Who is at fault here?
The answer is it depends. In theory the car in motion is at fault. Car B backed into an immobile object and thus the blame for damage lays with them. Hell, car A even went and attempted to warn off B from their current course. But life isnt usually that cut and dry. Was the accident recorded? Can you gain access to the recording? Will the driver of car B admit fault? Will the insurance of B pay up willingly? Were the police involved, and if so what did the report say? Were there witnesses? Will those witnesses care to testify, or even remember? Did someone get aggressive and fight, casting doubt on one sides story? It will appear clear BOTH cars had been in motion after the fact. Unless there is a recording car B may well say both vehicles were in motion. If the police report agrees than it will be a 50-50 split on blame. The real world things can get messy, and while it does appear a clean case for who pays it can quickly become a tangled web. So in short, who admits fault is at blame. Otherwise there is a fair chance all parties will be assigned some blame. Tossing out a small edit. As a commercial driver been the car A twice (thankfully never B). First time I got hit and sued, thankfully recordings existed that proved my case. The second time I used my airhorn and got everyone's full and undivided attention. Leading to Kyle Soli's answer to What is the purpose of the massive honking horns on semi-trucks?