8 ms·

Safety aside, roundabouts lower traffic significantly by never forcing cars to stop. Congestion increases as cars need to stop because of the inevitable latency

Safety aside, roundabouts lower traffic significantly by never forcing cars to stop. Congestion increases as cars need to stop because of the inevitable latency propagating along the tails, and having more than one traditional intersections along the way can only make things even worse, so any solution to prevent cars from slowing down to a stop would mitigate those effects.

sokoloff 3y ago
Until you do what Cambridge, MA has done twice near my house: put a traffic light just outside the roundabout, so when the light turns red, traffic inevitably backs up through the roundabout, locking it up entirely. https://www.google.com/maps/@42.3875777,-71.1422076,18z?entry=ttu https://www.google.com/maps/@42.3875777,-71.1422076,18z?entr...
- timc3 3y ago
  They do that in Sweden all the time. I have no idea how a country that has some amazing minds can come up with the worst road infrastructure designs possible. Most of it’s unsafe so they just lower the speed limit.
- securingsincity 3y ago
  I used to drive over there quite a bit. Always terrible.
- foobarian 3y ago
  The circles of death? :-)
- lumb63 3y ago
  I can decisively say anyone advocating for roundabouts has not lived anywhere near Boston.
  - InSteady 3y ago
    Let's be real, is this actually an indictment of roundabouts or is it just Boston?
  - throwaway562if1 3y ago
    Having lived in Boston and other areas with roundabouts, I can with confidence assert that this is a Boston traffic design problem, not a roundabout one.
- smegsicle 3y ago
  note that proper spinlock procedure is to not leave the roundabout unless there is room to do so completely this avoids complete deadlock until the entire roundabout is full of drivers who refuse to take any unblocked exit, while continually offering each the ability to do so
  - sokoloff 3y ago
    > not leave the roundabout unless there is room to do so completely I don't understand how that helps. The problem is there are too many cars in the rotary, so no amount of not leaving the rotary will unblock it. It seems like you should not enter unless there's room to exit, but on some of these, you can't both see the exiting traffic light and safely merge into moving rotary traffic, so I don't really blame drivers who enter the rotary and find the light turns amber then red on them.
    - smegsicle 3y ago
      as an ai language model, i cannot speak to your particular intersection, but the idea is that if say just one car is waiting for the blocked exit, circling would allow other exits to continue to function, by following the general rule of 'do not stop once entering the roundabout' i could also imagine that anyone waiting behind someone who is deadlocking might have success sending a signal to the blocking process, unless they have committed to a non-reentrant exit routine
Maxion 3y ago
There is an issue with them though. If there is very unbalance traffic flow in one direction through the roundabout, traffic from the perpendicular direction tends to be perpetually forced to yield. And only being allowed in the roundabout by some graceful soul. This can happen on routes where there are multiple chained roundabouts, that usually results in traffic coming at an even rate with no gaps in between.
macleginn 3y ago
With lively enough traffic, it’s not rare to see several cars waiting for their turn to enter the roundabout.
- ghaff 3y ago
  On my way into the city, there is a rotary that at rush hour can be a 15+ minute wait to enter the rotary in the direction of the heavy traffic flow.
belorn 3y ago
Roundabouts often lead to cars being forced to stop. The stop is however generally much shorter, occasionally not needed, and when cars leaves at the first turn then the other cars from other directions can simultaneously use the roundabout at the same time. Many roundabouts here where I live also have extra roads for some direct right turns where such traffic flow is common, which mean that those cars don't even need to enter the roundabout and just need to be observant of cars existing towards the same direction.
AmericanChopper 3y ago
They’re often worse in heavy traffic situations. Especially if you have two entrances that feed heavy traffic into the roundabout at peak times, the one that has to yield can end up basically locked out of the flow.
- hnick 3y ago
  Yes traffic lights can be far better in heavy traffic - at least everyone gets a turn. In some roundabouts I've driven through, where there is a "main" way the cars coming from the other directions often get locked out as people queue through it. It works out if people have some courtesy and take turns but that's not always a good basis for traffic planning.
- Maxion 3y ago
  If the heavy traffic or the locked out traffic does a right turn in the roundabout, then a solution to this is a bypass lane. This works very well for letting traffic through and opening up the roundabout. Multi-lane roundabouts help when theres a clog downstream of the roundabout. Roundabouts are also great if you have more than 4 roads meeting. But yeah, in general roundabouts shine the most in low-traffic intersections that previously had lights. They make driving in those are so much nicer, drivers basically never have to stop.
deleted 3y ago
[deleted]

sokoloff 3y ago

Until you do what Cambridge, MA has done twice near my house: put a traffic light just outside the roundabout, so when the light turns red, traffic inevitably backs up through the roundabout, locking it up entirely. https://www.google.com/maps/@42.3875777,-71.1422076,18z?entry=ttu https://www.google.com/maps/@42.3875777,-71.1422076,18z?entr...

