Cars pollute more at the beginning of the trip for two reasons. Under the assumption this is a somewhat long trip the car is likely staying around the speed limit for most of the trip while on the highway and cars are more efficient and release less pollution when driving faster, the stop and go driving on city and town roads are when a car is least efficient. Also driving for a little while helps clear out the engine and the carburetor, so at the beginning of the trip it's possible for the build up in the car to get released quicker.