My suggestion of not sending data to the PC in real time came from thinking about this scenario:
event 1: car A passes through its sensor
event 2: car B passes through its sensor
event 1 and event 2 are < 1ms away.
If you notify PC about event 1 as soon as it occurs, you'll lose precious ms, thus missing event 2. Using interrupts you'll count event 2 anyway, but you'll report its time with some ms error.
All of this gets worse when you have 4 events all very close to each other.
Serial transmission is "slow", see here:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1284458471/0#0