When one highway isn't enough: collector/distributor system design notes

[Aenarion]

Sometimes you'll find that one highway is insufficient for traffic volumes. Often the most effective solution is to build one or more additional carriageways, creating a collector/distributor (C/D) system.

It's actually very hard to get a C/D system working properly, so I thought it would be helpful to share a few thoughts. Maybe others have good ideas too.

1. In C:S as in real life, congestion is usually generated at the MERGE between two highways (or other busy roads). Plonking down an extra carriageway does not by itself increase merge capacity; at best it just moves the congestion slightly further down the road where the parallel carriageway merges back in. If you want to kill the congestion rather than just move it around you need to take traffic off the carriageways before they merge. This means creating exits on the new carriageway.

2. There is a specific issue with C:S at the merge. Most times you will want to start the new parallel carriageway with a merge between two ramps. Both ramps will often by default merge into one lane of the highway, which creates congestion. To get around this, you should make sure that the ramps at the start of the parallel carriageway come in from opposite sides. When the ramps are coming in at the correct angle for an orderly merge, one will have a left turn arrow and one will have a right turn arrow. If either ramp has a straight-on arrow it is likely that it will actually merge to the left or right and create congestion. Weird, but that's how it works.

3. You generally want to put similar amounts of traffic on the two carriagways so you using all the available capacity. Entries and exits should be allocated with this goal in mind.

4. In real life, the outer carriageway is usually for local traffic and has lots of entries and exits and the inner carriageway only has entries and exits at major interchanges. If you are building a C/D system from scratch in C:S, that is probably the best way to do it. But if you are upgrading an existing road, there is no reason to follow this rule. In my C/D systems (see below), there are a similar number of entries and exits on the inner and outercarriageways.

5. In real life, C/D systems usually have two carriageways in each direction. There is often no reason to do this in C:S as the game generally generates more congestion in one direction than the other. Hence my examples below all have two carriageways in one direction and only one in the other.

Here are three examples from my city:

Example A - two highways get funnelled into this section before the traffic splits at a major four way interchange. The less busy highway is put onto the outercarriageway; as I want to balance flows between the carriageways I also made the outercarriageway the only way to access the left turn at the four way interchange.

http://steamcommunity.com/sharedfiles/filedetails/?id=437374845
http://steamcommunity.com/sharedfiles/filedetails/?id=437375195

Example B - this is the biggest C/D section in my city. The new carriageway is mostly in the water, as this is the only place where there was room. This means it is on the 'wrong' side of the existing highway, i.e. it is going in the opposite direction to the nearest carriageway.
http://steamcommunity.com/sharedfiles/filedetails/?id=437375015
http://steamcommunity.com/sharedfiles/filedetails/?id=437375296

Example C - this section between a four way highway interchange and an industrial area was highly congested. The ramps from two of the highways form the outercarriageway, which has several exits in the industrial area.
http://steamcommunity.com/sharedfiles/filedetails/?id=437375118
http://steamcommunity.com/sharedfiles/filedetails/?id=437375377
Overall map of my highway network (example A is to the NW, example B is to the SW, example C is to the NE): http://steamcommunity.com/sharedfiles/filedetails/?id=437374762

There is some light congestion on the C/D sections but vastly improved from the long lines of stationary traffic that was there before I put in the extra carriageways :)

[[at] ajyto]

Put here: http://steamcommunity.com/app/255710/guides/

[leftbehind]

+1 for putting this in guides tab

[Aenarion]

Originally posted by aubergine18:

+1 for putting this in guides tab

Okay I made a guide here, based on my post but with some extra material. It's my first guide so not sure if I did it right :P http://steamcommunity.com/sharedfiles/filedetails/?id=437477626

[leftbehind]

looks good :) by having it as a guide I can mark it as favorite to easy find it again

[Aenarion]

Originally posted by aubergine18:

looks good :) by having it as a guide I can mark it as favorite to easy find it again

Cool, I made another guide here:
http://steamcommunity.com/sharedfiles/filedetails/?id=437734047