# Safety Effects of Freeway Hard Shoulder Running
###### Abstract  
Hard shoulder running (HSR) has been increasingly used as a sustainable and viable way to increase road capacity. This study investigated the safety effect of HSR on freeways in South Korea using the empirical Bayes method. This study found an increase in the total number of crashes. In terms of crash severity, a higher proportion of crashes (25.3%) on 2(3)-lane sections were found to be serious (involving injuries and/or fatalities) compared to those on 4(5)-lane sections (3.6%). Also, a positive relationship was found between the length of the hard shoulder running and changes in crash frequencies. Thus, hard shoulder running on lengthy 2(3)-lane freeways should be avoided. hard shoulder running; crashes involving injuries and/or fatalities; empirical Bayes method Article  
Safety Effects of Freeway Hard Shoulder Running  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON]. converting all or part of the shoulder into an additional lane and adjustment of the width of adjacent travel lanes is sometimes required as well. The lane created by this method is called hard shoulder running (HSR). Compared to the traditional method of increasing capacity by building more roads or constructing more lanes, HSR can be considered more sustainable because it utilizes the existing road shoulder during peak traffic hours to increase the road capacity. HSR has been increasingly used in Europe and North America to mitigate freeway congestion in a more sustainable way [9; 10; 11; 12]. Also, compared with roadway expansion, which usually costs about US$13.6 million per km in South Korea, a hard shoulder lane costs only about US$0.58 million per km [13; 14]. HSR was first adopted in South Korea a decade ago and is today gaining recognition within the general public as a favorable, low-cost alternative to tackling the congestion problem [15; 16]. On the other hand, transportation engineers in South Korea are divided in terms of their support for this design. The biggest concern for some engineers is a potential decrease in road safety. The purpose of having a road shoulder is to serve as an emergency stopping lane for motorists and to allow for recovery of errant vehicles before a potential collision occurs [17]. Its omission may therefore result in more frequent vehicle crashes [18; 19; 20]. The Korea Expressway Corporation (KEC), the central authority for operating the freeway system, acknowledges that vehicle crashes in freeway sections with HSR have increased [21]. As shown in the lower half of Figure 1, there were a total of 350 vehicle crashes in the three years before the introduction of HSR in the seven freeway corridors, but the number of vehicle crashes increased to 493 in the three years after the introduction of HSR in the same freeway corridors [21]. On the other hand, as shown in the top half of the figure, the number of crashes for the entire freeway (with and without HSR) has fallen for the corresponding before and after periods. Nevertheless, public support for the application of HSR remains strong in South Korea and proponents argue that there is no valid statistical evidence of more crashes with HSR in South Korea; also, there is international evidence that suggests it does not increase crashes [10]. Moreover, as shown in Figure 2, the repaved shoulder in South Korea is usually equipped with a lane control system that should mitigate safety concerns. Also, part of the increase in vehicle crashes could be attributed to an increase in traffic flow from an average of 119,793 vehicles/day to 134,480 vehicles/day. The increase in travel, whether for work or leisure, would contribute to the economy and improve the quality of life for users. Figure 1: Annual crash frequency before and after hard shoulder running. The authors' review of the literature suggests that the international evidence on HSR is mixed [9; 10; 11; 12; 18; 19; 20; 22; 23; 24]. More importantly, conflicting results and views have yet to be reconciled [19; 20; 24].