A number of new details about the crash involving an Uber self-driving Volvo XC90 in Arizona have emerged and they demonstrate how difficult it will be to perfect autonomous driving.

A report from the Tempe Police Department and testimony from Alexandra Cole, the driver of the Honda CR-V that caused the crash, she attempted to make a left turn across three lanes of traffic. However, while the first two lanes were clear, the third wasn’t and before Cole could brake, the Uber operating in self-driving mode made contact.

“As far as I could tell, the third lane had no one coming in it so I was clear to make my turn. Right as I got to the middle lane about to cross the third, I saw a car flying through the intersection but couldn’t brake fast enough to completely avoid collision,” Cole said.

The impact ultimately saw the Uber crash into a traffic police before flipping on its side and colliding with two other vehicles.

As The Washington Post notes, it is not uncommon for drivers to speed up to catch a green light before it turns red or to cut through various lanes of traffic and such behaviors are one of the greatest challenges which self-driving engineers have to overcome. After all, not all road users follow the letter of the law all the time.

According to Uber, the company’s autonomous prototypes will only cross an intersection at a yellow light if it determines that it can do so at its current speed. If the car believes it can’t make the light before it turns red, it will stop, if it can do so comfortably.