Take a deep breath, fellow Ford (NYSE: F) shareholders; the company isn't about to go over a cliff. But it is likely to lose a big chunk of a very visible market -- one that Ford has led for years, and one that has been the exclusive playground of the Detroit automakers for decades.

How visible? Very: Think about the last time you saw a police car.

You probably see police cars almost every day, though unless you're a police officer (or a crook), you likely don't think much about them -- until you see one looming behind you. And for quite a few years now, that maybe-not-so-welcome face in the mirror was likely to have been a Ford Crown Victoria Police Interceptor, the longtime undisputed leader of the U.S. police-car market.

The last bastion of the old Big Three
While Chrysler has recently made some inroads with its Dodge Charger -- the speedy Hemi-powered sedan is a favorite of a number of highway-patrol units -- and General Motors has picked up a few sales here and there, Ford is the clear king. More than 70% of U.S. police cars are Crown Vics, and about 75,000 new police cars are built and sold here every year.

But the Crown Vic is an elderly design, and next year it'll turn in its badge. All of the American automakers are launching new designs, and the battle for market share should be fierce. (And no, I don't see Toyota (NYSE: TM), Honda (NYSE: HMC), or any other non-U.S. automaker busting in on this party. It's anecdotal, but I know a few cops, and I suspect most of them would rather walk than drive an import while on duty, especially given Toyota's recent troubles.)

Now, I said "American automakers" -- but there's a twist: There are four companies entering this upcoming contest.

Yes, I said four. Ford, GM, Chrysler ... and one you've probably never heard of.

Get to know this new face
Unless you're a hardcore car geek or a big fan of police-car technology, I'm betting you haven't heard of Indiana-based Carbon Motors. Carbon Motors is an auto start-up with a twist: While most automotive startups have historically aimed at building unusually fast sports cars for unusually wealthy car enthusiasts, Carbon wants to be America's go-to maker for police cars, the sports car's natural nemesis.

And the upstart company, co-founded by a former police officer and a former Ford executive, might actually succeed.

Police cars might seem like a tricky niche market for a new manufacturer to break into. A newcomer have to compete with the Detroit automakers' economies of scale, and police departments understandably tend to favor proven solutions when lives are on the line. Put another way, there's a reason that almost all police cars are Crown Vics or Chargers: Cops know they work.

But as I said above, the Crown Vic is going away next year. That means those 75,000 sales a year will be up for grabs, and Carbon has been preparing for that moment for several years now. Their first model, the Carbon E7, has garnered some 12,500 preorders, the company says, on the strength of its sturdy design, cop-friendly features, and environmental friendliness.

And in a sign that they may have even more orders in their pocket, Carbon this week announced a deal to buy diesel engines and transmissions from BMW -- more than 240,000 engines, according to one source, which would power a lot of black-and-whites.

They're clearly serious. But then, so are the once-Big Three.

The established entries
Police cars have varied over the years, but they tend to be big, rear-wheel-drive four-door sedans with V8 power. Ford's new entry, however, is based on the Taurus -- a front-wheel-drive car powered by V6s. The company has clearly anticipated objections by making all-wheel-drive and a V8-like turbo engine available, at extra cost -- but given the conservative instincts of those who buy police cars, those options may not be enough.

GM and Chrysler, like Smith & Wesson (Nasdaq: SWHC), L-1 Identity Solutions (NYSE: ID), TASER International (Nasdaq: TASR), and other companies that sell to the police market, know that many police departments want the tried-and-true. Both are sticking with the traditional formula -- while Chrysler's revised Dodge Charger, due later this year, is expected to address officer complaints about the relatively small trunk and limited visibility in the current model, it will still be a large rear-wheel-drive sedan with (lots of) V8 power available.

Likewise the new fleet-only Chevrolet Caprice, which will be built on an extended version of the same rear-wheel-drive platform that underpins the Camaro. GM has said that the Caprice will be available with a high-horsepower V8 and cop-friendly features like special seats and an oversized trunk.

And then there's Carbon's E7, which is rear-wheel drive, diesel-powered, designed from the ground up as a police car, and will supposedly be available with decidedly non-civilian options like "radiation, chemical, and biological threat detection". It's an intriguing package, but until Carbon announces pricing and releases some samples for testing, it'll be hard to gauge its likely impact.

One thing's for sure, though: That face in your rear-view mirror is going to be changing soon.

Fool contributor John Rosevear owns shares of Ford. Ford Motor is a Motley Fool Stock Advisor selection. Try any of our Foolish newsletters today, free for 30 days. The Fool's disclosure policy lacks the right to remain silent, and that's a good thing.