Four hundred miles with Tesla’s autopilot

Supercharging at my usual stop in Columbus. Photo: Lee Hutchinson

Four hundred miles with Tesla’s autopilot

by Lee Hutchinson | Ars technica

A few weeks ago, I finally tried Tesla Motors’ “autopilot” feature. A Tesla rep and I tooled around Houston’s I-45 in a Model X crossover SUV for 15 minutes, just long enough to test the vehicle’s adaptive cruise/automatic lane-keeping wizardry. Once I toggled on the autopilot, the rep relaxed by checking e-mail on her phone. This sent a clear message: keep an eye on the dumb journalist when he’s driving the $140,000 SUV, but once the machine takes over, everything’s fine.

As we pulled back into the showroom (or whatever Texas’ insane dealership protection laws demand Tesla call the places it’s not allowed to sell or service vehicles), I told the rep that I was driving to Austin soon; Autopilot would be just the thing for the long stretches of empty road out on I-10 and TX-71. Without missing a beat, she offered me a loaner Model S.

Ars has officially driven a Model S with autopilot before, but only under controlled circumstances. The Austin trip would let me take the car out for nearly four hundred miles of driving in a big mix of traffic scenarios. Plus, I’d get to log more cockpit time in a Tesla. Of course I said yes. Who wouldn’t?

My chariot

I’ve driven two previous Model S sedans on the approximately 400 mile (about 640km) Houston-Austin round trip—first a 2013 model P85+ and more recently a dual-motor P85D. Both trips were fun, since both times Tesla provided the currently-most-powerful version of its sedan to hoon around in. True to form, the company again loaned me its top-end car: a dual-motor P90D with a 90kWh battery and the much ballyhooed “ludicrous mode” acceleration option (caution: there’s some NSFW language in that linked video).

The P90D looks and behaves much like previous Model S sedans. Its exterior is studded with sensors that can detect both the road and other vehicles, though unlike the P85D I drove, this P90D had the software installed to take full advantage of those sensors. (The entire Model S fleet produced with these extra sensors over the the past year has since been software-upgraded to match the new vehicle’s capabilities.)

As I gingerly placed my butt in the matte leather driver’s seat of the pearly white P90D, another friendly Tesla rep gave me an unnecessary tour of the cockpit. Yes, I knew how the climate control worked. Yes, I knew how to open the charge port. Yes, I knew how to engage the auto-cruise and auto-steer. I was an old hand at Tesla-ing. Let’s get this show on the road!

Perception, reality, and really good PR

My first jaunt was a 30-mile (48km) drive from the Houston Galleria showroom to my home on the southeast side of town. Houston’s west 610 loop is in a constant state of rush hour, but the traffic mid-day was as light as it gets. It flowed along at maybe 40 miles per hour (64km/h) until the loop bent east and things opened up. Once I merged with traffic on the loop, I pulled the cruise stalk toward me twice to engage auto-cruise and auto-steer.


Four hundred miles with Tesla’s autopilot

The Model S uses several sets of sensors, from optical to ultrasonic to radar, to maintain awareness of the traffic around you.


The car had already seen a 60mph (100km/h) speed limit sign, so it set that as its target speed. Since traffic was moving slower than the limit, my Tesla patiently paced the car ahead at a near-constant four car lengths. I twisted the end of the cruise stalk forward and reduced that distance to two car lengths, which curbed the constant flow of people jerking their cars angrily into the lane ahead of me (woe betide you if you leave a space in Houston traffic). I forced myself to relax and hovered my hands over the wheel.

The car was driving itself. All I needed was a leather jacket and a two-way radio watch, and I’d be living out theKnight Rider fantasy I’d had since age 5.

Well, OK, this system isn’t revolutionary; adaptive cruise control (where your car paces itself with the car ahead) has existed for a decade or more, and automatic lane-keeping is at least as old. Other car makers like Volvo, Mercedes, and Audi offer similar auto-cruise and auto-steer packages.

