The impact of child safety restraint legislation on child injuries in police-reported motor vehicle collisions in British Columbia: An interrupted time series analysis

Abstract

BACKGROUND/OBJECTIVE:

Motor vehicle collisions (MVCs) remain a leading cause of death and serious injury in Canadian children. In July 2008, British Columbia introduced child safety seat legislation that aimed to reduce the number of children killed or injured in MVCs. This legislation upgraded previous child seat legislation (introduced in 1985) and affected children zero to three and those four to eight years of age. The objective of the present study was to evaluate the effectiveness of this legislation.

METHODS:

Deidentified police reports for all MVCs involving zero- to 14-year-olds (2000 to 2012) were used to compare injury rates, booster seat use, and seating position among children before and after booster seat laws. An interrupted time series design was used to estimate the effect of the new law on injuries among children zero to three and four to eight years of age. Estimates were adjusted using children nine to 14 years of age as controls.

RESULTS:

The booster seat law was associated with a 10.8% (95% CI 2.7% to 18.9%) reduction in the monthly rate of injuries in four- to eight-year-old children (P=0.01). This was equivalent to a decrease of 14.3 injuries per 1,000,000 children. Similarly, the monthly injury rate among children zero to three years of age decreased by 13.0% (95% CI 1.5% to 24.6% [9.8 injuries per 1,000,000]; P=0.03).

CONCLUSION:

The results provide evidence that British Columbia’s new child safety restraint law was associated with fewer injuries among children covered by the new laws.

Motor vehicle collisions (MVCs) remain a leading cause of death and serious injury among children in Canada (1,2), the United States (3) and many other countries (4–6). Children zero to eight years of age are vulnerable to injury in MVCs, especially if unrestrained or inappropriately restrained. Lap and lap-shoulder belts are designed for adults, and their use by children can result in a pattern of injuries known as the ‘seat belt syndrome’. Injuries associated with the seat belt syndrome include hip and abdominal contusions, pelvic fractures, cervical and lumbar spine injuries, and intra-abdominal injuries involving both solid and hollow organs (7). Damage to the spinal cord can result in permanent disability (8).

To reduce the risk of injury, children travelling in a motor vehicle should use an appropriate child restraint until they reach nine years of age. Booster seats help position seat belts properly for children who have outgrown forward-facing child restraints with internal harnesses but who are still too small for adult seat belts (4). Belt-positioning booster seats improve the fit of the lap and shoulder portions of adult seat belts (9) and reduce injury risk among four- to eight-year-old children involved in MVCs (3,4). In children four to seven years of age, booster seat use reduces collision-related injury by 59%, and nearly eliminates the abdominal injuries seen in children using adult seat belts (9).

Booster seat laws, free distribution of booster seats to eligible families and education regarding proper use of booster seats can increase booster seat use (6), and reduce child occupant injury and fatality rates (10,11). Canadian parents report that legislation is an important factor in their decision to use booster seats (12), and that most parents and caregivers believe that child safety seat laws represent “best practice recommendations” (13). British Columbia (BC) has had laws mandating child seats for children zero to three years of age years since 1985. The 1985 laws exempted certain vehicles such as rental vehicles and passenger vans. Before 2008, children >3 years of age were not required to use booster seats. In 2004, an observational roadside survey found that booster seat use in BC was only 18% (14,15). In 2004, the BC Automobile Association Traffic Safety Foundation, in collaboration with the BC Ministry of Children and Family Development, launched a booster seat initiative that aimed to increase appropriate use of booster seats in BC (14,15). This initiative coincided with a national booster seat awareness campaign launched at the same time by Safe Kids Canada and included a media campaign designed to enhance a care giver’s knowledge about the safety value of appropriate booster seats use (14–16). This was followed by Boost BC, an ongoing booster seat campaign (2004 to 2014) that included public awareness and education, online resources and distribution of free booster seats to qualifying parents.

