It will be years before a tree is big enough to stop a car. My friend had this problem in my home town - people wrapped themselves around her barrier trees (gigantic Eucalyptus) regularly. The power company had to come out every year and top the trees. Finally they got tired of the maintenance and took out her trees but replaced them with a crash rated bollard barrier. She planted shrubs in between so you can't see the bollards but no cars in her livingroom either.