Roundabout Analysis: Congestion Tips & Solutions

An analysis of roundabout dynamics and optimisation. This guide should provide you with a variety of different ways of optimising traffic flow through large and congested roundabouts. It mainly focuses on how different joining road types and layouts effect the congestion of a single roundabout.

Introduction

When reading traffic tips most people simply suggest replacing intersections with roundabouts. However, believe it or not, some roundabouts can get congested. After a remodel of the surrounding area the original roundabout I had generated a lot of congestion and near gridlock. I will be using this roundabout as an example of how you can optimise traffic flow.

This guide covers the effectiveness of common solutions. If you just want to see my best working solution, skip to the Best Solution section.

Inner highway ramps

These were made popular by redsquizza's post on reddit. However I found that adding inner roundabout highway exits seems to have a very minimal effect on the gridlock problem. It does increase usage of the inner lane but will likely only serve as a small optimisation rather than a full congestion solution.



Removing outer exits will increase lane usage much more, but may result in congestion due to cars changing lanes to exit (Known as "Weaving"). However it would minimize the effects of the standard weaving problem common with clover leaf interchanges where vehicles entering have to cross with vehicles exiting.

In my opinion inner ramps aren't necessary but still look cool and doesn't hurt to add additional inner exits.

Joining Highways

My first working solution was to use 3 lane highways. Highways have no stoplights and let traffic flow straight in to the roundabout. However the road sides could not be zoned. This wasn't a major issue because I adjusted the surrounding area to fill in the space. Also I believe this may have slightly reduced congestion, due to not having cars stop and turn in to buildings alongside the road. The One major fault of this was that It required citizens to travel further to their location, due to highways being one way.

Best solution - Single lane highways for each joining road

After re-examining redsquizza's example I think I found what made it work so well. Using single lane highways as incoming and outgoing lanes for each road. The outcome being no stop lights, all roads could be both incoming and outgoing, it allowed faster speeds when entering and exiting the roundabout and almost as much zoneable roadside.


A neat side effect to there being only one incoming and outgoing lane you can make the joining road smaller, such as a simple two lane road. This frees up a little space for zoning. However this will result in more congestion when it reaches an intersection, but you can solve this by increasing the lanes just before the first intersection for roads leaving the roundabout, like in the example below.

Also if you don't require the roadside to be zoneable, you could use two parallel one lane highways all the way up to the first intersection, allowing cars to maintain a higher speed limit for longer.

Small roundabouts

When it comes to smaller roundabouts, I would suggest using one lane highways, due to their increased speed limit, however the benifits minimal if all joining roads have a slower speed limit. They also are slightly smaller. Using one way 2 lane roads is, in my opinion, usually not as good, since the inner lanes will rarely be used.

These smaller roundabouts can make great entrances and exits to highways or just flair up the look of your neighbourhood. If the roundabout isn't an entrance or exit to a highway the lower speed limit of one way 2 lane road isn't as much of an issue.

One way 2 lane roads can be handy for conjoined roundabouts due to higher lane switching like in dangerism's example on reddit. This is a great example of using multiple road types to manage lane usage.

Miscellaneous Tips / Conclusion

Hopefully this helps you fix your congested roundabout or create a new well flowing one.

Some other miscellaneous tips include:

• A common cause of congestion is a large number of nearby intersections. Joining them, removing them, or using one way roads can ease congestion, both on nearby roundabouts and in general.
  
• Since the 1.1.0 patch (19/05/2015) you can build underground tunnels. This allows for a lot more creative and effective roundabout designs, including underground roundabouts!
  
• Never use one way six roads. On a roundabout, everyone is turning and hence only use 3 of the six lanes. 6 lane roads in general usually wont do much to fix your traffic problems.
  
• Try not to zone the actual roundabout Inside or out. It can cause congestion. congestion on the roundabout block can block traffic to any exit on the roundabout, rather than just one road.
  
• Also zoning near the entrances and exits is usually not a good idea either since cars turning in and out might back up in to the roundabout.
  
• Here's a good guide on how to make perfectly round roundabouts.
  
• Roundabouts don't need to be circles. While rarer, they can come in various differents shapes as you will see in the examples section.

If you have any other advice or tips for roundabouts feel free to leave them in the comments and I might even add them to the guide. Also please upvote the examples I shared, they were a lot of help to this guide.

Inspiration

I decided to add some examples of roundabouts that you can use as inspiration.

Inner ramps with split single lane highway joins
by redsquizza
This roundabout that most closely resembles the example I used.

