When there are a lot of traffic on the road and it's moving very slowly or not at all, the front of one car is almost touching the back of another one. So it's bumper to bumper.
Cụm từ này thể hiện tình trạng kẹt xe trong các giờ cao điểm.
1. Traffic is on the northbound A12 near Greenwich Junction is bumper to bumper. Best to avoid the area if you can.
2. The approach to the Dartford Tunnel southbound entrance is bumper to bumper. Expect severe dalays.
3. By eight o'clock the traffic was bumper to bumper.
You can learn the idiom by video: