Last August, Apple sent several of its prototype self-driving cars on a roughly 40-mile trek through Montana. Aerial drones filmed the drive, from Bozeman to the ski resort town of Big Sky, so that Apple managers could produce a polished film, with picturesque mountains in the background, to show CEO Tim Cook how their costly and long-running autonomous car project, Titan, was making progress.

Inside Apple, executives hailed the demonstration as a success. The vehicles showed they could drive without relying on highly detailed, three-dimensional road maps, which most rival self-driving-car programs require. Titan managers hoped to ditch these costly and unscalable high-definition maps en route to one day building and selling a fully automated car that could work almost anywhere in the world without a steering wheel or pedals, making Apple responsible for the car’s behavior and the safety of its occupants.

The good vibes following the Bozeman demo didn’t last long. Apple’s test vehicles, which are modified Lexus SUVs, struggled to navigate streets near its Silicon Valley headquarters without the maps, smacking into curbs and sometimes having trouble staying in their lanes while crossing intersections, according to two people who worked on the program. And earlier this year, a test vehicle nearly hit a jogger who was crossing the street and had the right of way, one of these people said.