On the drag strip there's a 1/4 mile time and the car's speed at the end of the 1/4 mile. 1/4 mile times and 0-60s are generally the best measure of a car's actual speed.
Trap speed doesn't tell you much except that an unusually high speed for a certain 1/4 mile time means the car picked up during the 2nd half and had a slow start while an unusually low speed means the car didn't accelerate as much in the 2nd half and had a quick start.
A trap speed is a measure of the car's HP to weight ratio while in an instantaneous location. You can see lots of trap speeds being calculated at places like drag races or 1/4 Mile runways. :D