Highway(motorway) construction in map editor.
Just some general rules for noobs (like me) who didn't know already.
On/off ramps will only be "one way" if they start or terminate on the actual highway itself. Traffic direction on these ramps also depends on the direction in which they join/leave the highway. As has been stated above, if you construct off ramps that do not make contact with a highway, traffic will try and enter them from both directions and just get stuck in the middle.
Where large traffic jams on highways are concerned, I have found the best solution so far is to construct "slip roads" from the high congestion areas to other areas of the city that are densely populated/business/industry based areas. It doesnt really matter about their length, as long as at least one of the ends feeds from/to the highway. For example, I have one feeding off of the highway that runs through the centre of my city, crosses a river alongside the actual highway, and then spurs off into the centre of the industrial area on the other side of the river. I have a mirror slip road taking vehicles from this area back into the centre of town via the highway where the first slip road exits on the other side. You can even "stack" these slip roads on top of eachother if you plan and construct very carefully. These slip roads look really good running along side the highway and can even be built above the central reservation of the highway itself. This method has really helped alleviate traffic flow on highways in my city.
Junctions between highways - this is where highway construction gets really interesting! So, you have two highways crossing eachother and you want to connect them with off/on ramps. The best advice I can give you is to approach construction from the pov of a vehicle on the highway. I.e. you are here and you want to get there. There is a brilliant sticky on here about highway junctions and construction in general but as a rule of thumb, you want ramps to take traffic off and on for each lane of your highway and for every possible direction. For example, an off ramp from the right hand lane of highway A to the right hand lane of highway B is great, but then vehicles will also want to get to the left hand lane of highway B from the right hand lane of highway A, especially if there are no other general city roads around for them to connect to from the right hand lane of highway B. I can't post images currently and don't want to confuse the issue further so will shut up now.
I can also recommend sinking your highways down a little below the level of ordinary street roads, as this makes it easier to build bridges over them but also to tunnel under important building or areas that the highway may come into contact with. Where two highways meet and cross over, it's also useful as it means you dont have to raise the second highway too high above the first. Similarly, slightly raising a highway can also be useful in that ordinary streets can then be routed underneath it via tunnels or actually building highway bridges over normal roads. Depends on the area and the look you want to achieve. Personally, I prefer the highway to be set apart from ordinary streets and clearly defined as "highway", which is another reason I do it like this.
Other than that, I have noticed that vehicles tend to use a highway rather than a normal road where possible. For example, I built a normal road tunnel under a river connecting a residential and industrial area on either side of the river. I also constructed a highway that crossed the river a few metres down from this tunnel. Although some used this tunnel, the majority crossed on the highway, as I think the AI looks for optimum journey speeds, rather than shortest distances - could be wrong but it's just an observation.
All common sense stuff but it's stuff I had to learn for myself and thought it might help someone. :wacko: