in large commercial truck traffic on US highways. However, other factors exert significant influence; Canada has lower roadway death and injury rates despite a vehicle mix comparable to that of the US. 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. The US NHTSA has issued relatively few regulations since the mid-1980s; most of the vehicle-based reduction in vehicle fatality rates in the US during the last third of the 20th Century were gained by the initial NHTSA safety standards issued from 1968 to 1984 and subsequent voluntary changes in vehicle design and construction by vehicle manufacturers. Issues for particular demographic groups Pregnant women When pregnant, women should continue to use seatbelts and airbags properly. A University of Michigan study found that "unrestrained or improperly restrained pregnant women are 5.7 times more likely to have an adverse fetal outcome than properly restrained pregnant women". If seatbelts are not long enough, extensions are available from the car manufacturer or an aftermarket supplier. Infants and children Children present significant challenges in engineering and producing safe vehicles, because most children are significantly smaller and lighter than most adults. Additionally, children far from being just scaled down adults, still have an undeveloped skeletal system. This means that vehicle restraint systems such as airbags and seat belts, far from being effective, are hazardous if used to restrain young children. In recognition of this, many medical professionals and jurisdictions recommend or require that children under a particular age, height, and/or weight ride in a child seat and/or in the back seat, as applicable. Within Europe ECE Regulation R44 dictates that children below 150 cm must travel in a child restraint that is appropriate for their weight. Each country have their own adaptions of this Regulation. For instance, in the United Kingdom, children must travel in a child restraint until they are 135 cm tall or reach 12 years of age, which ever comes soonest. As another example, in Austria, the driver of passenger vehicles is responsible for people shorter than 150 cm and below 14 years to be seated in an adequate child safety seat. Moreover, it is not allowed