A car is a form of alternative transport for people that appears in approximately 1901 and is faster than ordinary pedestrians. It represent one person and will have a starting location and destination in the same way that ordinary passengers do with the exception that they travel on roads as opposed to footpaths.
The speed of cars is an issue for your existing transport links as they allows passengers to travel to further destinations not previously reachable in the 20 minute travel window. Existing transport links will need to be upgraded to compete with cars. Cars also affect truck routes by generating traffic on roads. It may be necessary to change roads and set waypoints to cope with heavy traffic. Cars appear to only travel between one town and another. If a person can walk to a destination then a pedestrian will be generated, otherwise it will become a car.