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Putting traffic lights on roundabouts has always struck me as a ridiculous
thing to do. Its as if the traffic planners didn't quite understand the
purpose of a roundabout or how it worked and assumed it was no different
to a 4 way junction. Once you've added the lights the roundabout is now
completely redundant and you'd probably get better traffic flow if you did
replace it with a simple junction.
Roundabouts work very well where there is a reasonably balanced traffic
flow on all 4 (or more) arms. They fail badly where the traffic is
highly directional, e.g. towards a city centre.
Consider a typical 4 arm roundabout with arms A, B, C and D positioned
at noon, 3, 6 and 9. If at certain times of day you have a very large
flow from arm B (3 o'clock) to arm D (9 o'clock), it is basically
impossible to get out onto the roundabout on arm C (6 o'clock) because
there is a constant traffic flow preventing this. There are a number
of roundabouts in Milton Keynes where this causes peak time queueing,
particularly as the A421 passes through eastbound in the morning peak
and westbound in the evening peak.
The way to prevent this is to place traffic lights on all but one of
the arms, having no lights on an arm that is not a "blocking" flow but
does have reasonable demand. In the example above, putting them on all
but arm C would allow continuous traffic flow, but would regulate arm B
such that those on arm C could get out and queueing is prevented.
This has an advantage over a traditional traffic light junction as
traffic is always flowing - when the lights are on amber or all on red
for the "overlap", C can flow. With a traditional junction there is
dead time, which with certain designs of junction can be as much of 25%
of the time - reducing overall capacity of the junction. With the
roundabout, this doesn't happen.
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