Is the cruise control functionality controlled with Accelerate (W) and Brake (S) to adjust the speed? Or is it a fixed, universal speed for all vehicles?

If it's adjustable on-the-fly, have you considered adding cruise control to trains? I know trains are meant to be automated, but I sometimes manually control a train. (Usually to test a new track section/design or to look over an area.) This typically means I need/want to go a lower speed than just holding down [W]. (Sometimes pretty slow, like to test lights at junctions.)

Cruise control's speed limit is set by pressing "L" (default key). Assuming you're in your vehicle, driving at a speed of 100 km/h, then pressing "L" will activate cruise control with a target speed of 100 km/h. Pressing "W" or "S" is no longer needed from this point on (except if you arrived at your destination or want to avoid a crash).

The value is stored for each player: If you stop your vehicle and exit while "cruise control" is still active (by not pressing "L" to disable it), then the value is still present. If you get into your car again and press "W" for a short time, then cruise control will automaticaly accelerate your vehicle to its already present value (in this case 100 km/h).

Unfortunately, PDA is not applicable to trains, as their speed is handled differently from that of a car (as far as i know). But this might be a good idea for a new mod though.

I find cruise control sets to whatever speed I'm already going. It's very easy to use once I get the hang of it, but I have to remember to turn it off when I stop the vehicle.

@StahnAileron: For better usability the latest version now includes a option to insert a specific cruise control speed limit into a small textfield. Hit CTRL + L to alter your speed limit this way.