Final answer:
At a railroad crossing, when there are flashing lights or ringing bells, a motorist must stop and wait for the train to pass.
Step-by-step explanation:
When there are flashing lights or ringing bells at a railroad crossing, a motorist must stop and wait for the train to pass. This is because the lights and bells indicate that a train is approaching, and it is unsafe to cross the tracks until the train has passed.
Ignoring the lights and bells or speeding up to cross the tracks quickly can be dangerous and result in a collision with the train.