Figure 2. The time series of standardized daily RTIs in Guangzhou, 2009–2012. Daily RTIs are standardized by a unit of 1 million population and 1 million vehicles. The vertical line indicates the enforcement of the law on May 1, 2011, in China. The curve represents the smooth lines of RTIs against the date (natural spline with 4 degrees of freedom per year). The straight lines represent the slopes before and after the intervention. The X-axis represents the calendar day during the study period. The Y-axis represents the number of standardized daily RTIs. RTI, road traffic injuries.