That fatal Tesla S vs. semitrailer accident in Florida back in May has become one of the more high-profile wrecks of the year. Mind you, not every truck-involved collision merits an investigation by the National Transportation Safety Board, but this one did, no doubt because the car was under autonomous control – Autopilot, Tesla calls it.

The event raised questions about the basic safety of the Tesla system as well as the pace of development of autonomous vehicles in general. How safe are they, really? A lot safer than now, where human drivers cause most of the traffic accidents that kill and injure many thousands of people every year.

Last year’s highway death toll of about 38,000 amounts to a 747 transport crashing every week, safety experts say. If jumbo jets were junking themselves and killing their occupants that often, you can bet aviation authorities would put a quick stop to it. But highway mayhem is so common that most of us just shrug. However, take the human element out of driving and much of the carnage would end, say autonomous-vehicle proponents.

Anyway, back to the Tesla-semitrailer wreck. The NTSB this week released a preliminary report that doesn’t name a cause but adds details to what happened. For example, it occurred along a stretch of U.S. 27A, west of Williston, Fla. This is a modern four-lane highway, but with cross traffic. The trailer was a 53-foot van filled with blueberries on the way to a local farm. (Strangely, the report identified the maker of the tractor but not the trailer.)

News stories have said that neither the Autopilot nor the Tesla’s driver, Joshua Brown, 40, of Canton, Ohio, saw the trailer, and the car ran right under it. The white trailer against a bright sky was invisible to the Autopilot’s camera, sensors and software, and the driver wasn’t paying attention because he trusted the auto-driving mechanism (and said so in YouTube posts).

A neighbor in Canton who knew Brown told reporters that he was a fast driver who had collected speeding tickets. The NTSB confirmed that Brown was speeding right up to the moment of the impact. Well, that explains why the tractor-trailer turned left in front of the approaching car; the truck driver thought he had more time than there really was.

NTSB said the Tesla was going 74 mph in a 65-mph zone – not that fast in light of how today’s traffic moves. But remember, U.S. 27A is not a limited-access Interstate. Still, it raises the possibility of a “failure to yield” charge against the truck driver.

Then again, it’s obvious that if Brown had been watching where his car was going instead of viewing a Harry Potter movie -- as the truck driver claimed after hearing audio playing in the car after the crash -- he’d have simply stepped on the brake pedal and probably avoided the crash. Tesla said that’s exactly what he should have done.

As an occasional driver of tractor-trailers, I have made left turns against oncoming vehicles. I always try to accurately judge their speed in deciding whether to go or wait for a longer break in traffic. And I always remember how long my trailer is and how much time it’ll take to clear the intersection. I recall one left-turn instance when the truck didn’t accelerate as fast as I thought it would, and oncoming traffic had to slow for me.

The truck driver in this incident probably assumed that the car would slow down if necessary, but it didn’t. So I can imagine his surprise when the Tesla tore under his trailer.

NTSB said it’s still investigating and will cite the accident’s cause in a later report. We’ll be watching for that. In the meantime, let’s be careful out there.

UPDATE (July 31): Tesla thinks its Autopilot might not be at fault in the accident. See this story.