## 1 Introduction
The length of the passing auxiliary lane is an important feature of the \"2+1\" design that would affect both safety and mobility [9] and is considered the main feature of the 2+1 designs, which allows drivers to comfortably overtake other vehicles [4]. A passing-lane length of less than 1000 m is not recommended [9; 10]. Internationally, the passing-lane length is between 800 to 3500 m [11]. [PERSON] (1995) recommended that passing-lane section lengths between 1000 and 1400 m for volumes of up to 1000 vehicles per hour per direction, with heavy vehicles (HVs) percentages of up to 15% [12]. Furthermore, the Swedish design guidelines recommend that overtaking sections be 1-2.50 km long [13]. Lastly, the range of lane widths used internationally for a single passing lane within the 2+1 design was between 3.25 and 3.75 m [11]. [PERSON] et al. (1988) provided instructions for identifying, constructing, and marking passing lanes efficiently to enhance traffic operations [14]. In addition, they provided, with other researchers, a method for determining passing lanes' operational efficiency in terms of enhanced service [14; 15]. [PERSON] (2017) simulated the effect of heavy vehicles (HVs) shares (up to 20%) in the traffic stream of a 2+1 long road bypass, using PTV VISSIM [16], and found that the 2+1 system appears to be reasonably effective when compared to the TLTW bypass for traffic volumes up to an average annual daily traffic (AADT) of 22,000 [17]. Furthermore, [PERSON] et al. (2014) recommended that speed limits for the 2+1 design be between 80 and 100 km/h [9]. Moreover, internationally, some 2+1 designs prohibit overtaking in the opposite direction, while others permit it [11]. One of the main benefits of the 2+1 design, which prohibits overtaking in the opposite direction, is the elimination of head-on collisions, which are the main threat in the conventional TLTW design. In Poland, the introduction of a 2+1 design instead of a conventional two-lane road design for treated sites that have 16 segments and extends 12.9 km long could reduce total crashes by 45% [18]. In addition, [PERSON] and [PERSON] (1985) found that using passing lanes could potentially reduce the accident rate by about 9% and 17% for total and fatal and injury crashes, respectively, based on data from 22 passing sites from different states in the USA [19]. They reported that these reductions were not statistically significant, as there was only one year of accident data in the after period [19]. The Swedish experience of installing a 2+1 fence with cable barriers along the median was examined by [PERSON] et al. (2003) [20], who reported reductions of up to 50% for all crash types and up to 90% for fatal crashes [20]. According to the NCHRP (2003) report, which focuses on the application of the European 2+1 roadway designs, there was a 25% decrease in all crash types in Finland, with a 45% decrease in fatal crashes [6]. As far as the authors are aware, there are presently no Middle Eastern countries that have single 2+1 roads. Hence, the primary objective of this study is to evaluate the 2+1 roads for Middle Eastern conditions and assess their safety and mobility performance compared to the conventional TLTW roads. Different design scenarios were considered: (1) a design that allows opposite-direction overtaking and (2) another design that prohibits opposite-direction overtaking. Moreover, different speed-limit strategies were investigated, namely, (1) the uniform speed-limit (USL) strategy, in which all vehicles (i.e., passenger cars and heavy vehicles) operate with the same speed limit and (2) the differential speed-limit (DSL) strategy, where large vehicles and other types of vehicles operate with different speed limits. The DSL strategy is the chosen strategy for speed limits on Eastern countries' highways. For example, on Egyptian highways, the truck trailers' speed limit is restricted to 30 km/h lower than the speed limit for passenger cars [21].