The aircraft HP is only set on motor start. As
@Kilrah pointed out the Dynamic HP is not a continuous dynamic update. You have to manually set it from the RC to the current RC coordinates. It is designed to allow you to bring the aircraft home to a new 'safe' location where it can land without damage. For example, it is used where your original HP is no longer safe to use because someone has moved into that area. In your road scenario, you would have to stop leave the car and then set HP to your location.
I personally would never fly from a moving vehicle for safety reasons, unless you are in a tank and can stick your head out of the turret to maintain VLOS at all times. Remember the Return to Home Point (RTH) functionality is there to allow you to automatically return the aircraft to the HP and land by itself or should the aircraft lose signal from the RC to either hover, land or fly to the HP and land (depending on settings) as a failsafe. There is also the Smart RTH based on remaining battery power and will activate RTH if the power drops to that needed to return to the set HP (ie. launch location). If you are driving along your road, you need to first switch off the Smart RTH and at 30% battery power stop the vehicle and set the Dynamic HP to a safe landing site (preferably off the road). The safest Failsafe RTH option is probably to Hover so you can reverse back until you regain signal control.