Driving the new Mercedes-Benz E-Class up and down San Francisco's highways with the semi-autonomous Drive Pilot system engaged, a remarkable thing happens: Traffic-spawned stress vanishes faster than water evaporating under the California sun.

The car monitors everything around you, working every millisecond to prevent any form of aggravated contact, and deftly doing your bidding if, say, you want to change lanes to pass that semi ahead of you. You can skim Twitter, tap out texts, or just kick back and enjoy the ride—as long as you check in at least once a minute by grabbing the steering wheel, and remain vigilant and ready to take over if the computer loses its way.

So when does that happen? It’s not quite clear—and that’s my key quibble with this freshly redesigned 2017 model year sedan. Overall, the E-Class is magnificent, a new front-runner of Teutonic tech that surpasses even the brand’s S-Class flagship and makes its $52,150 starting price seem totally reasonable. It automatically brakes in emergencies.

It helps you steer more aggressively around suddenly detected obstacles. It blares a static-like sound just before a crash that can protect your ears from loud noises. It inflates bladders in the seats to push you toward the center of the car in the event of an imminent side-impact. It has an exceptionally sporty air suspension and two bright 12.3-inch LCD displays—mounted beneath a single strip of glass—that seem to come straight from the future.

But the Drive Pilot option is the big news here, placing it essentially on the same level as Tesla's Autopilot system. Most of the time, it will make your commute a breeze, reading lane lines and spotting other cars to stay in its lane and a safe distance from fellow travelers.

The problems crop up when the system fails to immediately and clearly convey what's going on. Over two days of driving and riding hundreds of miles around the Monterey Peninsula, Big Sur, and Silicon Valley, the car’s decision logic repeatedly left me wondering who was in control, whether it knew what was truly happening, and what would happen next. Merge spots proved particularly tricky. In one case, the car didn’t seem to recognize there were two vehicles converging ahead of us, charging forward until I hit the brakes. This in spite of the fact that I was told the vehicle's sensors scan in all directions. So if there were any protocols in place preventing any action, I wasn't aware of them.

In several turns on the road, the car plowed straight ahead with no apparent awareness that it may have lost track of the road's arc, sending me toward either a wall or oncoming traffic. Each time, I saved it at the last second and put us back between the lines, where it happily took over as if nothing had happened. To the car, clearly, nothing had–though it was obviously no longer behaving appropriately.

The dashboard—featuring an artful graphic representation of the road in an array of colors and varying degrees of opacity—proved little help. A small green steering wheel icon lights up when the Drive Pilot system is on, along with a button on the steering wheel—not an obvious, helpful system for telegraphing the car's current awareness and intentions. Did some subtle colors change, or lines harden when things started to go awry? Not that I saw–which is to say, it wasn't obvious. So when do I jump in? Who was in charge?

Mercedes-Benz

The answer, Mercedes is quick to note, is me. I'm supposed to be there, always at the ready. I Tweet at my own peril. The reality of these systems, though, is that people will goof off. Tesla's Autopilot system demonstrated that nearly immediately upon reaching the public.

So hollering "buyer beware" and providing subtle visual and aural status cues isn't enough, especially in a system that can function at up to 130 mph. The car’s status needs to be obvious, particularly before it's perfect, and when it's sold to those who aren’t tech-savvy enthusiasts who’ve been briefed on the car’s operation directly by top-level management.

Taking Off

The weight of the quandary struck me when I returned the E-Class just around the corner from San Francisco International Airport, where, barely three years ago, Asiana Airlines Flight 214 crashed short of the runway. After the pilots failed to notice their descent rate was too fast and their airspeed too low, the Boeing 777 struck a seawall at the end of runway and cartwheeled violently, killing three passengers and injuring another 187. The National Transportation Safety Board pinned the crash on the pilots, who mistakenly thought the computer was controlling their throttles. It spanked Boeing and Asiana, too, for an autopilot system that did not obviously convey to the pilots exactly what it was doing.

If questionable design can lead professional pilots to slam a commercial airliner onto a seawall on a beautiful day, you can be sure you'll see smaller-scale incidents as autonomous cars fill our roads. Automakers are starting to address this problem, exploring new interfaces that combine bright and distinct colors with chimes, vibrations, and novel tricks like moving steering wheels–the automotive equivalent of an airplane's "stick shaker," which alerts pilots to an imminent stall.

I suggest a graphic representation of where the car is going, an accurate arcing of the road reflecting the computer’s intended trajectory—something Delphi experimented with in a recent concept. If the road is clearly going to the right and the graphic shows the intended path is straight as an arrow, I’ll know to intervene, with just a split-second glance at the instrument panel. I could even grasp the situation by sensing the graphic in my peripheral vision as I look out the windshield.

Mercedes' current autopilot system is a coup: It takes the anxiety and stress out of highway driving and is there to back you up when you lose focus. But the interface design doesn't rise to the challenge of letting its human passengers make the most of that capability.