A warning sign shows a low bridge on the planned route. What should a bus driver do before proceeding?

1. Proceed when the signed clearance equals the stated bus height, because signs include a safety margin
2. Compare the signed clearance with the bus's actual height and divert if adequate clearance is uncertain
3. Lower kneeling or air suspension before the bridge and treat that temporary height as the vehicle height
4. Continue on the scheduled route unless a circular prohibition sign is displayed beside the warning