why does adding more traffic lanes doesn’t help to alleviate traffic congestion?
In: 26
(Assuming an access-controlled highway. Normal surface streets have this and much more). Cars do not drive in a single lane straight down the highway for their whole trip. There is always entering/exiting the highway, which almost always requires merging into another lane. Merging and changing lanes causes cars to slow down, especially if the road is near saturation with other vehicles. If you add more lanes, that creates more merging/lane changing just to use them. Even if the far left HOV lane is moving, it will slow down the other lanes as vehicles have to cross them to get over. Imagine a 100lane wide road: cars would likely spend more time trying to get to the 100th lane than the normal trip in one lane would take to complete. Add in the behavior of people trying to merge early (causes the cars in the merging lane to slow down earlier and more, since this car is likely not up to speed) instead of zipper merging where the lane tells you to (cars are generally ready for the other vehicles to move in, and have the space available for them to and can maintain speed), or the other behavior of not allowing cars to merge (the refused car will likely move further ahead anyway and merge there, causing everyone behind this new merge point, still in front of the refuser, to slow down, possibly even more), and you get more extreme slowdowns regardless of how many lanes exist. Slowness in one lane often causes slowness in the neighboring lanes, again because cars have to merge between those lanes.
tl;rd: Cars have to change lanes, changing lanes causes slowdowns, adding lanes increases lane changes, increases the slowdowns after a certain point.
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