The situation isn’t helped by how “noisy” cars have become, with cellphones, iPods and Bluetooth devices, said Lissa Robins Kapust, a social worker and coordinator of a driving program at Beth Israel Deaconess Medical Center in Boston. “Driving is so busy on the inside and the outside of the car — it’s the most complex thing we do.”

But A.D.H.D. involves more than distractibility. Its other major trait is impulsiveness, which is often linked to high levels of risk-taking, said Dr. Barkley.

“It’s a bad combination” for young drivers, he said. “They’re more prone to crashes because of inattention, but the reason their crashes are so much worse is because they’re so often speeding.” Many drivers with A.D.H.D. overestimate their skills behind the wheel, Dr. Barkley noted.

Far better, researchers say, to have the attitude that Ms. Serpa does — not minimizing the difficulties or being daunted by them. “I am persistent,” said Ms. Serpa, now 21. “I don’t quit. And if there are people who think I am struggling with driving, I will tell them the truth.”

Ms. Serpa heads back to the road test on Thursday, with “a whole new level of confidence” after more intensive practice — plus a new string of kabbalah beads and a lucky pendant.

Fortunately, researchers and special instructors are discovering more tangible ways to help teenagers like her. The first step: deciding whether a 16-year-old is ready to learn, or really needs to drive at all.

Dr. Simons-Morton thinks that almost any reason to put off starting lessons is a good one. “If I were the parent of an A.D.H.D. or other special-needs kid, my goal would be to delay licensing,” he said. “They mature, they accommodate to their deficits and they’re more likely to take medication.”