ajpr,
given that you already need a driving license, and can drive a car ... they assume that you'd be okay driving cars and small vans air-side. If you're going for a job as a baggage handler then they will get you to drive a small electric baggage tug (about the size of a Smart car) ... flat out, with a tail-wind, you might be able to reach 20mph!
You may also get to drive a conveyor belt, which is about 20 feet long with overhangs both front and rear, so you have to be wary when turning corners (as I found to my cost and embarassment!).
They're looking to see if you can drive safely and within BAA guidlines in a variation of vehicles, so knowing the speed limits, complying with them, and know where and when you can cross taxi-ways is all important.