A 2006 study using 2004 data found that, in New York City alone, there were 25 taxi/livery crashes every day that caused injury. That study also found that when taxis were involved in a crash, the taxi passengers were nearly 2.5 times more likely to be seriously injured or killed than the passengers in the other vehicle in the crash.
So, best practice in taxis (and, in some states, the law) is to use a CR. Dr. Alisa Baer, pediatrician and cofounder of The Car Seat Lady, posts excellent advice on this subject at www.thecarseatlady.com.