But if Tesla isn’t the first OEM to the autopilot table, it is certainly the loudest. The company dominates the marketing war—to the point that even folks who don’t particularly care about cars know the company’s story.

That evening, when I caught up with my next-door neighbor—not a particularly tech-centric person—his eyes lit up. “That’s the electric car that drives itself, right?” he asked. He wanted a test drive.

The first leg: Getting out of town

I was due at my downtown Austin destination the next day at noon, so I set out at 7:30am with a fully charged car. From most points in Houston to most points in Austin takes three hours, but my trips begin with the disadvantage of starting southeast of Houston and having to fight through the city’s morning rush hour (easily adding an extra 60-90 minutes). I would also need at least 20-30 minutes at the Tesla Supercharger in Columbus, Texas to top off the battery.

Four hundred miles with Tesla’s autopilot

The steering wheel you won’t need to touch (except when the car tells you to touch the steering wheel to make sure you haven’t died or something).

For all the car’s software wizardry, calling its autopilot system an “autopilot system” isn’t really accurate. The car is very good at following the road and keeping pace with the car in front of it, but it does not follow the path you program into the navigation system nor do any other kind of advanced self-guidance. So even though I programmed the Columbus Supercharger as my first destination, I stayed hands-on with the car as I drove out of my neighborhood and onto the I-45 service road. Auto-steer had to wait for the freeway, where I toggled it on.

Unfortunately, I toggled it right back off. The portion of I-45 near my home is undergoing a multi-year $6 billion rebuild, with a long stretch spanning several exits being destroyed, reconfigured, widened, and otherwise “improved.” The result is a multi-mile segment of road that snakes back and forth across what will eventually be both sides of the finished project. For now, there’s no consistent surfacing, no real lane markers, and nothing for the lane-keeping system to “see.” I drove human-style until I could bail off of I-45 and onto the westbound Sam Houston Tollway.

Referred to by locals as “Beltway 8,” “the beltway,” or sometimes “the Houston Speedway” (people drive fast!), my section of the Sam Houston Tollway remains relatively un-cluttered for long stretches during the morning. I toggled on the auto-cruise and auto-steer, dialed the target speed up to 90mph and the desired follow distance to four cars, parked myself in the right lane, and pulled my hands slowly away from the wheel.

It takes a while to get used to this feeling. Instead of serving as the primary means of direction for a car, you’re now a meat-based backup and failsafe system. Instincts and impulses formed by more than two decades behind the wheel scream out a warning—”GRAB THE WHEEL NOW OR YOU’LL DIE”—while the rational forebrain fights back. Eventually, the voices quiet as the car starts to prove itself. When the road curves, the car follows. If the car is ever going too fast to negotiate the curve, it slows down and then accelerates smoothly back out of the turn.

Four hundred miles with Tesla’s autopilot

Waiting patiently to accelerate away from a stop. The blue speedometer and steering wheel icons at top indicate that auto-cruise and auto-steer are active.

Lane keeping was almost excellent. The car kept itself squarely planted in the middle of the lane without ping-ponging between the edges like some earlier systems did. The car would still sometimes “lunge” at exit ramps when lane markings would fall away to the right or left, though the behavior was never enough to make me grab the wheel and disengage. (Tesla’s 7.1 software update was supposed to eliminate the issue, though it hasn’t completely.) Other than the occasional lunge, however, the car was solid as long as there remained even the slightest trace of visible lane markings.

Sitting in the right lane on the tollway meant I frequently encountered other cars merging from the service road. Most of the time, the Model S would see them—with the speedometer display constantly updating to show silhouettes of other cars in their approximate positions around me—and slow down. The braking was perhaps a little later and a little harder than I would have done, but it was smooth and controlled every time. (The braking interval can be adjusted in the car’s settings). However, sometimes the person merging onto the road would be more beside me than in front of me. In those cases, I could force the system to initiate an automatic lane change by pushing up and holding the turn signal stalk, or I could scoot out of the way by manually mashing the accelerator.

