Cars increasingly roll off the assembly line with safety technology shown to prevent crashes.

But with the average U.S. car nearly 12 years old, most drivers aren't receiving the benefits of new collision avoidance systems. A vehicle fleet can take decades to turn over, and no automaker offers drivers a way to upgrade their vehicle with such systems. Some companies are now trying to fill the void for customers growing used to much faster upgrade cycles.

For instance, Rand McNally, best known to consumers for its maps and atlases, is releasing a $400 tablet for drivers to attach to their dashboard. The device offers driving directions, plays videos and includes a collision-warning system. A camera in the tablet monitors the road ahead and will alert a driver of an impending crash.

"Our goal was to capture all of the goodness that's found in luxury car entertainment systems and provide in a user friendly, affordable package," said Rand McNally chief executive Stephen Fletcher. "These high-end luxury cars are providing many different features and advances including collision and lane-departure warnings. We wanted to capture all of that."

Both Garmin and Brand Motion released similar devices in 2015. They record video and alert drivers when they stray from their lanes, or are about to be in a crash. Brand Motion said it has sold a couple thousand since the November release.

A Silicon Valley start-up, Navdy, is accepting preorders for a $500 dashboard device that projects directions and text messages onto the windshield. The idea is to keep a driver's eyes on the road rather than their smartphone.

"It's quite realistic people will start considering purchasing such devices to make things more safe," said Forrester analyst Frank Gillett. "We can expect to see a growing number of these type of offerings and at some point they will be interesting and useful enough to attract people's attention."

He expects insurance companies to have an interest in the device, yet cautioned that retrofits are never as good as systems built into a vehicle.

So far, automakers have shown little interest in upgrading existing cars with such tech on their own. They're focusing on bringing the latest technology to where the big money is - new cars. The Alliance of Auto Manufacturers doesn't have a position on the use of retrofitted safety systems, according to a spokesman. Ford, GM and Toyota all declined to comment for this story.

For a driver to appreciate the full collision-warning ability of the Rand McNally or Garmin device, they'll have to position the tablet straight ahead so the camera has a clear view of the road. But drivers will most likely want to angle the device to face themselves, which will hinder the device's effectiveness.

Additionally, an Insurance Institute for Highway Safety study released in January found that while automatic braking systems reduced crashes, systems that only issued a warning to drivers did not have a significant impact on crashes.

To upgrade one's vehicle with an automatic braking system adds another level of complexity, and drivers risk running afoul of their warranties. So far, no third parties have released safety systems that include emergency braking.

So while retrofit devices are popping up on the market, customers that want the very best will be forced to spend the money to buy a new car.