I have a similar Uniden unit, but I've never used it while driving for traffic prediction. Mostly I use it when there is a forest fire in the county, to keep me informed about the progress of the firefighters.

If I hear a lot of sirens in the 'hood, I'll use it to see what is going on. However, once you hear the sirens, the dispatch call has already gone out, and unless there is a lot of chatter, there may be little information over radio.

Pretty handy little gadget.

While in the office, I'll use one of the internet radio scanners, like http://www.radioreference.com/. This site is also good for getting frequencies of the things you are most interested in.