'They took into account thirty factors which it was thought might affect the death rate. Among these were included the annual consumption of wine, of spirits and of malt beverages — taken individually — the amount spent on road maintenance, the minimum temperature, certain of the legal measures such as the amount spent on police, the number of police per 100,000 inhabitants, the follow-up programme on dangerous drivers, the quality of driver testing, and so on.

The thirty factors were finally reduced to six on elimination of those which were found to have small or negligible effect. The final six were: * (a) The percentage of the total state highway mileage that is rural. * (b) The percent increase in motor vehicle registration. * (c) The extent of motor vehicle inspection. * (d) The percentage of state-administered highway that is surfaced. * (e) The average yearly minimum temperature. * (f) The income per capita.