On this page of the guide to The Witcher Monster Slayer, you will find information about whether you can play the game while driving a car or using public transport.
Fighting while traveling
You can't fight and complete missions when you move too fast. You can't play while riding a bike, scooter, let alone riding on a bus or driving a car. When you stop, the game will focus on the location you are currently in.
However, you can fight and clear the Nemetons while traveling on public transportation. You will receive 50 gold coins and over 500 experience points for clearing the ghostly tree.
You can, however, manage your equipment while traveling. There's nothing stopping you from creating new items, getting rid of unnecessary things, or buying new gear. You can do that while driving.
Remember not to use your phone if you are the one driving. Whether it's a car, bike or scooter. Take care of yourself and other road users.
If you stop in traffic, you will be able to play without any problem as long as there is a monster in your area. The map is virtually empty on the highways. This means you won't be able to fight anything while driving on a highway. There aren't too many monsters around, and the game doesn't see any places in the area to which it can move a mission.