# Safety Effects of Freeway Hard Shoulder Running
## 2 Methodology  
This study uses the EB method to evaluate the safety effect of HSR and to identify the factors contributing to crashes. The existing literature indicates that three years is the most common choice for the timeframe for before-after collision studies, because traffic flow for three years is unlikely to have changed a great deal [39] and this period is expected to provide a relatively reliable sample. In December 2015, there were eight freeway corridors with HSR [14], in which 32 different HSR sections were constructed. The total length was 222.6 km. Of the 32 sections, this study selected 22 HSR sections with a total length of 162.15 km, for which complete data were available (see Table 1). In terms of the number of traffic lanes, the selected sites consisted of 14 eight-lane freeways, one six-lane freeway, and seven four-lane freeways. Their construction period ranged from 2009 to 2013.  
In applying the EB method for before-after studies, one critical element is the availability of a safety performance function (SPF). An SPF is an equation/regression model that is used to predict the average number of crashes per year at a location as a function of exposure and geometric design elements (number of lanes, length of section, etc.). While the Highway Safety Manual [17] has specified SPFs for typical roadway sections developed from data in the USA, researchers can also develop SPFs that relate directly to their site-specific conditions. For this study, an SPF was developed using a select set of reference sites that have similar traffic and geometric conditions to those of the treatment sites. Therefore, this study selected 22 freeway sites located immediately upstream and downstream of a HSR, as well as 86 freeway sections located on the same freeways but farther away from the HSR sites (see Table 2). Data on these sites were used in the estimation of the safety performance function used in the EB method.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Site**} & \multirow{3}{*}{\
\begin{tabular}{c} **Length** \\ **(km)** \\ \end{tabular} } & \multirow{3}{*}{
\begin{tabular}{c} **Lanes per** \\ **Way*** \\ \end{tabular} } & \multicolumn{6}{c}{**Before HSR (2006–2008)**} & \multicolumn{6}{c}{**After HSR (2014–2010)**} \\ \cline{3-14}  & & & & **Ave.** & & & **Lane** & & **Ave.** & & & & \\  & & & & **Seed** & & **Seed** & & **Seed** & & **Seed** & & **Seed** & **Crashes per** \\  & & & & & **Seed** & & **Seed** & & **Seed** & & **Seed** & & **Seed** \\ \hline
1 & 6.7 & 4(5) & 53,500 & 93.2 & 12 & 3.1 & 3.50 & 2.85 & 60,400 & 95.2 & 26 & 5.9 \\
2 & 8.4 & 4(5) & 50,600 & 95.1  
Lastly, to determine the various safety effects of deploying HSR in freeways, this study examined its effects on serious crashes (those involving injuries and/or fatalities), property damage only (PDO) crashes, and total crashes. This approach was required because the crash impact from each independent variable on the dependent variables would be different in the safety performance functions. Moreover, it would also provide useful information on the effects of HSR on crashes of different severity levels.