What to do in the first five minutes
Turn the AC off and the heater on full — the heater core is a small radiator and will pull some heat out of the engine. Pull over somewhere safe and switch off. Do not open the radiator cap or the expansion tank while hot. Wait at least thirty minutes before looking. If there is steam or a sweet smell, you have lost coolant.
The usual causes
A split or perished hose. A leaking radiator, often at the plastic end tanks. A failed water pump, sometimes with bearing noise beforehand. A thermostat stuck shut. A failed cooling fan, which produces the distinctive pattern of overheating in traffic but running normally at speed. A blown head gasket, which is both a cause and a consequence.
Overheats in traffic, fine on the highway
That pattern points strongly at airflow rather than coolant flow — usually the cooling fan. At speed, air is forced through the radiator regardless; in traffic, only the fan moves it. It is a genuinely useful thing to notice and mention.
Why we check for head gasket damage
If the engine has already been run hot, we test for combustion gases in the coolant before assuming a simple repair will hold. Replacing a hose on an engine that has already cooked its gasket would just hand you a bigger bill a month later.