Tesla, take the wheel

Right around where the tollway intersects SH-288, traffic slammed to a halt. The car’s Google Maps-driven center display showed the entire 20-mile (32km) stretch of tollway between SH-288 and my exit point at I-10 as solid red.

The transition from cruising at 90mph to dead stop was extremely smooth. The car hauled itself down with a combo of regenerative braking and actual-for-real braking, coming to a halt about a meter behind the stopped car in front of me. Those familiar rush hour reflexes started to shout at me, and I could feel my blood pressure rising. Had I left early enough? Was I going to make the meeting on time?

But the most remarkable thing happened—instead of getting angrier and angrier, as I usually do when stuck in bumper-to-bumper traffic, I found myself relaxing. The Model S did all the annoying traffic creeping crap itself. Unlike some other OEM cruise systems, I didn’t have to touch the Tesla’s brake to come to a full stop nor tap the accelerator to resume moving. All I had to do was sit calmly and look out the window. After I felt satisfied that the Model S wouldn’t just accelerate through the car ahead of me, I took out my breakfast and enjoyed eating it. The Tesla crawled along.

When traffic sped up a bit, the Model S sped up with it, eventually re-establishing its preset car-length-following interval. When traffic slowed or stopped, the Tesla slowed or stopped with it, coming to a gentle halt just a meter away from the car ahead. Traversing 20 miles of clogged roadway took 90 minutes, but they were 90 relatively calm minutes—I was thoroughly engrossed in an audiobook and far less rage-filled than usual.

I was almost disappointed once it came time to take manual control and exit for I-10.Almost.

The second leg: devouring those country miles

Next came the 70-mile (112km) segment of I-10 between Houston and Columbus. Once through the western side of Houston and out past Katy, the drive turns into open skies and empty roads—except on this particular drive. I-10 was full of semi-trucks with no regard for the “Keep right except when passing” signs posted every few miles.

Here, though, another aspect of the auto-steer system came into its own: the semi-automatic lane change. By holding the turn signal stalk either up or down, you can tell the car to change lanes to the right or left. The car first checks to make sure that the lane you want to switch into is unoccupied, then it accelerates slightly and makes the change. I “drove” the car using the turn signal stalk for some considerable time on this leg, keeping the adaptive cruise set at 85mph (136km/h) and letting the car pace other cars where there was no opening to pass. It was fascinating.

four hundred miles with tesla's autopilot

Symbols on the assistive driving stick. Pull forward drive for Autopilot, move up for down for cruise control speed adjust, and twist for car following.

Even though the car largely takes care of itself, it must be stressed that this is not fully autonomous driving. Again, the car won’t follow the navigation system. You turn on auto-steer and auto-cruise and the car follows the road you’re on—nothing more. More importantly, you’re never meant to remove your hands fully from the wheel.

Though the temptation is to lean back and lace your hands behind your head (or to climb into the back seat like a damn lunatic, among other silly things), the car really, really wants you to pay attention. Unclasp your seat belt and the auto-steer and auto-cruise immediately disengage and the car slows to a stop. Keep your hands off the wheel for too long (about 90 seconds) and the car will sound an alert tone and display a dialog on the center console asking you to please grasp the wheel. If you ignore the warning, the car sounds another. If you ignore that one, the car will disengage the auto-cruise and auto-steer and slow to a stop (apparently on the assumption that you’re incapacitated, dead, or otherwise unable to grab the wheel).

When in auto-steer, the steering wheel tracks just as it would when being steered normally. You can rest your hands on it and feel the car making tiny adjustments to its course (or large adjustments as it turns). The wheel slightly resists your inputs when auto-steer is enabled, but if you get uneasy and want to take over, a small bit of extra effort will disengage the system (though the auto-cruise stays on). Alternatively, you can tap the brake pedal and both the cruise and steering revert to manual control.

