5 ms·

Roundabouts are awesome --- self-organising, self-managing, power-free traffic junctions! But they do have a failure mode, which is that they tend to starve low

Roundabouts are awesome --- self-organising, self-managing, power-free traffic junctions! But they do have a failure mode, which is that they tend to starve low-traffic loads if the traffic is too asymmetric. For example: consider a T-junction with a roundabout at the junction. (Very common in the UK.) Most of the traffic is travelling across the T in a straight line. But I'm entering the junction via the cross road. The rules are that traffic from the driver side has priority over me, but that I have priority over traffic on the passenger side. So I stop at the line and wait for a gap. With balanced traffic, I'll be guaranteed a gap because the oncoming traffic will have to stop to wait for someone else, which means I can move onto the roundabout ahead of the vehicle they're waiting for. But with unbalanced traffic, there won't be a gap, and I'll be stuck. Typically what the traffic planners do at this point is add traffic lights, which is basically giving up, and defeats the whole point of having a roundabout in the first place. ...I used to live near this monstrosity, which had lights at every stop line. Blech. https://www.google.co.uk/maps/@51.436168,-0.8971696,200m/data=!3m1!1e3 https://www.google.co.uk/maps/@51.436168,-0.8971696,200m/dat...

JoeAltmaier 10y ago
Roundabouts have a 'window of efficiency' where light-to-moderate irregular traffic works wonderfully well. But when an area grows and traffic increases, it can move out of this window. When you have regular moderate to-heavy traffic, it can be impossible to get onto a roundabout when facing say commuter traffic in the other direction. This is probably why rural American cities are installing roundabouts at the same time urban British cities are demolishing them.
- maxerickson 10y ago
  Rural and city are opposites. A city is a large urbanized area. Villages and small towns are installing roundabouts though.
  - david-given 10y ago
    The USA uses 'city' to refer to a district --- it's a unit of administration rather than strictly referring to an urban area. It's quite possible for a city to contain rural areas.
    - jsmthrowaway 10y ago
      Not really. A city is an administrative area, yes, but depending on where you are in the country the role you are describing might be a "township." In addition, there are often qualifiers to be a "city" in most states. Most cities end before reaching rural areas and you get into a township or "just a" county; we generally refer to "rural" as not in a city in casual conversation. Urban is different and not necessarily the direct antonym of rural; there's a middle ground. California seems to be an exception where my GPS will report a city quite far away as "where I am now" when I'm in the sticks, but that might be my GPS. California is also less particular about administrative division naming than, say, Michigan. City, town, village, county, parish, and township all have particular meanings that vary across the country. This is not an area where one can say "the USA" and be correct, as with many things in our really-50-countries patchwork. One example is that Wisconsin dubs "towns" what Michigan calls "civil townships." America is confusing, even before you get into the debate over New Urbanism.
      - russell 10y ago
        California is quite peculiar. California has 482 municipalities. Whether they call themselves cities or towns is local preference, although fewer than two dozen call themselves towns. The largest municipality is Los Angeles with 3.7 million people. The smallest is Vernon with 112 people. I live in the unincorporated town of Cambria with a population of about 6000. We are about 20 miles from the nearest city, but we have no local government at all. The closest we have to a government is the community services district which runs the water district, the sewerage treatment plant and the fire department. Cambria is in San Luisbo County which has an area of 3600 square miles and a population of over a quarter million but only 5 cities. California has a law that makes in almost impossible for a new city to incorporate. The new city must reimburse the county for any lost tax revenues caused by incorporation. If your GPS reports a city some distance away, it probably reporting the county seat of an unincorporated area.
7952 10y ago
A big issue is motorists failing to stop for bicycles already on the roundabout. This is particularly problematic with small roundabouts where traffic does not always come directly from the right. A well designed roundabout should make people slow down. Also, those big signalised roundabout are terrible for pedestrians. It is ridiculous to make someone wait to cross every single feeder road.
viperscape 10y ago
West Palm beach, fl had large round abouts installed about 10y ago and I thought that was great, except with little traffic at the time it seemed wasteful in time navigating and more importantly in space. I think the idea is great though, it's safe and usually gives everyone a fair shot to get moving without much delay.. I'd be interested in knowing how the diameter of the round about is decided on, specifically is it some algorithm based on predicted traffic congestion numbers.
- viperscape 10y ago
  If you're curious: https://goo.gl/maps/so6EDBad15K2 https://goo.gl/maps/so6EDBad15K2
  - spc476 10y ago
    South Florida has quite a few round-a-abouts. There's a few in Hollywood and Lake Worth as well.

