We were waiting for a stoplight to turn green in Beijing on Tuesday when a champagne-colored Mazda-6 with Snoopy painted on the right rear door screeched to a halt.

We could tell the driver was impatient. Sure enough, the car sped through the next stoplight and disappeared into the distance.

We were not surprised to notice that the speeding Mazda was missing its rear license plate. It is common knowledge that vehicles without a rear license plate often break the law, because they cannot be identified by police cameras. Scofflaws who don't have time to remove their license plates sometimes cover them with a CD, a piece of paper, or a strip of cloth.

On our way back from the airport recently, we spotted a taxi some 20 meters ahead of us whose rear license plate was all blue. Our taxi driver explained that the driver must have covered the numbers with toothpaste so they would be invisible to cameras. He was right; when we got closer, we could see the obscured numbers.

Beijing drivers did not invent these tricks. English language sites on the Internet offer ways to "avoid red light camera tickets", or "make your car's plate number invisible to cameras".

Radar detectors have been around for decades; license plate cover-ups may be the logical extension.

Police have launched crackdowns on cars whose rear license plates have been removed or obscured, but they seem to be losing the battle. To be fair, the police are short-handed and have better things to do than to chase these petty lawbreakers.

However, I believe such disregard for the law contributes significantly to Beijing's traffic woes. There are now more than 4.5 million motor vehicles in the city, yet traffic is much better in Tokyo, where there are some 8 million cars.