The Austrian was out riding with compatriot Bernie Eisel (Dimension Data) when disaster struck.
“It was my second day back on the bike, after I had to recover from a second viral infection in the 2018 season,” the 27-year-old said in a team statement.
“I was sitting in (Bernie's) wheel in a slight downhill, when suddenly a car, ignoring the stop sign, came at us from the right.
"Bernie just managed to escape him, but I could not do that and hit the driver’s door at full speed. My bike was completely destroyed and my knee as well.”
Haller was diagnosed with a double fracture of the left knee cap with further tests also revealing a fracture of the lateral side of the left femur.
He'll miss the Tour de France and leave a big hole in sprinter Marcel Kittel's lead-out train.
“I am very disappointed. Not only at missing the Tour de France but also because of - again - the attitude of some car drivers."
"Cars and cyclists share the road but this seems to be more and more difficult. Some mentality needs to change,” said Haller.
Haller will begin rehab in Munich once out of hospital, but it is not yet known when he can return to racing.
In this week's podcast, our host Christophe Mallet has a chat with Steel Von Hoff, recent winner of the Men's Commonwealth Games Road race and is joined by Sophie Smith, Matthew Keenan and Wes Sulzberger to review the queen of the classics Paris-Roubaix, and preview Flèche Wallonne and Liege-Bastogne-Liege.