Always have a Vehicle. Travelling on foot is not very convenient or safe, and should the Mayor require emergency swift evacuation from an Area to a safe location, Doing so becomes miles easier.
Ensure that you are driving Safely and following all traffic laws. The biggest threat to the Mayor's life should never be your awful driving.
Report the description of all vehicles which recklessly collide with your vehicle to Police rather than attempting to stop the vehicle yourselves. Detaining them yourself leaves the Mayor Stranded on the highway.
Ensure Secret Service agents are in a position to get out the vehicle at different angles and directions. This makes anyone forcing you out the car under gunpoint a less viable strategy of assassination.