I have lived here in CA(spelled a "C" not a k) for most of my 77 years and early on thru the 50's, there were a lot of cars with mechanica brakes. That is most of the reason for the 55mph speed limit. I don't think that revenue is why we keep it, I think it is so ingrained in some folks that they still feel speed kills.
Raising the National limit to70 did not raise the number of accidents or fatal injuries and they feel it is a big lie and the real numbers are hidden.
The truth is 60 is in general a better limit for 4 wheeled vehicles towing and maybe even 6 wheeled wheeled vehicles than 55 because the improvement in the mechanical and electrical has made vehicles much safer and because the impatient drivers don't do as many stupid and unsafe moves when they get behind folks obseving the speed limit.
I really don't believe it is a great revenue generator.
Just my never to be humble oplniol.