So as to the question of to whether it's legal to have a turtle as a pet, that really depends on what state you live in and what part of the world you live in. And the first think you'll want to do before you consider taking a turtle home and keeping him as a pet would be to actually research that species and find out if it's endangered or threatened. Many states also have laws protecting native turtles and tortoises and it would be illegal for you to keep one one of those as a pet. So you need to check out the laws where you live.
The other thing that is currently illegal in the U.S. is, it's illegal to sell turtles that don't have a shell that's larger than four inches. This is due to an F.D.A. regulation that is tying to control salmonella outbreaks in children. What they found out was that children that have the small turtles are much more likely to handle them and contract salmonella from them. Salmonella is a bacterial that's normal in the digestive track of reptiles, but it can cause severe illness in children, and older people, and immunocompromised people. If you have children at home or if you're an elderly person, do not buy a smaller turtle. You won't get in trouble. It's the seller who is actually breaking the law, but just be aware of that risk.
The other thing to be aware of when you're considering a turtle as a pet is to never take one from the wild and bring it into your home. It's illegal. It's also cruel to the animal. Most of them don't adjust very well to captivity. They often don't eat very well. They're not used to the captive diet. They're not used to people. And they're not used to being in enclosed spaces. So, again, if you see a turtle or a tortoise in the wild it's not recommended that you keep it as a pet. In fact it's illegal.
If you see a turtle that's trying to cross the road and there's traffic and you're worried about it being injured you can move that turtle. Help it across the road. Pay attention to which direction it's going. You might be able to help it get to safety without getting hit by a car. But, again, don't actually relocate that turtle. Many of them have home territories an if you move them out of that territory they're used to they may not be able to find food or the right environment to live in.
If you find a turtle that's injured, the best thing to do is note where you found it and to contact a wildlife rehabilitator or a veterinarian nearby.