Even if self-driving cars never make it to the streets in large numbers -- or if they are deployed only in geofenced areas -- the huge amount of money invested in the technology will have benefits that could improve all vehicles. Take, for example, the car's doors.

A visit to the North American headquarters of Brose (BROH'-sah), just north of Detroit, drove home the point that nearly every part of the car will change as self-driving technology matures.

Brose is a privately owned German company that specializes in mechatronics, a combination of mechanical and electrical engineering. The company, a supplier to almost every major automaker, makes around 200 million electric motors per year that power windows, seats, liftgates, air conditioners, etc. One of Brose's newest products is a self-closing door system for autonomous vehicles. It's nearly ready for production.

How hard can it be to engineer a system that automatically closes a car door, and why are such systems even necessary, you might wonder. Self-closing sliding doors are in use on minivans, trains and buses and elsewhere, so not exactly rocket science, right? But automobiles with swing open doors present a very different and larger challenge, I learned.

First, the why. Self-closing doors will be an important part of autonomous vehicles. If a passenger exits the vehicle and leaves before closing the door, the car won't move. If the door can't close, the car will be out of service until someone is sent to close the door. Downtime means the car isn't making money for its owner.

The technical challenge is significant. Real estate is at a premium inside a door. In addition to side-impact door beams, required for safety, the regulator, the mechanism that raises and lowers the window, is inside the door, sharing space with the lock and its parts, and in many cars, sound-system speakers.

But the tight confines are right in Brose's wheelhouse. Michael Kidd, Brose's development director for door systems, showed me how the company develops all the components inside a door as an integrated system, not a randomly packaged set of components.

Brose likely isn't the only supplier working on a self-closing door mechanism. Arnd Herwig, Brose vice president for development, said automakers have approached the company in search of the mechanism.

The prototype system I saw at Brose was in the door of a General Motors SUV. Engineers placed the small electric motor near the door's check strap, the device that prevents the door from opening too far. Two cables connected to the motor pull the door closed. If the door detects an object in the way, it stops automatically. When it gets close to the bodywork, an electrified latch pulls the door closed. Of course, all the action is controlled by a computer.

It might be years before automakers incorporate self-closing doors into light vehicles, but I made a suggestion to Herwig that was met with a positive nod: Why not first offer the self-closing door systems to upfitters who equip vehicles for drivers with disabilities? That would be a great way to get real-world experience.