ok ...here's my 2 cents worth ...
yep, mopeds pollute a lot since they burn oil directly and that makes for bad fumes. but ... they are somewhat more efficient and pollute less overall. i don't remember where all the evidence is to cite, but i know i've read it here and there.
fred's right about cars carrying 8 people. cars aren't really designed to do that ... but they could. also, vans and buses carry well over eight people.
now ... for a single person (e.g. me) a moped is way more efficient. if i had to drive a mile to work, a moped is better for me than a car. now, of course, if i had to take a passanger, perhaps then it evens out (as fred suggests). and i can't really try to carry three people ... so there you need cars.
let's say i have to buy many things. then a car is more efficient, since otherwise i might have to make more than one trip on a moped, which consumes time and pollutants.
if you really want to preserve the environment, you would only walk. not even a bicycle ... since bicycles also are manufactured (plastic, fiberglass, metal, rubber) and that pollutes. walking only requires shoes (though you could do it naked and barefoot) and you can carry just as much on your back walking as you could on a moped (sometimes i walk w/ my backpack of groceries instead of riding my moped, mostly when the weather is bad or my moped broke down).
ok ... i guess that's all i have to say. i just ride my moped because i fell in love w/ her (and mopeds in general). for years i only walked or rode a bicycle ... then i thought a moped/scooter would be convenient and still environmentally conscious ... so i did that. now i'm hooked.