The mostly automatic drive ended in Columbus, where I recharged the car’s battery knowing I had about 200 miles into Austin and then back again to Columbus. I also needed to adopt a more moderate pace. According to the car’s energy usage projections, if I did the drive in the same, er, spirited fashion as I’d done the first leg, I’d only get 180-ish miles out of the full charge. Even with the Model S’ extremely low drag coefficient (0.24, even lower than a Prius), pushing the 4,900lb (2,200kg) vehicle along at 90mph requires a hell of a lot of juice.

Exercising the DSLR, taking some pix at the Columbus Supercharger.

Exercising the DSLR, taking some pix at the Columbus Supercharger.

The third leg: driving like a normal person

With about 90 minutes until my meeting in Austin, I pointed the Model S north on TX-71. This time, I stuck religiously to the 75mph (120km/h) speed limit, which meant blowing past pickups with “FARM TRUCK” license plates or getting blown past by other cars doing 30mph over the limit.

Fortunately, I didn’t encounter a situation where the autopilot had to do a panic stop—although the system is perfectly capable of slamming on the brakes to avoid an accident. The car did have to rapidly decelerate because someone changed lanes or merged in front of me, but it always did so smoothly and authoritatively.

One thing the car doesn’t do is raise or lower its speed as the speed limit changes. There’s a particularly nasty speed limit drop from 75mph to 55mph at a little town called Ellinger(where, by the way, you can stop for some excellent kolaches). Even though the car’s sensors saw the speed limit change and flashed a “55MPH” sign on the dash, I had to manually pull down on the cruise stalk to drop the speed appropriately.

Eventually I exited out of the wide lonely hills and prairies and re-entered civilization (aka Bastrop, home of a giant Buc-Ees where I always stop for a bathroom break). I did most of the drive into Austin with the autopilot still engaged and set to 75mph—the Model S followed traffic, kept its pace, and behaved perfectly. If I wasn’t turning onto a new road, I was auto-steering all the way to the parking lot at my final destination.

A few hours later, interview concluded, I was talking with my interview subject about the Model S. He’d never ridden in one, so we hopped in and drove for 15 minutes around I-35. This time, I sat in the passenger seat as my interviewee drove. I admit that I was nervous, but once we were on the freeway, I encouraged him to turn on the autopilot. He made the same incredulous noises I had made the week prior. As he had a hard time letting go of the steering wheel, I took the opportunity to pull out my phone and check the e-mails and texts that piled up during the day’s drive.

And then I laughed—because I got it. I understood why the Tesla PR rep did the same thing on our first drive. With the car in control, I wasn’t worried about it wrecking itself no matter how many times my interviewee went, “Oh my God, this is insane!”

I had won the victory over myself. I trusted the machine.

The long road back…

The drive back mirrored the drive out—I took it easy from Austin to Columbus, making sure I wasn’t running out of juice. Next I sped like crazy (though very much with the flow of traffic!) back to Houston, and afternoon rush hour on I-10 eastbound into town shut that down cold. It was mostly stop and go around the Sam Houston Tollway and I-45 south, and the autopilot features carried the day—I leaned on them whenever I could.

I learned how much I missed the system when, a couple of weeks later, I made a second (now manual) trip to Austin for another interview. It’s hard to overstate how much the ability to kick back and let the car take care of itself improves long drives. Even though I was doing pretty much the exact same thing, the trip in the auto-steer and auto-cruise Model S was a lot less fatiguing than the normal hands-on drive.

What’s more, the Model S was remarkably energy efficient even with the cruise control pegged at 90mph. On the energy usage graphs, I could easily pick out segments where the car drove versus segments where I took over. The car doesn’t do jerky acceleration, and the computer keeps the energy usage sane and low even at high speeds. Even when I tried, I couldn’t maintain the same consistency and efficiency as the car. (Then again, I’m only human.)

The technology isn’t infallible. It still sometimes lunges a bit at freeway exits, and of course it’s not fully autonomous in any way, shape, or form. You have to keep your hands on the wheel and be mindful that you might have to take over if the computer interprets the road markings incorrectly or if someone swerves at you from the sides—or if, for some reason, the computer reboots mid-drive. The bones and scaffolding for a fully autonomous system are there, but that system doesn’t yet exist.

