# Safety Effects of Freeway Hard Shoulder Running
## 4 Discussion  
Our finding that installing HSR on freeways leads to increases in crashes is consistent with previous studies from the USA [48; 49] and Austria [50], but in contrast with the results obtained in another study from the USA [19], two studies from Germany [51; 52], a study from the United Kingdom [53], and one from France [23]. some of the differences in the results obtained may be attributed to differences in the driving culture between countries, while some of the differences may be due to the methods used to evaluate the safety impact. Most of the studies did not perform any before-after analysis, except one [23] that also used an empirical Bayes method. It should be noted that the number of freeway travel lanes also has an effect on crash frequency in sites with HSR. When total crash numbers are compared, the largest increase is found for sites with 4(5) lanes (four lanes without HSR and five lanes with HSR), with an estimated increase of 45.0 crashes. On the other hand, a moderate increase is found for 2(3) lanes, with an estimated increase of 8.7 crashes, and even a small reduction is found in the case of 3(4) lanes, with an estimated decrease of 1.4 crashes. However, in terms of crash severity, 25.3% of the estimated increase in crashes on 2(3)-lane sections is serious crashes (those involving injuries and/or fatalities), while only 3.6% of the increase in 4(5)-lane sites consists of serious crashes. The finding from this study on the effect of the number of lanes on crash frequency is consistent with a previous study that found that the increase in accident rate in 2(3)-lane freeways to be higher than the increase in accident rate for 3(4)-lane freeways [50]. Thus, it is advised that HSR should be applied with caution to freeways with two travel lanes. Moreover, further analysis of the results reveals a positive relationship between the length of the HSR and changes in crash frequencies. The estimated correlation coefficient between length and changes in total crashes (actual number of total crashes - expected number of total crashes) is positive and statistically significant (r = 0.468, \(p\) = 0.028). Similarly, the estimated correlation coefficient between length and changes in serious crashes (actual number of serious crashes - expected number of serious crashes) is positive and statistically significant (r = 0.591, \(p\) = 0.004). These findings are consistent with previous studies on the safety effects of HSR [22; 49; 50]. The results are expected as an increase in freeway length increases exposure, which is a significant contributing factor to crash frequency. Also, more traffic flow disturbance is expected to occur with longer designs than with shorter designs, and consequently, traffic incidents will be more likely to occur. Therefore, we recommend that lengthy HSR should be avoided. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Site**} & \multicolumn{3}{c}{**Actual Crashes**} & \multicolumn{3}{c}{**Expected Crashes**} \\  & \multicolumn{3}{c}{**(Crashes/3 years)**} & \multicolumn{3}{c}{**(Crashes/3 years)**} \\ \cline{2-7}  & **Total** & **Serious** & **PDO** & **Total** & **Serious** & **PDO** \\ \hline
10 & 67 & 14 & 53 & 59.1 & 13.2 & 46.7 \\
11 & 66 & 7 & 59 & 59.1 & 6.8 & 52.7 \\
12 & 33 & 5 & 28 & 30 & 4.9 & 25.4 \\
13 & 3 & 0 & 3 & 3 & 0.4 & 2.8 \\
14 & 1 & 1 & 0 & 2.6 & 1.3 & 1.8 \\
15 & 14 & 5 & 9 & 12.6 & 4.7 & 8.7 \\
16 & 33 & 20 & 13 & 29.3 & 16.7 & 14.2 \\
17 & 4 & 1 & 3 & 4.7 & 1.6 & 3.8 \\
18 & 5 & 1 & 4 & 5 & 1.3 & 4.1 \\
19 & 4 & 0 & 4 & 2.8 & 0.3 & 2.7 \\
20 & 17 & 7 & 10 & 14.7 & 6.2 & 9.5 \\
21 & 12 & 2 & 10 & 10.1 & 3.0 & 10.6 \\
22 & 20 & 2 & 18 & 7.1 & 3.2 & 7.8 \\ \hline Sum & 493 & 99 & 394 & 419.6 & 96.3 & 340.6 \\ \hline Average & 22.4 & 4.5 & 17.9 & 19.1 & 4.4 & 15.5 \\ \hline \hline \end{tabular}
\end{table}