I’ve always heard it said that building more lanes doesn’t help but I still don’t understand why. Obviously 8 wouldn’t help anymore than 7 but 3, 4, or maybe 5 for long eways helps traffic filter though especially with the varying speeds.
In: Other
Induced demand
Adding more roads and lanes encourages more people to drive vs taking alternative modes of transport (public transit, cycling, etc). So you end up with even more cars on the road.
Similarly if you improve public transit, or provide cycling paths, people will start to use them which reduces traffic.
Adding more lanes tends to make traffic worse overtime so it’s a losing battle. You also have to consider that you have to maintain all of that. Traffic lights, roads, overpasses, etc all cost money and require a lot of upkeep. Where-as a subway is also expensive but after the initial investment costs less to maintain compared to the number of people that use it on a daily basis. In the context of a city a subway is a more efficient way to move lots of people around quickly.
Having a mix of well developed public transport and roads has proven to be more efficient.
If you build it, they will come.
Austin built massive amounts of roads. They migrated traffic, so housing developers came along and built out a lot more housing. And those new people drive, so you are back at where you started.
They were successful in getting rents down in the cycle, but the excess road capacity ended up being used.
For most the US, mass transit is sufficiently low that it rounds to zero, and the big drivers in induced demand ends up being housing construction.
Once a road gets congested enough that you’re considering expanding it, people have already sought out alternative routes (or changed their driving patterns) because the traffic on that original route was so bad. When you go in and add lanes to that road, you encourage that traffic to return because now there’s more capacity, and the road fills right back up.
8 lanes won’t help if all the cars are going to the same place downtown where it gets narrowed into streets.
There is also the idea of induced demand. When you add more free roads, new people that were not commuting will consider using this new infrastructure. Basically, if you put so much money in car infrastructure, don’t be surprised when people choose cars over other means of transportation.
To add an example to what has already been said:
Near me there’s a highway with a lot of traffic during rush hours. There used to be two lanes all the way from start to end. To mitigate traffic, they made three lanes. But they couldn’t make it three lanes all the way. Now there is traffic during AND AROUND rush hours because merging slows everyone down even when it’s not peak traffic.
I don’t know if you drive, but have you ever though to yourself ”oh I don’t want to go to my destination by using road X, there is always a traffic jam at this hour, I will use road Y instead. Or maybe ”oh I don’t want to drive downtown it’s always a nightmare, I’ll use the subway instead’. Or maybe ”fuck that I stay home, I don’t want to get stuck in traffic”.
The way we perceive the road affect our driving habits. By opening more road, people that didn’t want or couldn’t use that road suddenly are happy to take it since it doesn’t have any traffic anymore. Word will spread that hey no traffic if you use that road now, which mean more and more people are using it until it reach a new equilibrium. If there is too much traffic, less people will want to use the road which will decrease the traffic. If there is less traffic, more people will want to use that road. It’s the reason why you typically end up with a similar amount of traffic, nobody get stuck 12 hours in traffic because then nobody would use that road. It always tend toward a manageable, but annoying level of traffic because of human behavior.
If you had infinite lanes, it would help. But we have far less than that. We have so few that people make conscious decisions to either travel at different times or using other options like public transit.
You basically have a giant queue of people who are waiting for there to be one more lane before they’re going to start driving instead. As soon as that lane opens, they show up.
Two main reasons:
1. Induced demand: if you add more lanes, more cars will drive on that particular roadway. There’s a sort of self-balancing equilibrium: there’s a threshold of a certain level of congestion or speed, at which a certain amount of people will or won’t be willing to drive on that roadway. If you add more lanes, suddenly the previously too-congested roadway became acceptable to people who previously wouldn’t have driven on it, so they will drive on it now. This happens until again the roadway becomes congested to the point where more people aren’t willing to drive on it.
2. At a certain point, a highway’s throughput is limited by other features, like the capacity of off ramps (which typically are signaled intersections onto fixed-lane local roads), interchanges, etc. That means you can’t keep increasing the lane count infinitely and expect throughput to go up infinitely. There are downstream bottlenecks. If you could also widen interchanges infinitely, and widen local roads onto which people get off the highway onto, then sure, maybe. But off ramps tend to only have 2 or so left turn lanes and and 2 or so right turn lanes.
OTOH, increasing the number of lanes, while it might not improve the experience of individual drivers from their individual perspectives, *does* increase the *overall throughput* of the roadway.
While the flux (measure of the amount of flow rate through a given cross sectional area of a surface) eventually stays relatively the same, the volumetric flow rate (the integral of flux) goes up because there is now more cross sectional area (more lanes) over which the same flux acts.
So overall, each car experiences the same speed, but there are now more cars, which means in a given day, more car-miles were moved through that highway.
Traffic backs up, so you go take the side roads. New lane is added, so traffic isn’t backed up anymore and you decide to not take the side roads. Enough people make the same decision that traffic backs up again. Repeat.
To add another wrinkle to this: every lane you add to a road adds less and less overall capacity to the road.
For example, adding a lane to a single lane road, going from one to two, effectively doubles the capacity of the road. But, going from two to three doesn’t do the same. Instead you have only increased the capacity 1.5x. Going from three to four is 1.3x. Four to five is 1.25x. Each time the capacity increase gets smaller and smaller.
Latest Answers