JoeAltmaier 10y ago

Roundabouts have a 'window of efficiency' where light-to-moderate irregular traffic works wonderfully well. But when an area grows and traffic increases, it can move out of this window. When you have regular moderate to-heavy traffic, it can be impossible to get onto a roundabout when facing say commuter traffic in the other direction. This is probably why rural American cities are installing roundabouts at the same time urban British cities are demolishing them.

maxerickson 10y ago
Rural and city are opposites. A city is a large urbanized area. Villages and small towns are installing roundabouts though.
- david-given 10y ago
  The USA uses 'city' to refer to a district --- it's a unit of administration rather than strictly referring to an urban area. It's quite possible for a city to contain rural areas.
  - jsmthrowaway 10y ago
    Not really. A city is an administrative area, yes, but depending on where you are in the country the role you are describing might be a "township." In addition, there are often qualifiers to be a "city" in most states. Most cities end before reaching rural areas and you get into a township or "just a" county; we generally refer to "rural" as not in a city in casual conversation. Urban is different and not necessarily the direct antonym of rural; there's a middle ground. California seems to be an exception where my GPS will report a city quite far away as "where I am now" when I'm in the sticks, but that might be my GPS. California is also less particular about administrative division naming than, say, Michigan. City, town, village, county, parish, and township all have particular meanings that vary across the country. This is not an area where one can say "the USA" and be correct, as with many things in our really-50-countries patchwork. One example is that Wisconsin dubs "towns" what Michigan calls "civil townships." America is confusing, even before you get into the debate over New Urbanism.
    - russell 10y ago
      California is quite peculiar. California has 482 municipalities. Whether they call themselves cities or towns is local preference, although fewer than two dozen call themselves towns. The largest municipality is Los Angeles with 3.7 million people. The smallest is Vernon with 112 people. I live in the unincorporated town of Cambria with a population of about 6000. We are about 20 miles from the nearest city, but we have no local government at all. The closest we have to a government is the community services district which runs the water district, the sewerage treatment plant and the fire department. Cambria is in San Luisbo County which has an area of 3600 square miles and a population of over a quarter million but only 5 cities. California has a law that makes in almost impossible for a new city to incorporate. The new city must reimburse the county for any lost tax revenues caused by incorporation. If your GPS reports a city some distance away, it probably reporting the county seat of an unincorporated area.

7952 10y ago

A big issue is motorists failing to stop for bicycles already on the roundabout. This is particularly problematic with small roundabouts where traffic does not always come directly from the right. A well designed roundabout should make people slow down. Also, those big signalised roundabout are terrible for pedestrians. It is ridiculous to make someone wait to cross every single feeder road.

viperscape 10y ago

West Palm beach, fl had large round abouts installed about 10y ago and I thought that was great, except with little traffic at the time it seemed wasteful in time navigating and more importantly in space. I think the idea is great though, it's safe and usually gives everyone a fair shot to get moving without much delay.. I'd be interested in knowing how the diameter of the round about is decided on, specifically is it some algorithm based on predicted traffic congestion numbers.

viperscape 10y ago
If you're curious: https://goo.gl/maps/so6EDBad15K2 https://goo.gl/maps/so6EDBad15K2
- spc476 10y ago
  South Florida has quite a few round-a-abouts. There's a few in Hollywood and Lake Worth as well.