Granting the speed status effect is unnecessary and conflicts with the actual speed status effect. Like soul speed, the attribute modifier should be affected directly.

A small suggestion while at Asphalt:
Rainbow Asphalt. That's... really it.

You know... rainbow road and that jazz.

Like I assume more rainbow blocks are coming at some point in future, but I just wanted to mention it while at the topic of Asphalt.

Perhaps having some sort of dyed dusts which you can spread/put on any blocks just like redstone as a way to mark the streets with specific colors and marks?

Howdy, just happened to see this. An easy way is to add a .slipperiness(0.50F) to the blocks properties. Will add a nice little pep in the step.

Soul speed is handled differently. Asphalt need to apply an attribute modifier to entities without going through an intermediate object (like a piece of armor). This means that it's the block that manages the speed. And to be honest, I have no idea how to do that. But indeed, I know that it would be much better and clean