There already exists Wide Area Augmentation System (WAAS) and European Geostationary Navigation Overlay Service (EGNOS). They augment GPS, GNSS and Golnass and provide accuracy, integrity, and availability. They are used in aviation.
EU's GNSS has PRS (Public Regulated Service). It is authenticated and can be used in sensitive applications for civilian uses. US has GPS modes that are protected but they seem to be only for military.
ps. Many road navigation systems already have accelerometers in them. Relatively easy counter against road routing spoofing is to detect turns and directions. System could flash a warning that something is wrong if crude INS is not matching GPS signal. There are spoofing techniques that fool integrated GPS/INS but they are hard or impossible to use in roads.
>All the components are available off-the-shelf. The total cost is about 223 US Dollars ($175+$35+$10+$3).
Then, reconsider the choice. Pretty sure the whole point of individual autonomy is life. Every bit we delegate is at our own peril.