how does increasing the length of a road help with traffic jam?

Question

I was watching a civil engineer’s video where he mentioned that in some cases where there’s a traffic jam but no space to widen the road or increase lanes, engineers resort to making the road longer and/or decreasing the speed limit. How does that help?

My first thought was that it acts as a buffer but since it’s still the same road technically, the cars entering and exiting is still the same therefore the buffer would eventually be filled up and the bottleneck will pop up again.

Edit: for more context. The road is a 2-lane highway in a game video (cities skylines 2) which has no traffic light and only 1 or 2 exit ramps at the end into the city.

In: Engineering

Answer 1

Widening the road/adding lanes usually doesn’t solve traffic jams, because jams don’t happen on those wide areas, but bottlenecks (exits and such). So adding more lanes would make motorists think that there’s even more room on the road – they can’t see the jam, thus even more of them end up in the eventual jam. Imagine trying to empty a bucket by making a small hole in its bottom. Making bucket wider doesn’t make water drip from the hole any quicker, but it’ll take even longer to empty, since you can put more water in and bucket looks more capable from the top end. So, if you can’t make the hole bigger, you’re just making flow problem worse.

Making road longer, thinner, less comfortable, etc might discourage drivers to use it, which means there’s less cars on it and bottleneck has less volume to deal with (thus it works better and disappoints/delays less drivers). If you watch civil engineer, quite often they actually do that part – force road to be one way, reduce lanes, add lights to cut long traffic spaghetti into smaller sections and so on. It’s more about managing the volume and cutting problems into timed chunks, than flattening more city to make room for the volume (road).

In terms of cities, it’s usually not about one road, cities have a lot of roads, but drivers tend to want to use most direct one. If you manage to get them to use more/other roads – (even if they’re a bit longer), you’re relieving pressure on bottlenecks and traffic flow becomes more fluid.