There’s a “city” where cars run around without a license plate and no one gets in trouble. This place is the Milford Proving Ground, in Michigan, U.S. A private complex with 4,000 acres that works as a secret “city” where General Motors try out their new models.
A world where cars run around unidentify
In this private city, cars circulate without a license plate, no one pays for gas and it’s very cautious of who gets in. That’s Milford Proving Ground, and it has been hiding in Michigan for more than a century.
But, why don’t cars need a license plate? The majority of the vehicles inside are actually prototypes, and they are usually covered to keep their designs in secret. GM doesn’t have to worry about identifying these vehicles since they are pre-production; they’re not ready to be sold yet.
However, what’s really cool about this place is; the gas station inside the complex, where the engineers can fill up the tanks as many times as they need, and completely free—well, the company covers it.
General Motors’ massive “city”
GM’s complex, the Milford Proving Ground was found in 1924 and it’s about 4,000 acres big. And it’s kind of a self-sufficient place—wich is why it’s called a “city.”
Inside, there’s more than 150 buildings, its own fire department, and a complex road network with paved circuits, gravel stretches, high-speed straights and demanding off-road tracks.
Since it’s a testing ground for vehicles, every single inch has been fully designed to take cars to the limit before they reach the dealers and become tomorrow’s transportation solutions.
Milford (the town in Michigan) is a community, with around 6,500 people (in 2025). It was chosen as the perfect location for this complex because it was located between two GM industrial poles, and because it had enough land to simulate highways, weather and safety tests in controlled conditions.
More than just a secret place
There are more “cities” like Milford Proving Ground around the world:
- Lang Lang Proving Ground (Victoria, Australia): Complex of 2.167 acres, built in 1957, and used for speed testing (bowl), driving and hard surfaces. It also work as an emission and safety areas’ laboratory. Today, is no longer owned by GM; But they continue to use it in a limited way.
- Cold Weather Development Center (Kapuskasing, Ontario, Canada): Smaller “city” with 272 acres, was property of GM in 1973 until October 2024, when it closed operations. Here they focused on winter components and extreme cold validation using cold cells, test tracks and “squeaks & rattles” tests.
- Cruz Alta Proving Ground (Indaiatuba, Brasil): Opened in 1974, with 2.718 acres, where General Motors validates vehicles for South America using multiple tracks and laboratories that replicate roads and local demands.
- Desert Proving Ground (Yuma, Arizona, EE. UU.): Complex of 2.400 acres. Operational since 2009, and dedicated to extreme heat testing, dust and durability (with oval, straights, “ride road”), dynamic zones and noise measurements.
- Guangde Proving Ground (Anhui, China – SAIC-GM/PATAC): This space is 1,408 acres, and is partially owned by GM with partners (Shanghai GM / SAIC and PATAC). It opened in 2012, with a test road network that simulates about 67 driving conditions (noise, durability, dynamics, etc.).
Protecting design—and legacy
It’s an exciting place; creativity, innovation, testing, and secret cars. Who wouldn’t want to go and at least have a look at the cars rolling on the tracks? However, as many would expect, a secret city has to be very secretive, and the access is nearly impossible.
GM imposes a very strict and strong security to avoid any leaks on what’s going on behind doors. In fact, a lot of employees have never been in one of these mini cities, and those who manage to do it have to face harsh rules: No phones, no cameras, no exceptions!