Smeed proposed the so-called Smeed formula to describe relationship between motorization level (V/P) and traffic accident level (D/V): D/V=α (V/P)β. Yearly number of serious road traffic accidents (fatalities) in China from the years 1951 to 2009 is analyzed. Its temporal evolution should be divided into two stages. At the first stage, spanning from 1951 to 1972, motorization level is very low, but traffic accidents fluctuate with time by a big margin. The higher motorization level is, the more traffic accidents statistically occur during this phase, and such a relation should be described more suitably by a linear formula than by the Smeed formula. The large temporal vibration of traffic accidents is inferred related to solar activity in this stage. In the second stage, from the years of 1973 to 2009, motorization level is obviously increased. Motorization level becomes higher and higher, but annual traffic accidents obviously decreasingly occur during this time period, and such a relation can be reflected suitably by the Smeed formula with α=1.618010310-4, and β=-0.692.