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
It depends on what’s causing the traffic jam. On highways if you have too many on/off ramps in too short of a strip, it causes congestion because people need to switch lanes to get on/off and they need to have room to speed or slow down. On streets, too many intersections in a short strip can cause congestion for similar reasons.
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.
There are more than one reason depending on the specific situation.
If we are talking about a road with traffic lights then your initial thought was correct. You want more capacity so the traffic can drain properly. If you cant add more lanes, than making the section longer provides capacity.
Decreasing the speed limit makes tis path less desirable, which means that the cars entering and exiting will not be the same. There will be less of them. Again, clearing the jam.
> My first thought was that it acts as a buffer
It can do. This is particularly important when you have junctions with traffic lights. If the stretches of roads between the junctions are too short, then you can end up with situations where an entrance to a junction completely empties long before the lights have turned red. Though another way of dealing this is to sync up the lights in such a way that traffic can continue flowing into these stretches of road from the previous junction whenever the lights are green.
> decreasing the speed limit
On motorways/freeways/whatever, a big problem is the phenomenon of traffic waves. Even without any external causes, traffic naturally tends to bunch up, so instead of flowing smoothly at a slightly slower speed, everyone goes through a cycle of driving at their top speed and then stopping. Some models suggest that reducing the speed limits on these roads can actually increase the average speed under certain conditions. But it’s complicated and not all models agree, and it obviously depends on how reliably people follow the speed limits. You can get all kinds of paradoxical scenarios like this in transportation networks: another famous one is Braess’s paradox, in which adding an extra road leads to increased congestion (even ignoring any slowdown at junctions or induced demand).
You are adding more room on the road for cars, but keeping the same amount of cars. Instead of having 1,000 cars in 1 mile of road you will have 1,000 cars in 2 miles of road. That being said if there is a choke point at the end of the road it can still back up.
I too watch RCE
Latest Answers