Final answer:
A large x painted on the pavement is a warning sign indicating a railroad crossing, alerting both drivers and pedestrians to be cautious for trains.
Step-by-step explanation:
A large x painted on the pavement typically indicates the presence of a railroad crossing. When drivers see this marking, it serves as a warning to be cautious and alert for possible trains crossing the road. This marking is critical for both vehicle and pedestrian safety, particularly in areas where the railroad tracks are close to the roadway, and it may be one of multiple signs or signals used to alert traffic to the presence of a railway.