No offense, but I believe massive road junctions are useless. The main reason is that, unlike trains, road vehicles don't reserve sections of the road. This means that:
- Road vehicles going in different directions just pass through each other without blocking, which means no need for overpasses.
- Road vehicles going in the same direction aren't blocked by moving traffic. They can queue up only if their path is blocked by something else* (such as full station or train crossing), and junctions don't address those situations.
- Road vehicles already use the second lane for overtaking on normal roads, and the third lane is for decoration only.
- If you have road jams not near a full load stop, this means that you have too many road vehicles and you should convert the route to rail or boat.**
- If conversion is not feasible (say, for a bus route in the middle of a city), then building a road junction is not feasible either.
If your goal is recreating real life junctions, then, of course, my arguments don't apply. But in that case a junctionary would be of no use.
Now, there's a related thing that is not useless, and that is design of road stations
. The only situation when road jams are hard to avoid is trucks queueing up in front of a full load station, because the very first loading slot is occupied. Actually, so far I haven't found an ideal solution for that.* In fact, even in a queue they sometimes magically pass through each other. it is possible to cramp an arbitrary number of trucks onto the exact same spot, then smash them all at once with a locomotive, mwahaha** The capacity of boat routes is limited only by the number of boats you can build.