In July 2008, BC introduced booster seat laws for children up to eight years of age and also updated its infant seat laws to require child passengers to be secured in a rear-facing child seat until at least one year of age. The upgraded child safety legislation also removed existing vehicle exemptions and prohibited placing a child safety seat in front of an active airbag. However, there is an exemption for children who cannot fit in a child seat for medical or physical reasons. In this case, the driver must have a medical certificate issued for the child by a medical practitioner (17).

The objective of the present study was to examine the safety benefits of the 2008 upgraded child safety restraint legislation in BC. Rather than investigate the prevalence of booster seat use, we investigated injury rates in children affected by the new laws.

METHODS

The present study was part of a comprehensive evaluation of the BC’s traffic laws approved by the University of British Columbia Research Ethics Board (Vancouver, BC). Deidentified BC traffic accident system (TAS) data were obtained for all MVCs that involved a child occupant zero to 14 years of age. The BC TAS contains details of all police-reported MVCs in BC including time, date, child passenger safety equipment use and child seating position, as well as information regarding the age of all motor vehicle occupants and their injuries. To estimate injury rates for child occupants, population data were obtained from Statistics Canada.

For the analysis, the change in rate of police-reported injuries in MVCs associated with the new legislation were estimated in two groups of children affected by the new laws: those zero to three years and those four to eight years of age. Included were all police-reported child passenger injuries that resulted from MVCs. Estimates were adjusted using the rate of injuries in MVCs in a control group of children not affected by the new legislation (those nine to 14 years of age). MVCs involving school buses and transit buses were excluded because booster seat laws did not apply to those vehicles.

An interrupted time series design was used to analyze traffic collision data from 2000 to 2012. The number of injured children zero to three, four to eight, and nine to 14 years of age were aggregated according to month. If more than one child was injured in a single collision, each child was counted separately. Injury counts were converted to monthly injury rates per 1,000,000 children, and rates were adjusted to account for the number of days in each month. Injury rates among children zero to three and four to eight years of age were analyzed separately using multiple linear regression models, in which the unit of analysis was time (months) and the response variable was injury rate. Each model included an indicator variable for the July 2008 child safety seat legislation intervention effect; a control group variable (injury rate among vehicle occupants nine to 14 years of age); and a seasonal ARIMA error term to control for seasonality, autocorrelation and data instability. The control group served as a surrogate for unmeasured confounders and trends unrelated to the intervention.

Estimates for reductions in injury rates were derived from the intervention indicator variable coefficient in the multiple regression models. The intervention coefficient was interpreted as the permanent and abrupt change in the average monthly injury rate following the new laws. This is similar to comparing the mean rate before and after the new laws; however, the estimate was adjusted for trends, seasonality and unmeasured confounders as explained above (18).

The use of data transformations and differencing were explored, but were not necessary to achieve a stationary time series that met the assumptions of the model. An iterative procedure was used to identify the order of the ARIMA error model. The initial model using a stepwise algorithm was selected to minimize the corrected Akaike information criterion (AIC). The correlation structures were explored via plots of the autocorrelation and partial autocorrelation functions. If the model fit was deemed inadequate, residual diagnostics were used to identify improved error structures. The model with the lowest AIC – that also satisfied residual assumptions – was selected. The error model was not constrained to be the same across age groups.

The models were fitted using conditional sum of squares to find starting values. The final estimates were obtained by maximum likelihood. The intervention effect was treated as abrupt and permanent. All regression analyses were performed using R version 3.0.1 (R Foundation for Statistical Computing, Austria); P<0.05 was considered to be statistically significant and all tests were two-sided.

RESULTS

Between January 1, 2000 and December 31, 2012, MVCs resulted in 1685 police-reported injuries and 17 fatalities in children zero to three, 3935 injuries and 28 fatalities in children four to eight, and 6376 injuries and 62 fatalities in children nine to 14 years of age.

