The most environmentally sound form of transport is probably the pushbike.
After that it is probably a modern Honda cub.
Most of the cars that are on the road are 4 seaters..yet rarely are all the seats filled.
Being single minded/honest/selfish even..in 20 years time I`ll be approaching age 70, so probably shouldn`t spend too much of my current time worrying if my car / bike is environmentally sound.
At the end of the day, being a tight arse/ late adopter/ make do and mend / reuse / repair/ don`t throw away type of person, I consider myself well "in credit" from and environmentalist point of view.
Too many projects not enough time....