Prostitutes in GTA III Era
Picking up hookers have pretty much been the same for Grand Theft Auto III and Vice City, Liberty City Stories and Vice City Stories; simply by pulling up to one. Honking your horn isn't really necessary in order to get a hooker in your car. You can only have sex with them on any ground that can kick up dirt or sand as the game doesn't consider seclusion as a factor. Having "sex" with a hooker is just an effect of your car bouncing up and down.
In Grand Theft Auto: San Andreas, however, if a hooker approaches your car, you have the option to accept or decline her offer to enter the car by simply using the "positive" or "negative" response buttons you normally use when speaking to pedestrians. This is useful, since many complained that they did not want hookers getting in their car when they simply pulled to the side of the road. Seclusion is now considered a factor. The effect is still the car bouncing up and down. If you complete the Pimping missions, prostitutes will pay you instead.
Prostitutes in GTA IV Era
In Grand Theft Auto IV, this is replaced with a "yes" option, or just don't press that button if you want "no". You can also have different types of sex in the game. It cost $20 for a "handjob", it costs $50 for a "blowjob", and it costs $70 for actual sex. You are unable to pick up prostitutes in emergency vehicles, the trashmaster, bikes and the bus, with Niko citing various reasons.