cities-skylines
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.
The simplest, easiest and most compact way to connect the starting highway to a smaller main road is to just use a pair of one-way roads - one set from the incoming highway to the end of the main road, and one from the end of the main road to the outgoing highway.
This isn't necessarily the best junction, but it avoids traffic lights and allows uninterrupted flow of traffic on to and off of the highway - all for very little effort. I've found it to be sufficient for quite a long while after starting a city, I get much more traffic build up elsewhere first.
At the start of the game you cannot build additional highways, so for now you can use one-way 2-lane roads (see left image). This isn't great though - as well as looking a bit odd, the 2-lane road also has a lower speed limit than both the highway and six-line that it connects. It still works pretty well, but you should upgrade these links to use highway once you unlock them (see right image).
Highways at the top. 6-lane at the bottom.
2-lane version is left image. Highway version is right image.
Also, as a bonus, you can see an extra outbound highway link coming from the left for trucks leaving my industrial area.
Note how that merges with the other highway section in a way that doesn't interrupt traffic.
As said, this still isn't perfect. If you note the inbound road has a 'turn right' arrow in its right hand lane - which allows traffic to U-turn back onto the highway. In the highway version, this actually means only the left two of the three lanes will be used to merge.
You could try a frontage road running parallel to your large one way
This allows for numerous entry and exit points to the highway without making a zillion intersections on your main avenue. The frontage road also "filters" the traffic to closer to where they actually want to go instead of having them all go in and out at the same place.
Here's a screenshot of one I made in game:
It's a bit goofy because I didn't have room to run on both sides of the highway but flows very well.
Best Answer
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:
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)
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.