in hot weather of course.... that's what my cab driver told me a couple days ago... that because of higher gas prices they wouldnt use it at all unless someone was in the car and requested it
Sometimes cab drivers prefer to just use the A/C so everyone is cool, which can lead to a better tip since the passengers are comfortable. Keeping the windows down in the front might keep the driver cool, but the wind will piss off backseat passengers. Besides, taxi companies can and will adjust their prices for gas prices and consumption.