Home |
Search |
Today's Posts |
|
London Transport (uk.transport.london) Discussion of all forms of transport in London. |
|
LinkBack | Thread Tools | Display Modes |
#11
|
|||
|
|||
Congestion charge cheat
In article m, Martin
Underwood writes Yes, the Magic Roundabout in Swindon is a pain in the bum: it's as if the road designers decided to make it as tortuous as possible - being cynical, I wonder if they decided to make it hazardous so as to keep the traffic speed down: which is silly because the deliberate hazards distract the drivers' attention from the hazards that they should be looking for - other road users! And then there's the roundabout in Hemel Hempstead. This started out as one big 6-way roundabout. It worked fairly well. [...] Actually, if you study both these junctions and other places where ring junctions (to give them the proper name) are installed (e.g. the A13/A130 intersection), you'll see that the junction was trying to handle too much traffic and snarling up. The basic point behind a ring junction is to reduce the average proportion of the roundabout that traffic has to go on, thus increasing the throughput of the junction. Consider a ring junction with 5 exits and assume that all 20 possible flows see equal traffic (this is to aid the explanation; at a real site you would of course take measurements). If you build it as a normal roundabout, the average distance that a car travels around the roundabout is half its circumference [1]. Thus the flow on the roundabout has to be 2.5 times the flow coming in from each road [2]. If you replace it with a ring junction, the average distance a car travels drops to 30% of the circumference [3] and the flow only has to be 1.5 times the incoming flows [4]. Put another way, you gain 66% capacity (though of course you then lose some because of the additional needs to give way, but it's still a net win). [1] 25% of the traffic goes 20% of the circumference, 25% goes 40%, 25% goes 60%, and 25% goes 80%. That works out as an average of 50%. [2] The section from road 4 to road 5 carries all the traffic entering at road 4, 75% of that entering at road 3, 50% of that entering at road 2, and 25% of that entering at road 1. That's 100+75+50+25 = 250% of the traffic entering at any one road. [3] Now 25% goes 20% of the circumference clockwise, 25% goes 40% of the circumference clockwise, 25% goes 20% anticlockwise, and 25% goes 40% anticlockwise. That's an average of 30%. [4] The section from road 4 to road 5 now carries (clockwise) 50% of the traffic from road 4 and 25% of that from road 3, plus (anticlockwise) 50% of that from road 5 and 25% of that from road 1. Total 150%. Moreover, if you are turning right, you go clockwise round each mini roundabout but *anti-clockwise* round the central roundabout, which feels very wrong: Live with it. Any unfamiliar layout feels wrong; what do you think about slip roads on the right instead of the left? -- Clive D.W. Feather, writing for himself | Home: Tel: +44 20 8371 1138 (work) | Web: http://www.davros.org Fax: +44 870 051 9937 | Work: Written on my laptop; please observe the Reply-To address |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Congestion Charge Fine | London Transport | |||
Congestion charge fine | London Transport | |||
Congestion Charge extension | London Transport | |||
Congestion Charge appeal question | London Transport | |||
Extending the congestion charge zone | London Transport |