Eli5: it’s said that creating larger highways doesn’t increase traffic flow because people who weren’t using it before will start. But isn’t that still a net gain?

Question

If people are being diverted from side streets to the highway because the highway is now wider, then that means side streets are cleared up. Not to mention the people who were taking side streets can now enjoy a quicker commute on the highway

In: Engineering

Answer 1

Induced demand is a well studied phenomena. I worked for a highway for a number of years and though I wasn’t a traffic engineer I had many conversations with them so my level of expertise on this is far higher than average but less than a civil engineer who works for a city. The connection between adding lanes and reducing service levels (defined by the total number of cars passing between two parts of a highway where there are no exits) is an unfortunate truth that no politician wants to hear about.

The simple explanation is that while the highway, in theory, can handle more cars per minute in perfect conditions – perfect conditions never exist. A couple of things collide to make this a reality. As more cars are added to the road, very predictably, the average speed drops in linear fashion. Car accidents become more common and have a larger impact on the overall service level. Popular exits backup even without accidents and impacts speed behind the exit in question. This is one of the reasons you see pictures of LA freeways that are like 12 lanes abreast and not one of them is moving very quickly.

The latest studies have shown that adding a lane does *reduce* commute times for an average of 5 years in the USA and after that it increases back to what it was, but now you are pissing off even more drivers. Considering the exorbitant cost of adding lanes and their proven ineffectiveness, it is surprising that local governments often insist upon them.

There is only one proven way to reduce traffic and commute times, reduce the total number of cars on the road. Thats it, right there, no other amount of gymnastics will ever match this very basic principle. If you are a metro area and you need to move a lot of people quickly in a wide variety of weather conditions, you need trains and dedicated (and preferably protected) bus lanes.