It depends. There are transits between guild cities... Except the Black City, since Moon Slicer never really bothered to establish any sort of arrangement with them. Most taxi services are player ran, and vehicles have to be obtained or made through the Deep Ground creation system and with a butt load of EXP. Or steal it from someone. And as for how common vehicles are, imagine it kinda like Mad Max. There are only so many you can scavenge in the world, and they're so easy to destroy sometimes... That and by no means is it still safe. Some outlaws have vehicles too, and there's a reason why Arms Slave's gang is called the War Machine.