Paul Walker's Family Believes Road Bumps Sealed His Fate

Paul Walker died after the speeding Porsche in which he was riding hydroplaned on reflective plastic markers in the street ... at least that's what his family and accident reconstruction experts believe.

Sources close to Paul's family tell TMZ ... the family has been in touch with stunt experts who have gone to the crash site and come back with what they believe is a solid theory -- a speeding car that hits the plastic markers -- called Botts' Dots -- will hydroplane, causing the driver to lose control.

We've learned the experts have told the family ... a car going speeds approaching 90 MPH will lose traction after hitting a series of Botts' Dots. It's akin to driving on ice -- and control is further compromised by the fact that the Porsche was outfitted with special racing tires that have very little tread.

And there's this ... Porsche sent out a memo to dealers back in 2004, warning about control problems with the Carrera GT, saying, "This vehicle cannot drive over a Foster Beer can that is lying on its side. It will crush the can and damage the carbon fiber under panels," adding, "You need to be aware of what type of road surface you are on (dips, pot holes, seam heights, etc.)."

Law enforcement sources say they have not concluded the exact cause of the accident, but it appears they're looking at the Botts' Dots ... as a viable theory for triggering the fatal crash.