My best friend is a highly experienced civil engineer, specialized in designing intersections. His take on roundabouts:
- They are adored by urban planners because they look nice and you can put a statue or fountain in the middle
- They reduce T-bone collisions
However:
- Small fender benders and sideswipes are much more frequent
- They use way more area
- They work where each direction has an approximately equal amount of traffic. However, usually, one direction (north/south, say) has a larger share of the traffic. In those cases, the bulk of traffic all needs to slow down, downshift, turn a bit in one direction and then turn a bit in the other direction for the minority of traffic that's bleeding on or off the main artery. In those cases, a traffic light is better because the timing can be adjusted to the proportion of traffic from each direction (and more flexible since it can be adjusted over time).
- Finally, they're also (mostly) worse for cyclists and pedestrians.
Essentially, they have use cases, especially in small towns, and they're pretty, but they're usually not the right engineering solution.
> They work where each direction has an approximately equal amount of traffic. However, usually, one direction (north/south, say) has a larger share of the traffic. In those cases, the bulk of traffic all needs to slow down, downshift, turn a bit in one direction and then turn a bit in the other direction for the minority of traffic that's bleeding on or off the main artery. In those cases, a traffic light is better because the timing can be adjusted to the proportion of traffic from each direction (and more flexible since it can be adjusted over time).
This point is so under-appreciated. Roundabouts are really good in some cases, but they're over-utilized in England. Imagine if the US had roundabouts on the interstate and you're getting close.
I think the main problem in England is that there is just too much traffic for too little road. I don't think there is a simple infrastructure solution to that problem.
For example I was going to point out that roundabout completely fail when one direction becomes nose to tail congested but when I thought about local examples, I'd rather take that over a set of traffic lights where it becomes near impossible to get out from one direction if the traffic keeps backing up onto the junction.
> I think the main problem in England is that there is just too much traffic for too little road.
I think Jevons paradox basically guarantees this in both countries. The difference between the two countries (in my theory) is how much responsibility they're willing to place on the drivers. The UK requires much more driver effort for good reasons I suggest in another thread.
> Imagine if the US had roundabouts on the interstate and you're getting close.
What's the problem with a roundabout on the interstate? Obviously it'd have to be huge to account for the speed, so people don't need to change speed too much, but given the right size, it'd work wonderfully.
Nothing depending on your perspective. Mine is a U.S. perspective and a disposition against. They slow things down and require more mental and physical capabilities than the U.S. approach. That’s not accidental or because U.S. people are dumber, it’s just a cultural difference between the countries in terms of driver expectations. I’ve proposed a rationale elsewhere.
> They slow things down and require more mental and physical capabilities than the U.S. approach.
Make the roundabout larger then, and you can keep just about the same speed. How can a roundabout take more mental and physical capabilities and slow you down less, than a stoplight on the interstate? That's very backwards, and not sure what roundabouts you've experienced, if that's the case.
There aren't stoplights on the interstate. That was the point I was making. (It got lost in the follow up.)
But to answer your question, roundabouts always require drivers approaching from all directions to slow down, lookout for cross traffic and assess other drivers' behavior. Stop lights have 2 modes that promise safe passage, and they can be set to prioritize routes.
I have actually seen some roundabout like constructs on parts of the interstate. They're giant, far larger than singular interchanges, and serve to handle multiple branches and belt loops.
Of course, they are augmented by allowing the main direction of travel to bypass the circle and go over/under it
propter_hoc · · focus · HN ↗
- They are adored by urban planners because they look nice and you can put a statue or fountain in the middle
- They reduce T-bone collisions
However:
- Small fender benders and sideswipes are much more frequent
- They use way more area
- They work where each direction has an approximately equal amount of traffic. However, usually, one direction (north/south, say) has a larger share of the traffic. In those cases, the bulk of traffic all needs to slow down, downshift, turn a bit in one direction and then turn a bit in the other direction for the minority of traffic that's bleeding on or off the main artery. In those cases, a traffic light is better because the timing can be adjusted to the proportion of traffic from each direction (and more flexible since it can be adjusted over time).
- Finally, they're also (mostly) worse for cyclists and pedestrians.
Essentially, they have use cases, especially in small towns, and they're pretty, but they're usually not the right engineering solution.
piker · · focus · HN ↗
This point is so under-appreciated. Roundabouts are really good in some cases, but they're over-utilized in England. Imagine if the US had roundabouts on the interstate and you're getting close.
VBprogrammer · · focus · HN ↗
For example I was going to point out that roundabout completely fail when one direction becomes nose to tail congested but when I thought about local examples, I'd rather take that over a set of traffic lights where it becomes near impossible to get out from one direction if the traffic keeps backing up onto the junction.
piker · · focus · HN ↗
I think Jevons paradox basically guarantees this in both countries. The difference between the two countries (in my theory) is how much responsibility they're willing to place on the drivers. The UK requires much more driver effort for good reasons I suggest in another thread.
embedding-shape · · focus · HN ↗
What's the problem with a roundabout on the interstate? Obviously it'd have to be huge to account for the speed, so people don't need to change speed too much, but given the right size, it'd work wonderfully.
piker · · focus · HN ↗
embedding-shape · · focus · HN ↗
Make the roundabout larger then, and you can keep just about the same speed. How can a roundabout take more mental and physical capabilities and slow you down less, than a stoplight on the interstate? That's very backwards, and not sure what roundabouts you've experienced, if that's the case.
piker · · focus · HN ↗
But to answer your question, roundabouts always require drivers approaching from all directions to slow down, lookout for cross traffic and assess other drivers' behavior. Stop lights have 2 modes that promise safe passage, and they can be set to prioritize routes.
paradox460 · · focus · HN ↗
Of course, they are augmented by allowing the main direction of travel to bypass the circle and go over/under it