I've felt this way before too
It's soooo sad- it always makes me so upset if I can't do anything.
One day there was a small Lab-ish looking dog running alongside a busy road. I pulled over and tried to get close but it ran straight across the three lane road!
I couldn't even see where it went and I wasn't about to get into an accident over it but I was so upset all day, thinking he probably ran off and got hit by a car.
Something similar happened once but near my house which is a bit more rural but a big dog ran right out in front of my car and I slammed on my breaks! Poor big dog ran off into the woods - I was hoping he belonged to someone that lives in the houses through the woods and that he would find his way back - the roads weren't too busy.
Then a couple of weeks ago, I saw a dead German Shepherd on the side of the road!! God it was devastating. I did pull over to check and make sure he was dead, called AC, and they said they hadn't heard of a report about it so were thankful I called.
Gosh it's just SOOO heartbreaking!! It always makes me so distraught and sad for the rest of the day and it's even harder when people you tell don't give much thought to it and don't seem to care.