Traveling by stretch limo may seem like the height of prudence for people going out to party, but it has hidden pitfalls.

Of course it makes great sense for a group of friends to set up a ride ahead of time so that no one who has been drinking — or planning to — gets behind the wheel.

But stretch limousines might be considered a kind of automotive zombie: They are former cars or sport utility vehicles that have been transformed after they leave the factory, and exist in a regulatory limbo. They usually do not have to meet arduous federal safety requirements and face a haphazard inspection system that varies from state to state.

“Without sufficient safety standards in place, it is nothing but a fine line between a stretch limousine and a hearse,” Thomas Spota, the former Suffolk County district attorney said in 2016.

Moreover, ordinarily cautious people who buckle up in their own cars may be less likely to do so in for-hire vehicles.

On Saturday, 17 people headed for a birthday celebration at a brewery near Cooperstown, NewYork, and had booked a party bus for the trip. They wound up in a stretch limousine, and all of them, along with the driver and two pedestrians, died in a crash in Schoharie, New York, about 40 miles west of Albany.

The cause remains under investigation, but Gov. Andrew M. Cuomo said Monday that the limousine had failed inspection last month and was deemed unfit for the road.

The stretch limo was a 2001 Ford Excursion that could seat nine people when it left the factory. Nearly two decades later, it held 18 — many of them seated along the sides.

Ordinary cars are strictly regulated by the federal government and most meet safety standards of the insurance industry. Not so for stretch limos.

WHAT IS A STRETCH LIMO?

There’s no official definition, but in the real world, it’s this: a regular car or SUV that has been cut in half, separating the front and rear. The body is extended on the top, bottom and sides. Additional seating is added, often along the sides.

HOW DOES THAT CHANGE THE SAFETY OF PASSENGERS?

One way is behavior. People who usually ride in hired vehicles are less careful about using seat belts than those who regularly travel in private cars, a survey by the Insurance Institute for Highway Safety found. While studies have not been done specifically of passengers in stretch limousines, the party atmosphere suggests that the same behavior applies.

The passengers in the Schoharie crash did not appear to have been wearing seat belts, according to Matt Deffer, chief of the Esperance Fire Department, which responded to the scene.

Unbelted passengers not only put themselves at risk but anyone else in the vehicle, including people who do wear their belts, said Raul Arbelaez, an engineer with the institute. Their bodies can be hurled through a vehicle’s interior at terrible speeds. Slightly more than 38,000 people died in crashes last year and about half were unbelted.

HOW ELSE DOES STRETCHING A VEHICLE CHANGE SAFETY?

By federal law, vehicle interiors are designed, in part, as cages that deflect the forces of a crash away from the passengers. The modifications to create a stretch limo eliminate some of those protections. In making the vehicle longer, side pillars and air bags are removed.

Chief Deffer said the Ford Excursion in Schoharie had extensive damage “on both sides, to the front, and the interior.”

Engineers take high-speed destructive forces into account when building vehicles.

“A regular car is designed from the ground up to meet all the safety criteria, from the very beginning when it first goes into a computer model to the very last bolt that’s put into it,” Arbelaez said. “You’re protected in a front crash, side crash, rollover.”

Those standards were set after studies of car crash fatalities, and from laboratory experiments on vehicles. Little to none of that experience applies to vehicles that have been turned into stretch limos. The modifications are rarely done by the original manufacturers, but by smaller companies after the vehicles have been sold.

“So the vehicle that was carefully designed to perform well in crashes is chopped up, extended, and most if not all of those safety standards get tossed out the window,” Arbelaez said.

HOW ABOUT AVOIDING CRASHES?

Keeping control of such large vehicles is a challenge for the driver. Fatalities from SUV rollovers led to a regulation that required the addition of electronic stability control. But that’s not really going to help a stretch limo.

“When you modify a vehicle this much, the algorithms for electronic stability control would not work because you’ve changed the length and mass,” Arbelaez said.

Brakes and tires are typically designed around the size of the vehicle, and increasing the mass changes the vehicle dynamics, he said.

Among other problems, the Ford Excursion had deficiencies in its braking system, Richard Azzopardi, a spokesman for the governor, said.

WHAT REGULATIONS APPLY?

Not many. Federal safety standards apply before the vehicle is sold to a consumer. For inspection purposes, New York state regulation treats vehicles that seat more than 10 people, including the driver, as buses. The Excursion failed to meet the bus standards last month.

IS THERE A PUSH FOR HIGHER SAFETY STANDARDS?

Sort of.

A Long Island grand jury issued a report in 2016 that said stretch limousines are underregulated. The report followed a collision between a stretch limousine and a pickup truck that killed four people who were traveling on a tour of wineries on the North Fork of Long Island. Sen. Chuck Schumer called for more regulation after those deaths.

Cuomo pointed out Monday, however, that in this case, regulations and inspections — which ordered the stretch limo off the road — were disregarded.

Arbelaez said that as awful as the crash was Saturday, highway safety specialists should stay focused on changes that can save large numbers of lives.

“I think there is little or no effort going into limo-type crashes,” Arbelaez said.