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I am having no luck with two-way roads. They seem to clog really easy, even with moderate traffic.

This shows a ring of 6-lane two-ways around an industrial zone, the traffic is already queueing up:

traffic

When I convert all the 6-lane roads into one-way roads, traffic seems to flow much better. However, I'm reluctant to do this for three reasons:

  • If two-way roads are in the game, they must have some purpose, right?
  • I can't ask my citizens if they are ok with the longer ways.
  • Most of the cities I have lived in have a ring road around the city, but none of them was ever one-way. (Some of them have grass in the middle and so not on every intersection you can turn in a way that would cross the other direction.)

How/in what situations would I use big two-way roads?

  • When your areas grow larger, try to have multiple entry/exit points to the main roads. This will off-load internal traffic somewhat, and will reduce the load on on/off ramps a lot. Try to optimize routes for specific traffic as well (e.g. disallow heavy traffic in residential districts, forcing trucks to take the highway around directly going to the commercial district). This separates traffic a lot and helps prevent congestion. Further improvements can be made with an efficient public transit system. Even footpaths help! Distance is not an issue, but they will take the shortest route. – Ivo Coumans Jul 30 '15 at 13:26
  • And a sidenote on ring roads in real-life: they probably are one-way roads. These types of roads are usually separated in the middle by grass or a fence, which disallows making a u-turn in the middle of the road (which two-way roads in the game DO allow). An efficient method to making a ring road is to run two parallel one-way roads, and placing a roundabout or intersection at places where they need to be. This also allows you to make an intersection on only one side of the road, without disturbing the opposite direction. – Ivo Coumans Jul 30 '15 at 13:35
  • @IvoCoumans lol Yes, they have grass in the middle and disallow u-turns or have "one-sided intersections. That was what I was about to write in that half-finished sentence there. ;) – AndreKR Jul 30 '15 at 14:19
  • One thing that helps is to minimize the number of intersections. I notice in the screenshot that you have a lot of two lane streets intersecting with larger roads. It's better to have more two-lane/two-lane and only a small number of exits onto the larger main road. – SMeznaric Dec 21 '15 at 1:51
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The general plan with the use of larger vs. smaller roads, mimicking real life road planning, is to create a "hierarchy" of roads. See this guide for general ideas on planning road networks. With this in mind, the larger four and six lane roads serve as the "trunks" to two lane road "branches". These larger roads handle a higher volume of traffic at higher speeds, allowing users to reach the point of their "branch" which will take them to their ultimate destination (though these "trunk" roads can also have things built on them, like commercial buildings, which like the customer flow of the larger roads).

As far as two way vs. one way is the concerned, there are two things to consider:

1) Bottlenecks. Trying to force the traffic to take certain, pre-planned one way routes can backfire big time if you're not providing enough different ways for the AI to get to its destination. This is a big cause of the infamous "every car is pilled into one lane" problem people complain about. They're doing that because they're all trying to turn the same way at the next light.

2) Services. A complex maze of one ways is difficult to navigate, and can cause major delays in service response time. Remember that service vehicles in this game have no "right of way", they have to follow all the same traffic rules as everyone else. Because of this, a building might burn down even though there's a fire station directly next door, but next door the wrong way on a one way, causing the fire engine to have to drive all the way around the block.

For your screenshot, two suggestions:

1) As far as I can see, there's only that one entrance and exit from your industrial area. This is a bottleneck, and can be alleviated by giving the traffic more options, either through adding more road connections, or other transport options, like freight trains.

2) The intersection of the the highway and the six-lane is very close to another intersection of the six-lane with another six-lane. The "stop, start, stop, start" from those intersections can jam up traffic. Try giving your traffic coming off the highway some "run-up" time before it encounters a traffic light.

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  • I have read that thing about road hierarchy before and I tried to follow it but I think my attempt at carrying over what I read about IRL road hierarchy to the game looks quite ridiculous: i.imgur.com/OWobAMh.jpg – AndreKR Jul 30 '15 at 15:06
  • The only thing I see that I would change there is, again, too many intersections too close to each other. There's nowhere for the vehicles to actually sit and wait for a light to change without clogging up another intersection. – JMR Jul 30 '15 at 15:28
  • But somewhere I have to connect those small roads, don't I? – AndreKR Jul 30 '15 at 15:37
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    Yes, but you don't have to connect every small road to the larger road. The small roads can branch off each other without much hassle because they shouldn't be handling lots of traffic. So what I'd do is eliminate three out of those five intersections of two-lanes to the four-lane, then have those two lane roads branch off the other two lane roads. – JMR Jul 30 '15 at 15:52
  • +1 because your answer gave valuble advice. However, I am reluctant to mark it accepted, because it doesn't really answer the question "what is the use case for 4-lane and 6-lane roads?" – AndreKR Aug 5 '15 at 3:21

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