Alas, the car still has rubber tires, and some of the rubber is worn away during braking, and some of it is actually turned into a gas. Most of what is worn away during braking becomes rubber dust, which is both made of and coated with petroleum products. Some of it is transferred to the road, and some of that will be liberated later by other vehicles passing over it.
Cars are lame. Really, really lame. I really enjoy driving, I adore the sound of a V8 with an open exhaust at full song- hell, I thrill to hear the little growl that my W126 300SD makes during a full-throttle change-up, and that's a diesel with 120HP. I really love cars. But they are a really, really crap way to move people around. We should be replacing them with a combination of PRT and rail, and relegating cars to motorsport — where they should be run on carbon-neutral fuels or electricity, and run on biodegradable, carbon-neutral tires. That sounds like a bunch of hippie bullshit, but every tire manufacturer is actually working on such things in expectation of future environmental protection regulations which will effectively outlaw the compounds they are using today, and carbon taxes which will make them unprofitable in any case. And of course, we already have some electric racing series. We have carbon-neutral or even carbon-negative replacements for both diesel and gasoline (green diesel or biodiesel and butanol, respectively) so all of the pieces are there. As usual, the only thing missing is the will.