Is the Tesla Model S really the safest car on the road?

One key fact went unnoticed this week as Tesla Motors Inc. trumpeted its “best” crash-test rating in the media: The federal government doesn’t test most other luxury cars.

So the Model S may be safer than many cars costing far less. But whether it’s safer than direct competitors from BMW, Audi and Mercedes-Benz — considered among the safest cars available — remains a mystery.

For the third time in six weeks, it has become apparent that a