marked by a long line of cars moving slowly or with many stops and starts, one behind the other: bumper-to-bumper traffic.
Informal. following one another in profusion: bumper-to-bumper worries.

Origin of bumper-to-bumper

First recorded in 1935–40
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019