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So I noticed that I can basically connect any type of road to a highway, and this got me thinking. What advantages does using a highway ramp actually bring?

Is it just that traffic can leave the highway at higher speed? If so why not just use a highway to connect right to your road? Does the one-way nature of a highway ramp make it more efficient?

Plus it seems that highway ramps are single lane, which would make them less efficient for higher density traffic.

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If you use any multi-lane one-way road as an exit to a highway, you effectively reduces the highway to two lanes at that point. This is because the lane on that side of the highway typically becomes turn-only, while the middle lane will become turn/straight. Unsurprisingly, this can be bad for traffic flow on the highway. Note that this also applies for splitting a highway into two highways.

You will find similar problems when merging a multi-lane one-way road into a highway (again, this includes highways merging into highways), with traffic trying to merge across multiple lanes and potentially interrupting the flow of the highway.

Lane switching (to avoid turn-only lanes or merging traffic) has a negative effect on traffic flow on any highway with a moderate throughput of traffic. This is why junctions like this exist.

Other considerations:

  • Faster than all one-way roads, with a limit of 80 (down from 100 for highways).
    Note that this is twice the speed of two-lane roads, and thus should have about the same overall traffic capacity (at least while traffic is moving at full speed)
  • Cheap to build (30/cell; cheapest of all roads except gravel)
  • Cheap to maintain (0.32/cell; same as regular two-lane)
  • Cannot be zoned
  • No pavement for pedestrians to walk along
  • Traffic will never park on them

With all that said, personally I tend to use ramps as the entry/exit road from highways everywhere, but will often have exit ramps change into one-way two lane roads before they reach a junction, this can help the traffic to spread out as it waits at the junction and hopefully not back up down the ramp as far.

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Well, the main advantage of highway ramp is its price, lower than a regular one-way road. But it term of function, it is the same as the regular one. But on the other hand, I don't think highway ramps are suitable for in-city use.

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