why does adding more traffic lanes doesn’t help to alleviate traffic congestion?
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Drivers tend to be self-centered, and will gladly jam up all of the lanes if they think they can get past the vehicle in front of them.
A number of reasons but here’s a couple big ones.
1. More lanes DOES lead to less congestion in any given lane, however because of this it leads to more people driving which more or less gets you back in the same neighborhood of where you started
2. Congestion is caused by way more than just “too many cars for a given number of lanes” – driver inefficiency/error (braking too hard/too early, not staying right if not passing, etc), onramps/offramps, conditions, and various other factors contribute. Adding more lanes doesn’t fix any of these problems (and in some cases can make things worse)
In the vast majority of cases more lanes do reduce congestion. There are a small number of instances where it doesn’t due to poor road network design, but those are very much the exception. Unfortunately a lot of NIMBYs will use facts like this to block any new infrastructure construction, which is probably why you believe this misnomer.
A big part of it has to do with a phenomenon called induced demand. When you make driving easier and quicker, like by adding lanes or bypasses or freeways or reconfiguring intersections, you get more people into cars, until it reaches capacity and then you are back at square one, and then you end up with Los Angeles.
Because to beat peak congestion you would have to add like 12 lanes and 8 lane ramps and 6 lane side streets. You can’t build for peak hours because the rest of the time it is empty.
On normally sized roads, peak congestion causes the most cars possible to fill the road. At this traffic density, average traffic speed is about 40 mph. And due to variance in the speed of individual vehicles, speeds will drop as people brake and the braking action chain reacts backwards.
Cars do not run into traffic, cars are the traffic.
If you simulate a road where the only rules are 1) speed up if there is nothing in front 2) slow down if something is in front then you will get traffic bunching up just on the random speeds and braking action. Traffic clears from the front of a jam and builds from the back.
Add in one accident and it reduces lanes by at least 1 lane and makes everyone have to zipper over, limiting speeds all the way across the lanes.
One thing not mentioned yet: exits. Folks are on the road to get somewhere. If enough other people are going to that same somewhere, it is impossible to create enough exit ramp to accommodate them all. So things will clog up.
I have a city planning degree. Adding more lanes will always reduce congestion in the short term. In the long term it will never reduce congestion. But “long term” is defined very differently depending on the area.
Others have used the term induced demand, and it’s the correct answer, but it helps to understand exactly what that means. If roads are congested, some people will stay home rather than drive, or, more particularly … they will live somewhere else.
Now widen the road into a beautiful new wide thoroughfare. People suddenly are fine with driving since the congestion is lower ***and*** developers are suddenly eyeing the area a lot more closely looking to build housing and market the “ease” of driving on the new highway. So they build more, and more people move in, and the people who didn’t drive suddenly start driving … and hello, the road is right back where it was, after $500 million of taxpayer money was spent.
This process can take a very short time or a long time. In hot cities, like Atlanta or Houston, highway widening projects are doomed to fail almost immediately — within two years, they are probably going to be the same level of congestion or worse. Expansion projects in slightly less “desirable” or hot markets will take longer to reach this stage. Either way, the long term investment does not pay off.
It is important to note that this will happen with construction of commuter rail as well, which is something people bring up a lot. If you build commuter rail instead of widening the road, then yes, congestion will ease a little bit, until, whoops, more cars suddenly show up to fill the space vacated by the new train riders.
Of course adding more lanes alleviates traffic. How on earth can people think that it doesn’t. Alleviated traffic leads to more people choosing to drive at any given time, but again more people are getting where they need to go per unit time.
Widening a 10 lane highway to 12 will NOT increase flow through by 20%, it will be significantly less. However, to argue that it wouldn’t increase flow through at all is nonsense.
1) Induced demand – if you make the road ‘better’ then more people will drive on it, meaning it will quickly reach some sort of capacity constraint again.
Now *maybe* some of these people were already driving but along alternative routes to avoid the congestion. So if you upgrade one road, it’s possible you will actually help alleviate congestion on the alternative routes rather than on the road you actually upgraded.
2) People don’t just randomly drive on a road/freeway, they have a destination in mind, and there may still be constraints at the destination.
e.g. perhaps upgrading a freeway makes a smoother ride from the suburbs to the city, but people eventually need to get off the freeway in the city. Capacity constraints on local city roads can still cause traffic to back up onto the main upgraded route.
3) Traffic jams can still be caused by poor driving, accidents, confusion etc. This is a human issue and will exist no matter how many times you upgrade the road.
4) People still mostly start and end work broadly at the same time. So even if the road can handle a normal traffic volume without congestion, there will always be ‘peak’ traffic volumes when people are commuting that will be beyond this.
A. If you build it. They will come. And usually road Improvements are way behind the population. As soon as a new bridge, new highway is opened, hundreds living in expensive areas realize those cheaper areas are not so bad nor is the commute at the moment.
B. There are often congestion points that are unavoidable due to curves, multiple convergence on ramps, poorly designed or legacy roads that can’t be fixed.
C. Improvements here just push the problem down the road
D. After they spend a billion to add a lane, it turns out to be a carpool lane. Which no matter how low the bar it is to qualify, here in Washington state you only need two in a car, they are sparsely used.
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