With what level of precision are you trying to maintain constant speed? At .25 mph/s it takes 4 seconds to have a 1 mph change in speed. At highway speeds the c

With what level of precision are you trying to maintain constant speed? At .25 mph/s it takes 4 seconds to have a 1 mph change in speed. At highway speeds the change in your position from where you'd expect to be at perfectly constant velocity would be 0.8%, or about 3 feet off. That's nothing compared to the variability of your speedometer, which is only accurate to about +/- 10%. If you're a blue angel trying to fly in close formation at hundreds of miles per hour, yeah that's not good enough, but for a normal person driving on a road it is trivially simple to keep your speed within the margin of error of your speedometer.