adherents.
I would suggest that the current faddish popularity is as much based on the desire to have something new and cool and foreign ( because they always handle urban planning issues so much better outside the US, at least that's a basic tenet of belief) as it is on actual experience from actual intersections. All I know is that wherever they've been in place a long time, whatever the level of traffic, actual local residents want them gone.
Psh roundabouts are nowhere near as trendy as other interchanges/intersections such as displaced left turns/continuous flow intersections, or median u-turns/superstreets or diverging diamond interchanges. Now those are the current fads (and in comparison, roundabouts are much simpler and easier to replace if something goes awry)
Last edited by pierretong1991; 03-03-2021 at 07:31 PM..
Psh roundabouts are nowhere near as trendy as other interchanges/intersections such as displaced left turns/continuous flow intersections, or median u-turns/superstreets or diverging diamond interchanges. Now those are the current fads (and in comparison, roundabouts are much simpler and easier to replace if something goes awry)
If you come on one of those unexpectedly it can throw you for a loop.....
DC has about 40 circles including 4-6 with underpasses. Dupont has 10 ingress egress points, including an underpass, traffic lights in the circle, plus express lanes. It has four lanes anti-clockwise. On a bad day in the local lanes it can take 20 minutes to do a 360. There's a Thomas Circle, and a Dave Thomas Circle (soon to be no more), so named because it has a Wendy's in the center.
The primary advantage I see is they are unaffected by blackouts and software glitches. The long list of disadvantages starts with extra pavement to maintain. Where I live, battered potholed roads are more common than smooth pavement.
The first roundabout I saw is one of the five most dangerous in Michigan, 18 1/2 at Van Dyke. I mostly avoided it in cars and now I absolutely avoid it on a bike. That's how they reduce traffic, by diverting it to other routes.
Another thing about roundabouts is that here in North Carolina, we've started using them at rural interchanges (at the intersection of the on/off ramps and the intersecting road) because they prevent wrong-way driving onto the freeway.
Another thing about roundabouts is that here in North Carolina, we've started using them at rural interchanges (at the intersection of the on/off ramps and the intersecting road) because they prevent wrong-way driving onto the freeway.
I don't think so. The (incontinently drunk) person who'll turn left and go UP the on-ramp to get on the freeway the wrong way, will have no trouble turning left onto the circle and going up the on-ramp to get on the freeway the wrong way.
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