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I think the way this is presented (including in the Wikipedia article) is overly confusing and leans too much on irrelevant game-theory issues. Adding a route

I think the way this is presented (including in the Wikipedia article) is overly confusing and leans too much on irrelevant game-theory issues. Adding a route to a road network does not itself reduce total capacity, of course. It's actually a queueing theory issue--misusing a road as a dysfunctional queue by putting more cars on it than can get through causes head-of-line blocking and allows cars taking the congested, shorter route to obstruct the clear, longer route. (this is concealed in the Wikipedia article by describing the travel time as Travellers/100, the latency of a fast, very wide road queueing into a narrow intersection.) Cutting the route can "solve" the problem by eliminating that particular congestion source, but only in the case of an extra, redundant shortcut route. Other options -- like rate-limiting at the start point, or queueing separately for the shortcut route (an express lane), or forbidding queuing on the main road for the shortcut at all, both fix the general case and this case in specific. The general version of the issue is: When you build a network, never queue in the core. Either drop traffic early and often at the point of congestion, or rate limit as close to the source as possible. Or change links at random and hope your network reconverges into a less broken state, I guess.

bollu 5y ago
Nice explanation, I hadn't t heard this perspective, and it makes it a lot more intuitive! What's your favourite way to learn queueing theory? I feel like I'll get cool insights from this :D
alimw 5y ago
As a game theorist I find your description utterly impenetrable :)
calvinmorrison 5y ago
Philly has this! Our newest highway - the blue route runs wide around the city and connects the turnpike to close to Delaware. It has a queue system which only let's on X cars per minute, preventing a heavy merge area from slowing everything down. Basically it's a stop sign with a red light at the on ramp. It goes green for a second, two cars go, it goes red. Interesting concept. More recently 76 has implenented variable speed limits to prevent backups - though it's entirely ignored unfortunately - you can't safely follow a 30mph limit when everyone is going 65 speeding their way into a jam
xenadu02 5y ago
This is what the Bay Area highway on-ramp traffic lights do. They rate limit traffic merging onto the highway thereby avoiding stop and go jams. That results in an overall increase in throughput on the highway at the cost of forcing vehicles to queue at the ramps. The overall travel time is lower despite the time spent waiting in the on-ramp queue.

bollu 5y ago

Nice explanation, I hadn't t heard this perspective, and it makes it a lot more intuitive! What's your favourite way to learn queueing theory? I feel like I'll get cool insights from this :D

alimw 5y ago

As a game theorist I find your description utterly impenetrable :)

calvinmorrison 5y ago

Philly has this! Our newest highway - the blue route runs wide around the city and connects the turnpike to close to Delaware. It has a queue system which only let's on X cars per minute, preventing a heavy merge area from slowing everything down. Basically it's a stop sign with a red light at the on ramp. It goes green for a second, two cars go, it goes red. Interesting concept. More recently 76 has implenented variable speed limits to prevent backups - though it's entirely ignored unfortunately - you can't safely follow a 30mph limit when everyone is going 65 speeding their way into a jam

xenadu02 5y ago

This is what the Bay Area highway on-ramp traffic lights do. They rate limit traffic merging onto the highway thereby avoiding stop and go jams. That results in an overall increase in throughput on the highway at the cost of forcing vehicles to queue at the ramps. The overall travel time is lower despite the time spent waiting in the on-ramp queue.