Research on the trends in use of heavy vehicles indicate that a significant difference between the . and other countries is the relatively high prevalence of pickup trucks and SUVs in the . A 2003 study by the . Transportation Research Board found that SUVs and pickup trucks are significantly less safe than passenger cars, that imported-brand vehicles tend to be safer than American-brand vehicles, and that the size and weight of a vehicle has a significantly smaller effect on safety than the quality of the vehicle's engineering.  The level of large commercial truck traffic has substantially increased since the 1960s, while highway capacity has not kept pace with the increase in large commercial truck traffic on . highways.   However, other factors exert significant influence; Canada has lower roadway death and injury rates despite a vehicle mix comparable to that of the .  Nevertheless, the widespread use of truck-based vehicles as passenger carriers is correlated with roadway deaths and injuries not only directly by dint of vehicular safety performance per se , but also indirectly through the relatively low fuel costs that facilitate the use of such vehicles in North America; motor vehicle fatalities decline as fuel prices increase.