As it is the number of cars that is increasing, rather than the cars' size, we would generally refer to the number or volume of cars, or simply to increasing traffic.

To be more specific, you could say, "the number of cars on the road are increasing". Another alternative might be something like this - "There is an increase in the number of cars on the road from 8 to 10 a.m." "There is an increase in the traffic on the road during the summer". "The amount of traffic has increased". Hope this helps

Not really. It's a little ambiguous because it could be read as meaning the individual cars themselves are increasing in size. Better to say "The number of cars on the road is increasing". (Note the use of "is" because it's in agreement with the word "number" not "cars").

The cars are getting increased on the road. sound better.