If not secured with either a seat belt harness, or in a cage, even if the dog survives an accident, they are most likely going to get loose. Now you have a panicked dog who, even if they avoid getting hit by a car as they run out into traffic, is gone, perhaps never to be seen again.
Please think about that the next time you get in the car with your unsecured dog.
We're RVers and I cringe everytime I hear or see about people who let their pets run around loose inside their motorhomes while they're driving down the road! One person on one of the RV forums I read tells a story about his wife sitting on the bed in the back of the motorhome, and when he had to brake hard to avoid an accident, she flew the length of the motorhome to the front. Fortunately, she didn't go through the windshield, nor was she seriously hurt, but just think about a pet flying around inside a vehicle!