<movietrailervoice>"In a world where all words are five letters... one man can make the difference between brilliance and darkness..."</movietrailervoice>
"Innna world where allll words arrre fiive lttrs... onnne mannn cannn maake theee dfrnc betwn brlnc annnd drkns..."
The best way would probably be to install GPS transmitters on all vehicles, and then write some code to plot them all on a map, but the price and privacy issues would probably prove this too difficult.
A second option would be to point video cameras at a stretch of road and then use some image recognition software to get the information you need.
Another option would be to get a group of small children, have them play the game where they pick a color or model of car and count how many they say, and then just tally their results.