The warning about unmarked police cars is half right

A message that has circulated on social media for years tells drivers not to stop for an unmarked car until they reach a lighted, populated place, and to dial 112 on a cell phone to check whether the stop is real. The first half is sound. The second half is not.

112 is the emergency number in Europe. The National Emergency Number Association has said repeatedly that it does nothing special in this country, that some carriers convert it to 911 and some do not, and that nobody here should count on it. Dial 911 and no other number.

The part worth keeping is the part about where to stop. The Texas Department of Public Safety driver handbook tells an uncertain driver to turn on the hazard lights, drive below the speed limit, call 911 and continue to a well-lighted, populated place before stopping. Rhode Island State Police make the same point, saying most officers understand a short drive to a driveway, a side street or a parking lot. The hazard lights are doing the work. They say you have seen the lights behind you and are not running.

A dispatcher will want your location, described by landmarks or business names if no street sign is handy, a description of the car behind you, and where you intend to stop. Texas DPS adds a caution that is easy to miss: police jurisdictions overlap, and a 911 center cannot always tell right away which officer is working a given stretch of road.

The handbook covers the opposite problem as well. A driver who stops somewhere genuinely dangerous, on a bridge or in a lane of fast traffic, may hear the officer's public address speaker telling him to move along to a better spot.