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> (Cambridge also has far too many sets of traffic lights for its size, which introduces an entirely artificial ceiling on the capacity of its road network for

> (Cambridge also has far too many sets of traffic lights for its size, which introduces an entirely artificial ceiling on the capacity of its road network for reasons no-one seems to quite understand. You can usually tell when the lights fail at a major intersection, because the drivers all start being polite and taking turns to proceed, there are hardly any queues in the area, and journeys are invariably quicker and more pleasant. Note to people wanting saner traffic systems for cities in future: competitive routes and stop/go lights are an evil that will cripple your efficiency.) Incidentally, I did hear about a study that indicates that roundabouts are far better for safety than traffic lights. Unfortunately, I don't remember the reason they concluded for this. Suffice to say... I don't really see much reason to ever use traffic lights if they can be avoided.

shalmanese 15y ago
It's because it's only possible to have an angular impact at a roundabout which is far safer than a T-bone impact which is common at traffic lights.
- deleted 15y ago
  [deleted]
willyt 15y ago
Roundabouts typically take up a bit more room. You would need to cut chamfers off (demolish and rebuild) the corners of buildings at junctions to make enough room for a roundabout that's more efficient than traffic lights.
anthonyb 15y ago
Only if you're in a car. For cyclists they're far more dangerous than traffic lights (where you can do a hook turn), and also pedestrians (you can install pedestrian lights, and the cars will be going slower anyway).
- Silhouette 15y ago
  It depends a lot on the design of the roundabout. Our traditional medium-sized model here in the UK, with two or three lanes coming in as one block from feeder roads, certainly isn't great for cyclists. However, we seem to be moving towards a more continental style, where for a medium-sized four-road roundabout you'd have one lane peeling off to the "first exit" and a physically separated lane onto the roundabout for those going further round. This is generally an improvement for everyone, because you only ever have to worry about merging with traffic from one other lane at once. It's also an improvement for cyclists in particular, because you can construct those single lanes so that either they are wide enough to pass a cyclist with a good clearance or they are narrow enough that overtaking is clearly not an option. When there are several adjacent lanes, an aggressive driver will often force a cyclist into the next lane over (even if there's another vehicle there). When doing so requires driving through solid concrete lane dividers, funnily enough it doesn't happen so often...
  - anthonyb 15y ago
    It doesn't help when making right hand turns* or going straight. In that case, a cyclist has to cut in front of two lanes of traffic (left and straight/right), which isn't very good for either them or the cars. You also have to consider that the cyclist needs to give way to the traffic from the right. If they've stopped, they need an extra large gap in the cross traffic, since acceleration on a bike isn't quite as good. * - This is for areas where you drive on the left, obviously.
    - Silhouette 15y ago
      The risks you describe are all true, but mostly apply at traffic lights as well: a cyclist still has to navigate into the right-hand lane at a junction if turning right from a two-lane (or more) entry road. In practice, modern design practice for roundabouts ends any separate cycle lanes well before entrances to the roundabout, thus allowing everyone to merge on approach in plenty of time. Also, the roundabout itself should be designed to limit both the number of potential points of conflict and the speed of traffic crossing those points if shared use is expected, for example by the use of solid islands and dedicated lanes that must be chosen on approach and then followed throughout in a predetermined spiral path around the roundabout. That means cycling on such roads is usually both more efficint and safer than cycling through signal-controlled crossroads of a similar scale. For larger roundabouts, cycling around with several lanes of traffic is rarely advisable anyway and alternative provisions will need to be made. Although dedicated cycle routes like underpasses come with their own safety concerns, particularly at night and not necessarily anything to do with traffic, most roundabouts on that scale are signal-controlled anyway these days (at least here in the UK) so a separate system of crossings to allow cyclists to move around the outside like pedestrians can be provided.

shalmanese 15y ago

It's because it's only possible to have an angular impact at a roundabout which is far safer than a T-bone impact which is common at traffic lights.

deleted 15y ago
[deleted]

willyt 15y ago

Roundabouts typically take up a bit more room. You would need to cut chamfers off (demolish and rebuild) the corners of buildings at junctions to make enough room for a roundabout that's more efficient than traffic lights.

anthonyb 15y ago

Only if you're in a car. For cyclists they're far more dangerous than traffic lights (where you can do a hook turn), and also pedestrians (you can install pedestrian lights, and the cars will be going slower anyway).

Silhouette 15y ago
It depends a lot on the design of the roundabout. Our traditional medium-sized model here in the UK, with two or three lanes coming in as one block from feeder roads, certainly isn't great for cyclists. However, we seem to be moving towards a more continental style, where for a medium-sized four-road roundabout you'd have one lane peeling off to the "first exit" and a physically separated lane onto the roundabout for those going further round. This is generally an improvement for everyone, because you only ever have to worry about merging with traffic from one other lane at once. It's also an improvement for cyclists in particular, because you can construct those single lanes so that either they are wide enough to pass a cyclist with a good clearance or they are narrow enough that overtaking is clearly not an option. When there are several adjacent lanes, an aggressive driver will often force a cyclist into the next lane over (even if there's another vehicle there). When doing so requires driving through solid concrete lane dividers, funnily enough it doesn't happen so often...
- anthonyb 15y ago
  It doesn't help when making right hand turns* or going straight. In that case, a cyclist has to cut in front of two lanes of traffic (left and straight/right), which isn't very good for either them or the cars. You also have to consider that the cyclist needs to give way to the traffic from the right. If they've stopped, they need an extra large gap in the cross traffic, since acceleration on a bike isn't quite as good. * - This is for areas where you drive on the left, obviously.
  - Silhouette 15y ago
    The risks you describe are all true, but mostly apply at traffic lights as well: a cyclist still has to navigate into the right-hand lane at a junction if turning right from a two-lane (or more) entry road. In practice, modern design practice for roundabouts ends any separate cycle lanes well before entrances to the roundabout, thus allowing everyone to merge on approach in plenty of time. Also, the roundabout itself should be designed to limit both the number of potential points of conflict and the speed of traffic crossing those points if shared use is expected, for example by the use of solid islands and dedicated lanes that must be chosen on approach and then followed throughout in a predetermined spiral path around the roundabout. That means cycling on such roads is usually both more efficint and safer than cycling through signal-controlled crossroads of a similar scale. For larger roundabouts, cycling around with several lanes of traffic is rarely advisable anyway and alternative provisions will need to be made. Although dedicated cycle routes like underpasses come with their own safety concerns, particularly at night and not necessarily anything to do with traffic, most roundabouts on that scale are signal-controlled anyway these days (at least here in the UK) so a separate system of crossings to allow cyclists to move around the outside like pedestrians can be provided.