Thanks for the clarification. You mean to say that at 90km/h, nobody survives, so the question of pedestrian safety ratings is rubbish.
It is NOT rubbish for 2 reasons:
1) One is every bit as probably to hit a pedestrian at 90km/h or 50km/h. So your artificial environment of 90km/h though is a good academic point, is meaningless in this context.
2) It may be to survive a crash at 90km/h, and therefore, the possibility increases logically if you have higher NCAP pedestrian rating. At higher speeds, the rate of death increases exponentially.
A deeper look, it's not only the speed, but it's the momentum and deformation of the car also. There comes the concept of NCAP pedestrian safety rating. And this question of the relationship between speed of impact and pedestrian fatality is politically charged as it is highly relevant to speed limits. Many of the fatality charts end at 90km/h from the government's perspectives, but many allege that government left out data in their studies.
In short, the data I found so far is ambigious, and the principle of higher pedestrian safety rating higher survivalability holds because one can impact at whatever speed, the better chance of survival is always valuable.
References for point (2) in a quick search:
http://usww.com/homepage/starteam/speed.html#s2
For example, the rate of severe injury for people involved in crashes at impact speeds of 21-30 mph is 11.1 - a rate that increases to 27.9 at impact speeds of 31-40 mph and to 54.3 at speeds of 50 mph or more. (The rate is calculated as the number of occupants at a certain impact speed with severe injuries, divided by the total number of occupants in crashes at that impact level times 100.)
Physics of car impact speed
http://www.science.org.au/nova/058/058print.htm
Safety vs. Convenience
http://portlandtransport.com/archives/2006/04/safety_vs_conve.html
Speed Management in Urban Areas
http://www.vti.se/nordic/2-99mapp/299dk1.htm
Speed Limits and Speed-Related Issues
http://www.driveandstayalive.com/articles%20and%20topics/speed/aa-index_speed.htm