I used this for the first time today - some of the streets underneath seem broken and show the unjoined 1-way error.

No, the lanes are kept seperate to reduce conflict points. The weaving is kept to just traffic turning left onto the highway. I don't really like roundabouts.

If you're wondering about u-turns, traffic can perform a u-turn to reverse directions on the highway. To add a u-turn to the surface streets you can wait to bend the surface lanes as they come out from under the interchange, and connect it to the ramp for turning right onto the highway.

As to putting the lanes on top of the highway, I could do one as access for a tunneled highway if you guys want.

@Twilight: It's probably best if the highway is elevated, since it's impossible to zone on strips of highway.

@Employee 432: Is there a roundabout underneath that highway by any chance?

@Twilight

Thanks, I'm not sure. I think you would need the no pillars mod which I haven't used. Traffic turning off the highway runs under the highway. The pillar where it intersects with the surface road would be right in the middle of the highway. Maybe, I'll give it a try. For similar capacity, with the surface streets running over the traffic, I would suggest a DCMI (double crossover and merge) interchange.