Upon further thought...
have you seen any aftermarket companies that employ sonar range detection? Most vehicle manufacturers that I've kept up to date with use sonar for parking assistance systems, and I think I read that a few are modifying it for use in the adaptive cruise control.
It seems this would be the least obtrusive method, and probably more effective for safety concerns: as I read over my first post, I realized that a focused laser is probably the least appropriate means for a safe range finder. Directional is bad in this case because the cruise control wouldn't detect a vehicle in an adjacent lane or one leading you through a curve in the road. Something that emits a dispersed 'flashlight' beam like radar or sonar is more effective in this application. But I think sonar bests radar in this case, as per the reason I discussed in my first post.