Eyewitnesses tell us the singer was riding passenger in an Escalade Saturday along PCH in Malibu near Point Dume and Zuma Beach. We're told a woman who was driving -- which we've confirmed was his fiancee and baby mama, April Love Geary -- somehow collided with another car full of what we're told were younger people.

Eyewitnesses say that Robin got out of the SUV and walked over to the other vehicle, which was lying on the side of the highway ... nearly tipped on its side. Luckily, this looks a lot worse than it actually was, as nobody seems to have suffered major injuries.

Law enforcement sources tell us no one was transported to a hospital in the two-vehicle collision, and that a report was taken on the scene. Fault will be determined at a later date.

In video of the scene, obtained by TMZ, you see Robin picking up car parts off the road and attempting to direct traffic away from the accident. At one point, you see Robin's SUV pulling out ... and the extent of the damage on it is on full display.

Based on what Robin posted on IG a little while ago, it doesn't appear their kids were with them -- looks like he and April did a solo lunch date, and they both seem okay.