Emphasis on the yet. The week before, when testing out the Model X crossover, I had asked the Tesla PR person what made the company’s auto-steer and auto-drive system stand out from other solutions. The answer, as might be expected, had to do with those over-the-air software updates. Each Model S is heavily instrumented and telemetered, and each stays in regular communication with Tesla Motors in California. (In fact, Tesla buyers sign a form agreeing to telematic data collection when they buy or lease a car, though it is possible to opt out).

That link is used to push updates to cars, but it’s also used to regularly upload data about driving habits back to Tesla. If a driver disengages auto-steer at a certain location, for example, Tesla engineers can look at where that happened and what the driving conditions were—especially if the driver regularly travels that route.

The entire fleet—save owners who opt out—is therefore one giant research project. And while telematics aren’t exclusive to Tesla, no other company is as vocal about its collection and use of this data; it’s fuel for the self-driving learning engine. Rather than guessing at how to build an autonomous vehicle, Tesla conducts a live, worldwide experiment in machine learning and development. In other words, we’re teaching Tesla cars how to eventually drive themselves. Or at least we’re helping.

Enlarge / A new crossover SUV Model X, sitting in Tesla's service bay in Houston.

Enlarge / A new crossover SUV Model X, sitting in Tesla’s service bay in Houston.

…and everything after

This was the third time I had returned a top-of-the-line Tesla loaner, and getting back into my normal internal combustion-powered car gets harder each time. The Model S isn’t perfect—the interior still isn’t up to spec for the price range, for example, and those oh-so-sexy 21″ Y-rated Conti summer tires look great but anecdotally only last about 5,000 miles (8,000km) and cost $1,600 to replace—but at this point it’s firmly lodged in my brain as the dream car I’d spring for if I had the means.

I’ve gone this entire time with just one mention of “Ludicrous Mode,” but if you’ve stuck with me this long, we’ll go a little farther.

I talked in the P85D review about that car’s “insane mode” button; the P90D’s “ludicrous mode” is an improvement, utilizing some tweaked software parameters to launch the heavy sedan from 0-60mph (about 0-100 km/h) in a reported 2.8 seconds. That’s almost a half-second quicker than the P85D’s 3.2 second 0-60 time.

You can read those numbers, but it’s difficult to internalize the feel. There are tons of Tesla “launch reaction” videos on YouTube, and people repeat many of the same phrases. “It’s like a roller coaster!” comes close to the experience, but it’s still not quite right.

Here’s my best attempt to make you feel something: from zero to maybe 30 or 40mph, the car is untouchable, even by the fastest production sportbike. It teleports to that speed. It pulls so hard and so brutally that your muscles all tighten up and just for a moment you can’t breathe because your entire body involuntarily tenses. From there through 60mph, you’re in a rarefied area of acceleration that only some top-end sportbike riders and the fastest of supercar owners experience. If you’re the passenger, you’ll involuntarily gasp, grin, or try to grab something (though the Model S still lacks interior “oh shit” handles). Even if you’re prepared, you’re not prepared. The acceleration pushes so hard that it triggers your body’s “I’m falling!” response. It’s downrightdisconcerting.

And then it’s over. Above 60 or 70mph, the massive torque of the big Tesla’s dual motors and their single reduction gears tapers. You re-enter the world of normalcy, a world where internal combustion engines with multi-gear transmissions still rule. A lot of supercars (and a lot of non-exotics with giant engines, like Dodge’s Hellcat line of tuner cars) trounce the P90D in a quarter mile, and of course, Tesla’s track performance islegendarily poor due to battery overheating.

But, oh, those first intoxicating seconds. You mash on the pedal and feel like you’re sitting in the Millennium Falcon and Chewie’s just punched in the hyperdrive. It’s the closest thing most people will ever get to finding out what happens when Kirk looks at Sulu and says, “Let’s see what she’s got.”

The fact that such a car can almost drive itself is just cake.