If the GG is independent and guides the car without the need to connect to wifi, then I can turn off wifi at any time without penalty.
A wireless disabled vehicle will not automatically recharge and it's mapsofts will not be automatically updated (you will not know the latest traffic jam situation, know about the accidents that might have impact on traffic, know if the bridge up ahead is about to open or not etc).
If anyone turns off the wifi while driving, then it leads to the automatic stop of the vehicle, the car is taken over by the Pilot and he guides him to a safe stop.
If you are connected to GridLink then the autopilot of the vehicle cannot exceed speed limits and the vehicle can be remotely shut down.
If you are connected to GridLink but run a GridLink Override (or if you turn wireless off) then the autopilot of the vehicle can exceed speed limits and the vehicle cannot be remotely shut down.
Or the car continues in its original direction until it hits something.
No...
That pretty much only happen if nobody is controlling the vehicle and the vehicle's autopilot have been disabled. Or perhaps if the vehicle have been bricked.
A vehicle with an Autopilot that have access to the correct autosofts (in a vehicle that runs GridLink Override) can take all sorts of actions (evasive maneuvers, perform stunts, participate in car chases, firing on-board weapons etc) and unless the vehicle have an active metahuman pilot it will also automatically take over control to make sure you don't crash into stuff.
You can also manually take control over the vehicle. Driving it like people used to drive vehicles back in 2020. You never need a wireless connection for physically driving something yourself. At least in 5th edition, manual control override autopilot.
And you can also control it via AR or VR. Even if it is wireless disabled (via direct connection). At least in 5th edition, this override both autopilot and manual control.
And if the vehicle have a rigger adaption and you have an implanted control rig or is a technomancer with a machine mind echo you can also control while being jumped in. Even if it is wireless disabled (via direct connection). And at least in 5th edition this would override autopilot, manual control as well as regular remote control.
However, if Autopilot loses connection and does not know the exact conditions of traffic, then its primary directive is to stop the car safely.
Where did you read this?
Sensors (such as cameras and range finders) and most vehicle features (such as lane assist, adaptive cruise control and various map softs) and it's own onboard autopilot (and it's locally loaded autosofts) don't suddenly stop working just because the vehicle is not connected via wifi....