There are two mistakes in this sentence.
The first mistake is the use of the word "less" when used with the noun "cars." Instead, the correct word should be "fewer." In general, use the word "fewer" when the number of objects can be counted; use the word "less" when the objects cannot be counted. For example: fewer cars on the road (cars can be counted), but less traffic (traffic is a general noun and cannot be counted).
Also, note that the verb "means" does not agree with the subject "cars." Therefore, the correct sentence should read: Fewer cars on the road mean less traffic.