I was just chatting with a friend about this, and she said that she thought that the law was that you have to have a child in a booster/car seat in a taxi if they are on the highway, but if the taxi is only on surface streets, you don't need the booster, but the child must be in the backseat. I don't have a drivers license so I always use public transit or taxis, but I haven't ever taken DS in a taxi because of the booster thing, so this was news to me. I did a little research, and it does sound like she might be right. Here's one website I found with a discussion on it:
http://www.akuaku.org/2008/10/childseats-are-not-required-in-taxis-in-california.html
I'm certainly not an expert on this, and like I said, I have never taken DS in a taxi because I thought we needed a car seat or booster (and frankly, I feel better if he's in a safety seat), but I am curious now to learn more about the law and see what the actual requirements are for taxis.
and edited to add that another friend just said that she tried to take a cab in San Francisco with her 5 year old and didn't have a booster, and even though it was just on surface streets, the driver said he wouldn't take her because the law says they needed the booster. So even if the law says it's only needed on the highway, odds are that taxi drivers would rather be safe than sorry.