In Phoenix, we don't have the MPH written on the road in our highways, but we do have the highway that this is an exit-only lane for written on the road. For example, you're travelling on the 202W, and there's an exit for the 101N coming up. Before the exit, there's an on-ramp from another street and after that on-ramp (and before the exit) there's an off-ramp for a different street.
Of course, there are signs above with yellow sections and black arrows pointing down at which lanes are for what--which is an exit-only for the new off-ramp, which are exit-only for the 101N, and which will allow you to continue on the 202W. I think it's safe to say there's a lot going on here.
In addition to this signage, there's also writing in the lane that says:
as you drive along, so it's clear that you're in the proper (or improper!) lane. This is much more clear, and I think that of all the use cases for writing on the highway, this is probably one of the most useful.
I've tried to find a picture of it, but apparently all of us here in Phoenix take it for granted! :)
Next time I'm driving on the highway as a passenger I'll try to snap a picture of it for you to see.
Anyway, it doesn't have to do with speed limits, but it does have to do with a different kind of signage so I thought it might be interesting to chip in.