5-way magic roundabout
by DeanoMachino14
This is based on the real life "Magic roundabout" in Swindon. Here's a video of it. In the C:S comunity a magic roundabout now usually refers to a roundabout with other roundabouts attached to it. This particular example 3 lane highways throughout the roundabout.

8-way magic roundabout
by DrTardigrade
Same design as above but much bigger. It also includes a public transport hub in the centre of the roundabout.

4-way magic roundabout with varying lanes
by dangerism
This is a great example of using varying amounts of lanes to manage traffic appropriately. As you can see all the lanes see moderate usage.

Single three layer spiral roundabout
by hellmonkey
This is an amazing roundabout. It is all once single roundabout that spirals up 3 layers. This allows for much more joins and for the roundabout to hold more vehicles at one time.

Inner roundabout exits
by jsj1985
This is a unique example of using inner exit ramps. This is a simple 4-way roundabout but only inner ramps can be used to exit the roundabout and only outer ramps to join roundabout.

4-way roundabout with underpass
by Kallipoliz
A simple 4-way ropundabout with a tunnel through the centre, to allow cars on the main joining road to bypass the roundabout. You can do the same thing elevated roads or use an overpass.

3-way magic roundabout with inner roundabout overpass
by wezed
Quite an complex magic roundabout. Has an elevated smaller roundabout in the centre which a overpass join to. Personally I believe this is too complex for such a simple problem. This leads it to have many wasted lanes/roads that are barely used. None the less, it is still quite impressive.

3-way roundabout with inner roundabout
by Dewnis
Similar to one above but much more simpler and practical. Roads are less wasted. The inner roundabout flows in an opposite direction to the main roundabout and allows cars faster access to the further exit and u-turns.

Trafic light controled roundabout
by thingshappen-xuras
I believe this uses a mod to add traffic lights to highway intersections. This allows traffic lights to manage the weaving on traffic joining and leaving the roundabout.

4-way roundabout with sharp intersections
by shortglass
This uses the sharp intersections mod to allow for shallower angles on joining roads. This minimizes the amount a car needs to slow down to turn on to the roundabout. This is how a real roundabout looks, as opposed a typical C:S roundabout which is actually a turning circle. Here's an explanation of the difference.

If you've got a unique roundabout design, add it to the comments.

[ItsCody]

The whole aspect his brings up about not zoning roundabouts... I think it's more within the confines of the game rather than the realistic aspect of it. That being said just because it's common in more than a few parts of the world doesn't make it right... lol. I say this knowing full well the round about is a rather old concept that's been around for a long time... and I've just started seeing them pop up in my small, mid-western town. Bless us we're trying... lol.

But i digress, this article is well put together and inherently calmed my nerves of my impending congestion in my city. ty <3

[♔Richard♔]

Would you know why my cars would go in but wont go out of my city since i try to add this Roundabout in it http://imgur.com/a/vk8cj?

[No]

in some cases i find it helps to have "shortcuts" that allow people to skip the roundabout's main bit, maybe a path just outside, where the road splits into the 1 lane ramps, this is useful if a lot of people seem to be going from area A to area B instead of evenly splitting

[Professor]

Really? Theres businesses where the entrance or exit to the property is on the roundabout? I've never heard of that. I should have guessed someone would have done it though. Just like the real life underground roundabout I linked in the guide. Anyway changed the wording a bit. Also it's pretty weird this is all of a sudden a top guide. When I posted it on reddit months ago it got like 3 responses. lol.

[Dylan]

"Never, ever, zone the actual roundabout Inside or out. It causes heaps of congestion and makes no sense. For those of you that don't have roundabouts where you live, this would never happend in real life."

Not saying your wrong about the game but around me some round-abounts have bussnisses on them. (Although each one has an auto body place because of how poorly they are designed and the ammount of accidents on them)

Also your guide is curently #2 on the main screen guides list. That may explain the suden rush of commenters. lol

[Professor]

Did this guide suddenly get posted somewhere popular? Its getting lots of hits all of a sudden. Well since people are interested in it now I did some editing and added some new information as well as some links to great roundabout examples.

[MangoMango]

I found adding an entire underground road network makes managing traffic incredibly straight forward; underground roadabouts can be particularly suited to replacing some quite cumbersome highway interchanges

[jPRO]

Nice article, good job :-)

[Octopian]

Good discussion! Especially adding exit lanes from the roundabout should help "flush" it out.

[A Fat, Angry Serval]

I actually have some advice about the usage of roundabouts vs other interchanges - a roundabout is not always the best solution for the job at hand. Roundabouts are best used as central hubs for collector roads and when joining multiple collectors into an expressway. If you have two expressways crossing eachother, however, you'll be better off with a flyover/cyclone/cloverleaf/hybrid interchange.