A tailback is a long line of traffic stretching back along a road, which moves very slowly or not at all, for example because of road works or an accident.
[British]
The flooding led to six-mile tailbacks between west London and Heathrow Airport.
regional note: in AM, use backup
tailback in British English
(ˈteɪlˌbæk)
noun
a queue of traffic stretching back from an obstruction
tailback in American English
(ˈteɪlˌbæk)
US
noun
American Football
the running back farthest from the line of scrimmage, positioned directly behind the center