If an evacuation order has been issued, your original plan is pretty good. It may be that a potential hazard could quickly turn in a major disaster, and the authorities want everyone out of the danger zone ASAP.

Otherwise, without knowing the location of the spill, what has been spilled, and a clear indication of wind direction, hopping in the car is potentially a death trap. A toxic, asphixiating, or explosive cloud will knock out the car and its occupants. I guess if you can move fast and know the wind direction, it's a personal choice.

For sheltering in place, anything that can be done beforehand to reduce air movement between the interior and exterior of a building would be time well spent. Caulk is cheap (haha). A plan to kill all HVAC and ventilation fans, shut of natural/propane gas (if any) and retreat to a safe room can be executed very quickly. For the safe room, pre-cut plastic sheets and duct tape applied to the inside of doors and windows can seal things up very quickly.