My car overheats

Either water flow is restricted/incomplete or air flow across the radiator is restricted/incomplete. Assuming the car was not being asked to do more than it was designed for, these are the only 2 possibilities.

Impeded water flow can be caused by a sticking thermostat, clogged or partially clogged radiator flues, air mixed with the coolant, or combustion entering the cooling system due to a leaking head gasket.
Impeded air flow can be caused by obstructions in front of the radiator in the form of bugs, leaves, trash blown in from the road, or maybe a nice Christmas decoration hung from the hood over the grille area. Also, a radiator fan may not be working, though if you were at highway speed while the car was overheating, this should have provided the air movement you need even without the fan's help. An extra "help" when a car is overheating is to try this: roll down the windows and turn the heater on, with the blower on full speed and temperature at full hot. This will act as an extra auxilliary radiator to help dissipate excess engne heat, though may not do much for the comfort within the car. If the car is front-wheel-drive, you may try selecting AC on(instead of heater), full hot, full blower, as this will often activate a second fan at the radiator, and overide the temperature switch for the primary fan. If the car is rear wheel drive without electrical fan(s) at the radiator, using the AC would only be a hindrance, so use the heater method. Note that this is not a "fix"... it is simply a method to "help" get you to a safe place, and will not always work.

In the case of a leak, be aware that with a malfunctioning cooling system your plastic coolant tank will often show to be full, when the radiator and engine passages are empty.