How to fix highway traffic slowdown on line change in Cities Skyline?

If that's your main channel from the Industrial area leading to the Commercial, it's wise to either make a separate highway for the industry (either underground or overground), or upgrade that highway off-ramp to a three lane highway. (Sometimes it helps, sometimes it just pushes the congestion farther down the highway).

Yes, in fact there are many ways in which 4-way highway intersections can be improved or upgraded.

You can take a look at what is done in real life. The accuracy of the traffic model in Skylines is such that often solutions that work well in the real world, work well there too. Most commonly, in order to improve the throughput of a cloverleaf these are done:

1) Conversion to a partial cloverleaf / partial stack interchange.

When a left turn is congested, this can help. In an standard cloverleaf, traffic is first added, then removed. If you convert two of the left turns by removing them and instead having them pass over the structure (making it a 3-level interchange), you can improve the flow. Unfortunately, this method won't help in your particular case.

2) Add slip roads.

Many cloverleafs in the real world use this method, for it does not require modifying the elevated (or tunnelled) parts, which are the most expensive bits to build or change in an intersection. (In C:SL you will have to modify these parts though, as you can't change the lane configuration on the bridge) What you do is the following (assuming a 3-lane x 3-lane highway cloverleaf):

• Split up both highways into 2x2 + 2x2, so your lane config is LL|LL|RR|RR.
• Expand the outer 2-lane highway to 3-lane.
• Replace the off-ramps with 3-lane to 2-lane splits, then 2-lane to 1-lane splits for the left turns.
• Put a higher speed limit on the center roadway to make sure through traffic uses that instead of the slip roads.

3) Modify the main branch

If your cloverleaf has more right-bound traffic than straight traffic, you could replace the intersection with one where both highways turn right. In this case, the easiest way to do it is to move the highways so that they both make a parallel S-bend, causing them to run straight, then put a parallel highway interchange to connect them.

4) Weave intersection

This is a variation on the intersection from the previous paragraph.

Alternatively, you can 'weave' the highways for a smaller footprint. Raise or lower the right part of the left highway, interchange it with the left part of the right highway. Then add splits of 3+3 -> 6 lanes and back to both.

Note that this type of intersection does have weaving like a cloverleaf, but on different lanes. Make sure to never have a situation where traffic flow is higher for the cars that have to change roads than the cars that go straight (this will bottleneck this type of intersection).

Images in this post courtesy of Wikipedia. Also see this article.