Using injury rates in children nine to 14 years of age as controls, a 10.8% (95% CI 2.7% to 18.9%) reduction in the rate of injuries in four- to eight-year-old children was found (P=0.01). This is equivalent to a decrease of 14.3 injuries per 1,000,000 children per month (Figure 1). Similarly, the monthly injury rate among children zero to three years of age decreased by 13.0% (95% CI 1.5% to 24.6% [9.8 injuries per 1,000,000 children per month]; P=0.03) (Figure 2). The results also show that the percentage of children four to eight years of age involved in a police-reported MVC who were using a booster seat gradually increased from 11% in 2000 to 35% in 2008 (Figure 3). In addition, between 2000 and 2008, the percentage of four- to eight-year olds sitting in the rear seat gradually increased from 65% in 2000 to 80% in 2008 (Figure 4).

Figure 1).

Monthly rates of injured children in police-reported motor vehicle collisions (2000 to 2012) before and after the upgraded child safety legislation (July 2008). The black lines indicate injuries in children four to eight years of age; the grey lines indicate injuries in nine- to 14-year-olds (control group). The dotted lines represent smoothed monthly averages. For illustrative purposes, the smoothed averages were calculated from a fitted model in the treatment group but as observed rates in control group

Figure 2).

Monthly rates of injured children in police-reported collisions (2000 to 2012) before and after the upgraded child safety legislation (July 2008). The black lines indicate injuries in children zero to three years of age and the grey lines indicate injuries in nine- to 14-year-olds (control group). The dotted lines represent smoothed monthly averages. For illustrative purposes, the smoothed averages were calculated from a fitted model in the treatment group but as observed rates in control group

Figure 3).

Trends in the use of child safety equipment in child passengers involved in police-reported motor vehicle collisions. The top panel shows use of child restraints (either infant or booster seats) according to age group. Unfortunately, police collision reports do not differentiate between booster seats and infant seats; therefore, it is not certain that these children (especially younger ones) were in age-appropriate restraints. The bottom panel depicts the use of adult lap or lap-shoulder belts according to age group. These restraints are not appropriate for children younger than eight years of age

Figure 4).

Trends in seating position in child passengers involved in police reported motor vehicle collisions. To prevent air-bag related injuries, both the American Academy of Pediatrics and the Canadian Paediatric Society recommend that children younger than 13 years of age sit in rear seats (2,3)

DISCUSSION

The present study was the first to evaluate the effects of BC’s upgraded child safety seat legislation of July 2008. We searched the literature and could not find any other evaluations of Canadian child passenger safety laws based on police-reported MVCs invovling injuries. Worldwide, we found only five studies that reported a reduction in injuries or death following booster seat legislation (10,19–22). These studies used either a before-after design (20–22) or a case-control design (10,19). According to this research, child safety legislation based on the American Academy of Pediatrics best practice guidelines (with provisions for children up to eight-years-old and 4 ft, 9 in tall) (3) appears to result in the greatest safety benefits (20). Our study, the first to control for long-term trends using an interrupted time series design, provides further evidence that booster seat laws reduce childhood injury. We found statistically significant decreases in injuries in booster seat-age children (four to eight years) as well as in other child safety seat age children (zero to three years) after enactment of the new legislation. Using the injury rate in nine- to 14-year-old children as a control, we found that children zero to three years of age had a 13.0% decrease in MVC-related injuries, equivalent to 9.8 fewer injuries per 1,000,000 children per month. Similarly, children four to eight years of age had a 10.8% decrease in MVC-related injuries, equivalent to a decrease of 14.3 injuries per 1,000,000 children per month. Although the new laws primarily targeted booster seat-age children (four to eight years), we believe that the benefits seen in younger children are explained by the fact that the new laws also targeted children zero to three years of age by removing some exemptions to infant seat use and by stipulating that infant seats could not be placed in front of an active airbag. In addition, an extensive media, education and awareness campaign was launched to promote the new legislation, which likely had the effect of increasing caregiver awareness and compliance with appropriate child restraints in all age groups.

