Factors relating to the road
In an Australian in depth study with 205 accident cases as reported by Haworth et. al. 15% of the cases were found in which the road surface had contributed to the accident. In more than half of all cases some site factor (of various kinds, including lack of vision) was involved. The role of road design and maintenance in PTW accidents may vary from country to country.
The MAIDS study  with 921 PTW accidents reports roadway defects in 30% of all cases. This does not necessarily mean that the defect contributed to the accidents. In a report by the European Motorcycle Industry (2006) some more results are presented. In 25 of all 921 cases a roadway maintenance defect was judged to have contributed to the accident. The total number of cases in which the road environment contributed was 72, including road design defects as well as temporary obstacles.
Both studies were based on officially reported accidents. It could well be that the role of poor road surface is more prominent in accidents that are not reported because no other road user was involved or the damage and or injuries were minor.
Brendicke et. al.  presents an overview of the problems of PTW riders with different road surfaces. They show the considerable, adverse consequences of poor road surface on skid resistance and thus on braking and cornering of PTW’s