Generally the roads are 4 lanes each side, with a partition of trees, and another lane for parking and bicycles. All the roads are very wide like this, and big sidewalks. It almost seemed luxurious after being in Busan, where sidewalks are whats left over after everything else is considered (i.e. inefficient, many obstacles). Even with the extra lanes, there are still many traffic jams. One foreigner magazine I read while there made mention of this problem, and the push to make more subway lines to alleviate the congestion. Mind you, all these projects are expected to be done in time for the Olympics next year.
People in Beijing drive like they walk. I though traffic was bad in Busan, but in Beijing they don't really stop; everyone drives really slow (Busan they drive fast as possible and shove their way through, but everyone drives the same so its okay). No one waits for the crosswalk signal, bikes are mixed in with cars (even though there is a designated lane for bikes) and you cross when you can, always making sure the driver coming at you sees you. Its much more chaotic then anything in the States, but again, they all drive the same so its usually okay. There's just so many people that's its hard to enforce any of these rules.
Another note on the cars: many of the cars look new, but they are actually factory models from 10 years back, with a few fascia modifications. There were many Volkswagen for taxis, and overall a good mixture of cars from all over the world (contrast with Korea, where 80% of the cars are Hyundai or Kia or Samsung). Overall there was a certain bleakness about this big city; many of the buildings were actually painted grey (with red accents). However, don't be persuaded by my opinion, I still enjoyed visiting there. Food is cheap and tasty, clothes are cheap (once you learn how to bargain the prices down). But its definitely a place to visit at least once, also due to its proximity to The Great Wall...