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.
I live in the UK, which has roundabouts, mini roundabouts, double and triple roundabouts, traffic-light controlled roundabouts, and magic roundabouts. They work perfectly fine.
The difference is that traffic flows very differently in the UK to the US. In fact, the way traffic flows in those two countries captures a lot of their cultural differences. In the US, everything needs a grand overarching theory and imposed order, which is why Americans need traffic lights, and why they’re so keen on a very specific set of liberties that allow them to freely live such similar lifestyles. The UK embraces mild chaos and ruthless pragmatism, with everyone finding locally optimal solutions to their own problems, so things like roundabouts fit well with the national mindset.
The fact that you mention light-controlled roundabouts highlights that they do not, actually, "work perfectly fine".
Roundabouts are a practical example of the tyranny of the majority; minor traffic flows suffer against major. The centre of Salisbury is a perfect example, constantly gridlocked at two roundabouts except at odd hours on a Sunday.
Adding traffic lights to 'fix' this problem then leads to tailbacks on the major routes and congestion on the roundabout, such that the timing sequence for the minor routes are subsequently reduced, returning to the original problem.
Roundabouts, in the plurality, are used as a lazy 'solution' to at-grade traffic crossing, which the road designers themselves presumably don't have to suffer. They also cause pollution from brake dust from slowing and exhaust fumes from accelerating.
> The fact that you mention light-controlled roundabouts highlights that they do not, actually, "work perfectly fine".
I don't think anyone is claiming that roundabouts work in every and all situations: design handbooks often say they're good 'up to X vehicles per hour/day'.[1] But that does not mean they do not work, or not useful, for <X.
Intersections are an engineering problem, and there is a hardly ever an engineering solution that works everywhere and always. There is usually a spectrum of solutions for the various situations that are faced.
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.
cjs_ac · · focus · HN ↗
The difference is that traffic flows very differently in the UK to the US. In fact, the way traffic flows in those two countries captures a lot of their cultural differences. In the US, everything needs a grand overarching theory and imposed order, which is why Americans need traffic lights, and why they’re so keen on a very specific set of liberties that allow them to freely live such similar lifestyles. The UK embraces mild chaos and ruthless pragmatism, with everyone finding locally optimal solutions to their own problems, so things like roundabouts fit well with the national mindset.
dingaling · · focus · HN ↗
The fact that you mention light-controlled roundabouts highlights that they do not, actually, "work perfectly fine".
Roundabouts are a practical example of the tyranny of the majority; minor traffic flows suffer against major. The centre of Salisbury is a perfect example, constantly gridlocked at two roundabouts except at odd hours on a Sunday.
Adding traffic lights to 'fix' this problem then leads to tailbacks on the major routes and congestion on the roundabout, such that the timing sequence for the minor routes are subsequently reduced, returning to the original problem.
Roundabouts, in the plurality, are used as a lazy 'solution' to at-grade traffic crossing, which the road designers themselves presumably don't have to suffer. They also cause pollution from brake dust from slowing and exhaust fumes from accelerating.
throw0101c · · focus · HN ↗
I don't think anyone is claiming that roundabouts work in every and all situations: design handbooks often say they're good 'up to X vehicles per hour/day'.[1] But that does not mean they do not work, or not useful, for <X.
Intersections are an engineering problem, and there is a hardly ever an engineering solution that works everywhere and always. There is usually a spectrum of solutions for the various situations that are faced.
[1] <a href="https://www.fhwa.dot.gov/publications/research/safety/00067/00067.pdf" rel="nofollow">https://www.fhwa.dot.gov/publications/research/safety/00067/... § 1.6.1, 3.3.1