I explained that even though there may be other trash on the road, we should never throw garbage anywhere but in a garbage can. We did not pass any trash cans outside so he held onto it until we arrived home where he promptly tossed it.
Sometimes we forget that kids have to be taught everything or they will simply follow what they observe. Kids think: There's trash in the road? That must be where it belongs. Take ten to teach about litter bugs!