1No.: The front of the car just got on the bridge (the car hasn't got on the bridge yet) and the rear of the car just got off the bridge (the car got off the bridge completely). At this time: S=L 1+L2.

No.2: The rear of the car just got on the bridge (the car completely got on the bridge), and the timing started, and the rear of the car just got off the bridge (the car completely got off the bridge). At this time: S=L 1.

No.3: The car just got on the bridge (the car didn't get on the bridge) and started to time, and the car just got off the bridge (the whole car is still on the bridge). At this time: S=L 1.

No.4: The rear just got on the bridge (the car completely got on the bridge) and the front just got off the bridge (the whole car is still on the bridge). At this time: S=L 1-L2.