Re: What's the environmental impact of a car?
'Zero' is the answer. A car (like the plants or any living organism) consists entirely of the Earths natural resources shaped into a transport machine by man and, as Batty has clearly stated, transfers stored energy (from oil) into movement. Man does not 'consume' energy he is merely transferring it. After approx 12yrs of productive life said automobile is buried in the Earth from where it came. "Dust to dust" as the saying goes. Just 'dust' doing something mighty productive and useful in between two states of laying about in the ground doing zero.
The better question would be to ask where your question came from. It is, like Green angst, sourced from a double negative. You cannot get a positive/useful answer from 2 negatives like asking how can I live longer while flogging myself!
Negative 1 is how can we consume less which is not a question life ever asks itself except in times of hardship (desert, famine, cold and snow, war etc). Negative 2 is the issue of consumption. Either this is 'emotional' such as the Greens guilt and pea brained confusion with mans incredible complexity and ability to extract and make useful Earths resources (and turn it into a progressive, comfortable lifestyle) or, as an engineer, probably driven by your intellectual aim for efficiency and weight saving (as Pete917 displays).
In your post you demonstrate a mix of thouhts but mainly the engineers obsession with weight saving which is admirable but if we applied it to our lives would result in minimalist cell blocks for our homes. So do not confuse weight saving for engineering purposes with luxury and comfort adding weight to our cars. It is not the same discipline. If a Yukka plant had the option, he'd have a big sofa, a flatscreen tv and ticked all the options on his Mercedes parked outside his Penthouse.
Can I unravel a few of your thoughts; you say "they can feel like mobile prisons in a traffic jam"...why blame the car for a congested road network that is jamming you in? ...you say "..and deny one the simple pleasure of walking from A to B, or shopping locally"... why blame the machine/car for you not using your feet or where you shop? ....you say ".to rely too much on.. and diminish the variety of life..." ..the car provides more variety to life, not less, than just having your feet.
All life is (more than) the sum of its natural resources and all life consumes (energy and resources) and indeed produces limited waste. All life is by its nature consumptive. Life is an opportunity and indeed opportunistic. But life does not consume or destroy its enviroment as an end-game. Life is the enviroment shaped into moving (intelligent) parts. And in consumption of its enviroment it merely 'transfers' the enviroments contents from one state of being to another state of being (plants consumes CO2 into Oxygen, man consumes Oxygen into CO2).
So the answer to your question is your question is an anomily. The Greens would ask a monkey to put down a stone used as a tool to crack a coconut as the monkey was "exploiting" the rock. Man is just a very clever monkey with very clever tools. And the truth is in 100yrs of mans most industrial expansion we have not changed a thing on the Earth. Not one atom of Earths resources has been lost except for a few thousand tons of material sent to space.