7 top road-trip tech gadgets

Cruise control can help you save gas by keeping your car at a constant speed. But on today's congested roadways, it's rarely convenient: When you get stuck behind a slower car, you have to turn it off.

But with "active" cruise control, radar in the nose of your car detects the slower-going vehicle ahead. Your car will automatically slow down to maintain a safe following distance. You may not maintain a constant speed, but it's better than slamming on the brakes.

What most of these systems won't do is bring your car to a complete stop. If there's a traffic jam up ahead, it's still up to you to hit the brakes. And they mostly work only at speeds over about 30 miles per hour.

The Distronic Plus system offered in high-end Mercedes-Benz luxury cars is an exception. It uses two radar systems, one for far-away objects and another that detects hazards closer in. The Distronic Plus system will handle the brakes and gas all the way down to a dead stop. We still recommend you take over in an emergency, though.

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