timc3 3y ago
They do that in Sweden all the time. I have no idea how a country that has some amazing minds can come up with the worst road infrastructure designs possible. Most of it’s unsafe so they just lower the speed limit.
securingsincity 3y ago
I used to drive over there quite a bit. Always terrible.
foobarian 3y ago
The circles of death? :-)
lumb63 3y ago
I can decisively say anyone advocating for roundabouts has not lived anywhere near Boston.
- InSteady 3y ago
  Let's be real, is this actually an indictment of roundabouts or is it just Boston?
- throwaway562if1 3y ago
  Having lived in Boston and other areas with roundabouts, I can with confidence assert that this is a Boston traffic design problem, not a roundabout one.
smegsicle 3y ago
note that proper spinlock procedure is to not leave the roundabout unless there is room to do so completely this avoids complete deadlock until the entire roundabout is full of drivers who refuse to take any unblocked exit, while continually offering each the ability to do so
- sokoloff 3y ago
  > not leave the roundabout unless there is room to do so completely I don't understand how that helps. The problem is there are too many cars in the rotary, so no amount of not leaving the rotary will unblock it. It seems like you should not enter unless there's room to exit, but on some of these, you can't both see the exiting traffic light and safely merge into moving rotary traffic, so I don't really blame drivers who enter the rotary and find the light turns amber then red on them.
  - smegsicle 3y ago
    as an ai language model, i cannot speak to your particular intersection, but the idea is that if say just one car is waiting for the blocked exit, circling would allow other exits to continue to function, by following the general rule of 'do not stop once entering the roundabout' i could also imagine that anyone waiting behind someone who is deadlocking might have success sending a signal to the blocking process, unless they have committed to a non-reentrant exit routine

Maxion 3y ago

There is an issue with them though. If there is very unbalance traffic flow in one direction through the roundabout, traffic from the perpendicular direction tends to be perpetually forced to yield. And only being allowed in the roundabout by some graceful soul. This can happen on routes where there are multiple chained roundabouts, that usually results in traffic coming at an even rate with no gaps in between.

macleginn 3y ago

With lively enough traffic, it’s not rare to see several cars waiting for their turn to enter the roundabout.

ghaff 3y ago
On my way into the city, there is a rotary that at rush hour can be a 15+ minute wait to enter the rotary in the direction of the heavy traffic flow.

belorn 3y ago

Roundabouts often lead to cars being forced to stop. The stop is however generally much shorter, occasionally not needed, and when cars leaves at the first turn then the other cars from other directions can simultaneously use the roundabout at the same time. Many roundabouts here where I live also have extra roads for some direct right turns where such traffic flow is common, which mean that those cars don't even need to enter the roundabout and just need to be observant of cars existing towards the same direction.

AmericanChopper 3y ago

They’re often worse in heavy traffic situations. Especially if you have two entrances that feed heavy traffic into the roundabout at peak times, the one that has to yield can end up basically locked out of the flow.

hnick 3y ago
Yes traffic lights can be far better in heavy traffic - at least everyone gets a turn. In some roundabouts I've driven through, where there is a "main" way the cars coming from the other directions often get locked out as people queue through it. It works out if people have some courtesy and take turns but that's not always a good basis for traffic planning.
Maxion 3y ago
If the heavy traffic or the locked out traffic does a right turn in the roundabout, then a solution to this is a bypass lane. This works very well for letting traffic through and opening up the roundabout. Multi-lane roundabouts help when theres a clog downstream of the roundabout. Roundabouts are also great if you have more than 4 roads meeting. But yeah, in general roundabouts shine the most in low-traffic intersections that previously had lights. They make driving in those are so much nicer, drivers basically never have to stop.

deleted 3y ago

[deleted]