The traffic safety benefits observed in our study were likely mostly due to switching of children from inappropriate restraints (ie, seat belts) to age-appropriate infant or booster seats. Other research also provides evidence of increased booster seat use in four- to eight-year-old children in BC. An observational study conducted by the Child Passenger Safety Program in British Columbia in 2004 found that only 18% of four- to eight-year-old children were using booster seats (14,15). In contrast, a roadside observational study in 2010 (23), found that 40% of four- to eight-year-old children in BC were using an appropriate child safety seat. Finally, although not measured in our study, the ongoing education campaign associated with the new laws may have resulted in more caregivers applying child restraints correctly. Snowden et al (24) reported that in provinces with booster seat legislation, booster seats were more likely to be used and, when used, more likely to be used correctly.

We believe that the safety benefits and improved use of age-appropriate safety restraints found in our study resulted from a combination of the new laws and an ongoing education and awareness campaign that included free distribution of booster seats. There is strong evidence that caregivers are more likely to secure young children in safety restraints if their use is mandated by law (7,9,11,22,25,26). Our finding of a gradual increase in child restraint use in children four to eight years of age (Figure 3) suggests that some of the benefits attributed to the new legislation may be explained by pre-existing trends such as the ongoing education and awareness campaign, which was launched in 2004 before the new laws (15). This campaign was reinforced with free booster seat distribution and continued for at least six years after enactment of the 2008 booster seat legislation (27). Other researchers found that education and public awareness campaigns, especially when coupled with distribution of free child seats or other incentives, are effective in increasing child seat use (6). A Cochrane review (6) concluded that booster seat giveaways combined with laws mandating their use is a best evidence practice.

In addition to events occurring in BC, there may be increasing awareness across Canada of the need for age-appropriate child restraints. In 2002, many caregivers were unaware of the need for proper child restraints for children four to eight years of age (28). A more recent Canadian poll, published in 2011 (29), found that most parents believe that a booster seat is necessary until a child is physically able to use a seat belt on their own, and that 78% of Canadian households support booster seat laws.

Although most countries have infant and toddler restraint laws, child safety legislation for older children has not been implemented universally, despite evidence of effectiveness. According to a recent WHO report, only 96 countries, including only 32% of the world’s population, have enacted child safety seat legislation (30). It should be mentioned that, despite its proven effectiveness, booster seat legislation is not currently enacted in Alberta, the Yukon, the Northwest Territories or Nunavut (www.parachutecan-ada.org). The Canadian Paediatric Society encourages clinicians to advocate for enhanced child safety seat legislation (2). Similarly, the WHO strongly recommends that all countries enhance child passenger safety by mandating appropriate child safety seat use in their traffic regulations (30).

Strengths and limitations

During the study period, there were too few fatal MVCs involving young children to enable a meaningful analysis of the effects of new legislation on fatal crashes. Therefore, our evaluation was based on police-reported MVCs involving injuries. It is widely recognized that under-reporting is a limitation in police MVC data. However, under-reporting is less of a problem in injury MVCs (which we studied) compared with property damage-only collisions. Our study was strengthened by the use of ARIMA models and an internal control group consisting of nine- to 14-year-old children, who were not directly affected by the upgraded child safety seat legislation in 2008. This accounts for unmeasured confounders and trends unrelated to the intervention. A limitation of using children nine to 14 years of age as a control group is that some beneficial effect of the legislation may spillover for some smaller children in this age group (ie, those who are too small to be fitted with a regular seatbelt); however, we believe that this effect is very small.

CONCLUSION

Our findings suggest that BC’s new child safety restraint law, together with an ongoing public education and awareness campaign that included free distribution of booster seats, was associated with improved child occupant safety in the province. After enactment of this legislation in July 2008, there were significant reductions in child occupant injuries in both the zero- to three-year-old age group (child seat age) and in the four- to eight-year-old age group (booster seat age). Our findings provide further evidence to support upgrading child safety restraint legislation.