In this era, mourners do not need to walk to the cemetery anymore. The mourners, instead, form a vehicle procession with the hearse ahead of the fleet of cars that follow.
Different states in the US have different laws that govern these processions. In some major cities, a police escort is always in front to clear traffic. The escort also acts as an indication that a procession is ongoing.
Again, cars have to turn on their hazards and headlights to show that they are part of the procession.
For veterans, organizations such as Patriot Guard Riders use motorcycle escorts to help in alerting other drivers.