What percent of road collisions occur because of mechanical failures?

The percentage of road collisions caused by mechanical failures really varies from country to country, and also we have to take in consideration the drive park of a certain country. The percentages of this type of collisions are 2-3% in more developed countries, and as the economy of the countries is poorer usually the percentage increases up to 12% and even 20% in some cases. This is due to the fact that in more developed countries people usually drive new cars, in the transitioning countries big portion of the drive park is consisted of second hand vehicles, thus the chances of mechanical failure increases, and in the developing countries there's often vehicles older then 20 years on the road so the chances of mechanical failures are much higher.