A straightforward noun for a person who drives a car, bus, truck, etc. Neutral in register.
The driver must wear a seatbelt.
The bus driver is responsible for the passengers' safety.
歩行者 refers to a person traveling on foot, while 運転者 refers to the person operating a vehicle, a common contrast in traffic contexts.
Compound of 運転 (driving, operation) and 者 (person). A straightforward Sino-Japanese word.