I have a question about the effect of highway onramps (or one-way roads) and congestion, when it comes to two way roads.

The idea is, if I have a large two way road, and I want to create some highway onramps, from a congestion point of view - I don't want have a situation where traffic is crossing traffic to get onto the onramp.

That is, assuming it's right-hand traffic, to get onto the onramp, the traffic just needs to turn right onto the onramp, (and thus never obstructing oncoming traffic), and similarly to get off an onramp, they turn right and merge with the traffic flow.

So total, I'd have four onramps, two each for each traffic direction.

The question is - is the only way for this to happen to create a pair of one way roads? Or is there a way to do it smartly with a two way road?

  • I am not sure I understand the question correctly or if it's actually answerable since it all depends on your city's layout. If you have 2 2-lane one-way streets next to each other and you want both directions to cover both highway directions, then you need in total 4 onramps roads since onramps are one-way (if I am not mistaken). Could you elaborate a bit more?
    – Xander
    Mar 28, 2019 at 7:59
  • 3
    If I'm understanding correctly (and I'm not sure I am), then you may want to look into mods that can limit what turns drivers can make at any given intersection.
    – MBraedley
    Mar 28, 2019 at 14:30
  • Without mods, traffic can turn left onto the onramp, similarly traffic can turn left from the offramp. Mar 28, 2019 at 20:15
  • 1
    @AlexeyIvanov While true, you can "bait" the majority of cars into taking the offramp you want. If you branch early enough the speed of the offramp is high enough that most cars will take a ramp branching from a right turn and curving across the road to go "left" instead of crossing traffic to turn left at the opposing direction's offramp. It seems left turns have a slight penalty when determining route so as long as the right-turn offramp has a lower cost, most cars will prefer it. There's always that stupid straggler that won't, but depending on conditions might be enough.
    – Troyen
    Apr 5, 2019 at 7:38
  • 1
    It will depend a bit on the type of two-way road you're branching from (different types have different speeds which weigh into the travel cost calculation - doing this from a two-lane two-way highway is harder than e.g. a four-lane two-way road because the highway has the same speed as the offramp), how much space you have, and how messy of an intersection you're willing to tolerate. If you can post an example of your setup it may be easier to provide an answer.
    – Troyen
    Apr 5, 2019 at 7:41


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