He was supposed to be in hiding, and it was just a matter of luck that he wasn't caught... the police didn't check the inside of the cars, and 'his' was covered in snow.
It wasn't one of his clever plans... Walt called the police from the bar because he was ready to give up after Junior refused to take any money from him and told him to just die already.
Seeing Gretchen and Elliott on TV gave him the idea to use them to get the money to Junior, so he changed his mind and ran. Being in the car under the snow when the police arrived was just luck.
Walt got lucky a lot of times throughout the show, and never really understood that luck was as important to his success as careful planning.