- The traffic is bumper-to-bumper. I don't think I will be home in time for dinner.
- The cars on the freeway are so close together. I won't get to school on time in this bumper-to-bumper traffic.
A "bumper" is the part of the car at the very front and the very back. When cars are very close to one another in a traffic jam, the front bumper of one car can almost touch the back bumper of the car ahead of it.
This idiom comes from Reading Savvy, 1st Edition. For more information, please visit www.languagesystems.com