A semi? You mean like this?
Yeah, there isn't much on the highway you can't just run over with near impunity with that bad boy and enough weight in the trailer. But the same is true with a bus or many RVs.
The Tesla Model S has achieved the
best safety rating ever handed out by the NHTSA. But it is really more a matter of bragging rights than anything else.
As you can see from this graph, beyond a certain point you are just talking about a few percentage points safer in their tests:
Keep in mind people are supposed to survive these tests with only slight chance of injury, so it obviously doesn't mean that you are nearly impervious to any traffic accident. If you really want to be a lot safer, get a roll cage, safety harnesses, and always wear a Snell-approved helmet.
The real questions regarding the Model S are if it is worth to you the nearly $70K price tag, do you want an electric car, and can you get by with the maximum miles it offers without a recharge at home.
The Model S is also a very heavy car due to all the batteries. This is also why it did so well in the safety tests. The more massive object in a 2-vehicle collision typically fares much better.
But if you want a car that is known to be safer than others in its segment, here are the IIHS top-rated cars by vehicle type:
IIHS Gives 33 Vehicles Its Top Safety Rating For 2015
Not sure why the Tesla isn't listed, but any of them would be good choices for their particular segment based on safety.