In New Jersey, a newly licensed teenage driver can take only one passenger from outside his household for a ride, a provision soon to become even more stringent.
Delaware also imposes limits on teen drivers' passengers, as do 41 other states and the District of Columbia.
But in Pennsylvania, a new young driver can load up as many friends as there are seat belts in the car.
The deaths in the last month of six Chester and Montgomery County teens, in three crashes, have again raised questions about the state's laws for young drivers, among the least restrictive in the nation.
In two of the cars, there were six teen passengers; in the other, three.
The deaths devastated the teens' families and their friends in the Pottstown and Coatesville areas.
"A vehicle becomes a virtual party barge when you start adding teen passengers," said Catherine Rossi, a spokeswoman for Mid-Atlantic AAA. "Pennsylvania is lax when it comes to safety."
There is no shortage of data on teen driving:
A Johns Hopkins University study found that the chances of a 16-year-old's dying in a car with a teen driver increased 39 percent with a single passenger, 86 percent with two passengers, and 182 percent with three or more passengers, Rossi said.
Motor vehicle crashes are the leading cause of death for 15- to 20-year-olds, according to the National Highway Transportation Safety Administration, killing more than 4,000 teens a year.
In Pennsylvania, between 1998 and 2007, 380 teen drivers were killed. In addition, teen crashes killed 600 people who were passengers, occupants of other vehicles, and nonmotorists.
Ways to prevent such statistics are also well-researched.
The NHTSA says that teenage drivers and passengers are among those least likely to wear seat belts and that a failure to buckle up should be a primary offense - "click it or ticket," the NHTSA calls it.
In Pennsylvania, police can issue seat-belt citations only if a vehicle is stopped for another offense.
And handheld cell phones are still legal except in cities that have imposed their own bans, such as Philadelphia.
State Rep. Katharine M. Watson (R., Bucks), who for four years has pushed for tighter teen-driver laws, says colleagues have derided her as a "hysterical mother."
Her House Bill 289 is among four teen-driving bills pending in the state.
To facilitate passage, Watson said, she has kept the language "narrowly focused" on well-documented safety issues.
Currently, a Pennsylvania teenager with a learner's permit must drive 50 hours with an experienced adult driver before obtaining a junior or provisional license. Watson's bill would add 15 hours - 10 spent driving at night and five in inclement weather.
The bill would also restrict a junior-license driver to only one nonhousehold passenger under age 18. Household relatives under 18 would be permitted with parental approval.
New Jersey, which has allowed one nonhousehold passenger (along with multiple family members), passed a law in April that allows provisional drivers to transport just one other person - family or not - unless a parent is in the car. It will take effect in the spring. And it bars cell phones, hands-free or not - a particular hazard for young drivers. Also, failure to buckle up becomes a primary offense.
In the spring, New Jersey also will become the only state where drivers under 21 without full-privilege licenses must display a decal on their vehicle identifying them as new drivers subject to special restrictions. The practice is used in other countries, such as Australia.
Watson said her colleagues have argued that they did not want to deprive young people of having fun in groups, as the lawmakers did when they were young.
"Things are different today," she said, citing distractions such as cell phones. "I want them to live to have a good time when they are 30, 40."
Pennsylvania State Police Commissioner Frank E. Pawlowski said that for years, police have pleaded unsuccessfully for a law making failure to use seat belts a primary offense. People ejected in crashes rarely survive, he said.
"We can't hand over the keys to inexperienced kids, who are easily distracted, and expect a positive outcome. We're fooling ourselves if we do," Pawlowski said.
He and others say parents must enforce the rules.
"Somebody's got to be the bad guy and say no," he said.
A 2009 study by Children's Hospital of Philadelphia and State Farm Insurance found that if parents set clear rules, teens are half as likely to crash.
Flaura Koplin Winston, founder and scientific director of the Center for Injury Research and Prevention at Children's, has been a leading international proponent of teen-driving safety for almost a decade.
"Honestly, it is embarrassing that Pennsylvania is behind other states and other countries," she said. "We have to begin to recognize that teen-driving deaths are not accidents. Most are preventable."
If safety data have no impact, Winston said, she hopes lawmakers will be motivated by the fact that these "preventable injuries and deaths" drive up health-care costs.
State Rep. Barbara McIlvaine Smith (D., Chester), who recently announced she would not seek another term, said that she would support a primary seat-belt or restricted-teen-passenger law, but that she believed passage was unlikely.
"You're never going to get 253 votes if we can't even pass a law banning texting," she said. Pennsylvania lawmakers are "not interested in trying to legislate behavior; they feel it's burdensome."
"That's one of the reasons I'm leaving," said Smith. "We can't get things done."