Ahh that's the common thinking, conventional wisdom that says 2/4 = 50% of the original capacity. But in reality, that's not the case. Add in some illogical human sociology, the practicalities of left-hand and right-hand turns, and you end up arriving at a much different conclusion.hoborich wrote:Wrong! Three laneing a four lane road cuts it's carrying capacity by 50%! What part of that do you "smart folks with four year degrees" not understand?Three-landing roads reduces accidents and improves operational efficiency. This is no different than goobs trying to stop a roundabout.
From: http://nacto.org/docs/usdg/conversion_o ... adways.pdf
The Statistics Block shows, for an example roadway, signalized traffic control has a much greater impact on delays, but in general, delay time for 4-lanes is 21 sec, 3 lanes is 29, and five lanes is about 16 (being relative and generalized).DISADVANTAGES
Increased Travel Delay
Increased travel delay along the corridor is the primary concern many have with converting a four-lane
roadway to a three-lane facility. Many assume there will be a 50% reduction in corridor capacity because the
number of “through lanes” are reduced by half. In reality the capacity of a three-lane facility is very near that of a
four-lane undivided roadway. Envision a four-lane undivided roadway in a commercial area during the peak hour of
the day. Drivers who want to travel through the corridor generally stay in the outside curb lane to avoid getting
caught behind mid-block left-turning vehicles. During these peak hours the inside lanes are generally used by leftturning
vehicles and very few through trips are made in those lanes. As such, only one lane in each direction is
accommodating most of the through trips – which is similar to a three-lane facility.
So according to the "50%" logic, going from four lanes to three would result in a doubling of delay time, which would be 42 seconds. But that simply isn't the case.
Now five-lanes does even better, but in most cases that requires a complete rebuild, and roads are often boxed in by other things such as buildings, signs, etc, which makes them unfeasible, before you even get to cost.
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Now, if you still think a 4-way intersection is better than a roundabout in terms of traffic capacity, here's a nice video clip from Mythbusters of them totally debunking that theory. And, not only does capacity increase, but the average delay time drops. Real scientific studies bear this out, too. Anecdotally, they recently installed a roundabout on US41 in Negaunee, and even through it isn't yet finished - only one lane open where there will be two - I can already tell you that I've never stopped, whereas the light would likely catch US41 often to let the side street go. By eliminating hard stops, traffic collisions drop.
http://www.wimp.com/mythbusters-test-a- ... oundabout/
So, in summary: conventional wisdom is usually trumped by science...which is rightfully carried about by a bunch of fine people who went to school to learn some things instead of going right to 2/4 = 50%.
Sorry to derail the thread, but hopefully this ends the tangent.