Until we all start riding around in Google's self-driving cars, or getting ourselves delivered by Amazon's drones, accident-avoidance systems will loom as the ultimate in vehicle-safety technology. Rather than just protecting people in a crash, such as seat belts and air bags, they hold the promise of preventing crashes entirely.

They're also a huge challenge to create, a classic example of easier said than done. Sure, combinations of sensors - video cameras, radar, lasers - and software can detect conditions that predict an imminent accident risk. But how do you communicate that instantly to a driver in a way that guides corrective action?

Most automobile alerts use sound or sight - warning beeps, or flashing red alerts on a dashboard or windshield. But another category relies on a different sense: touch. Milliseconds matter in these situations, and studies suggest that so-called haptic warnings can break through to a driver's attention more quickly than alternatives.

That's the theory behind the Safety Alert Seat that I tested recently in a 2015 Chevrolet Tahoe - one of three large Chevys now offering a safety system that General Motors debuted in Cadillacs and also offers on its Buick LaCrosse.

The safety seat doesn't totally replace other kinds of alerts. If you drift across a lane marker at 35 m.p.h. or above, you'll see a green dashboard icon suddenly change to amber and start flashing. But rather than an audible warning, you'll feel three quick pulses from the left or right side of your seat - depending on the direction of your drift.

The feeling is more subtle than rolling across one of the rumble strips often installed at the edge of highway shoulders. It reminded me more of hitting the raised reflectors that some road builders - better-funded than Pennsylvania's Department of Transportation - use to demarcate highway lanes.

The GM system also pulses to warn a driver of an imminent forward collision when it detects that your vehicle is approaching another too quickly. In that case, the vibrations supplement a flashing red alert on the windshield.

How well do these systems work?

The jury is still out, says Russ Rader, a spokesman for the Insurance Institute for Highway Safety (IIHS), which continually pores over accident data. But there's one exception: Growing evidence supports the value of forward-collision warning systems, particularly those that include automatic braking.

Auto-braking systems were available on one in five vehicle models in 2014, and are reaching such mainstream brands as Subaru. (The 2015 Tahoe offers auto-braking, too.) When the insurance institute publishes its 2015 ratings this month, only vehicles offering an effective auto-braking system will be eligible for its "Top Safety Pick Plus" rating.

Rader says some systems haven't met their promise, earning only the same "basic" rating IIHS awards vehicles with frontal-crash systems that warn drivers rather than actually stopping a car.

For example, Rader says the 2014 Toyota Avalon and Infiniti QX60 SUV both "provided minimal braking in IIHS tests." He says Toyota's 2014 Prius V wagon failed both for auto-braking and as a warning system.

Rader says the institute hasn't given up on the potential of lane-departure warnings, which it once estimated could prevent or mitigate about 179,000 crashes a year, including 37,000 that cause injuries and 7,500 that cause deaths. But so far, the evidence is mixed.

"We don't have enough real-world data yet to say how it's working," Rader told me Wednesday.

Nor has the institute evaluated the relative value of haptic alerts vs. other kinds of warnings, or compared different approaches to haptics. Some warn with steering-wheel pulses. Others give more active feedback - resistance, essentially - to help drivers avert risky lane changes and crashes.

The good news, Rader says, is that automakers "are now competing on safety." That may be the most welcome innovation of all.

215-854-2776 @jeffgelles