## 1 Introduction  
Tractor-trailers (TTs) are types of trucks (i.e., large trucks) that are mainly used for transporting different kind of goods and materials from one place to another. TTs can be classified into single-trailer trucks and multi-trailer trucks, with total length ranges between 21 m (69 ft) and 29 m (95 ft) [1]. Large trucks are essential to Egypt's economy, as the majority of goods are transported by them.  
For highways, there are two distinguished speed limit rules: the first is the uniform speed limit (USL), in which all vehicles operate with the same speed limit; and the second is the differential speed limit (DSL), where large vehicles and other types of vehicles operate with different speed limits. All Egyptian highways use the DSL rule, where large truck speed is restricted to be 30 km/h less than the highway speed limit for passenger cars (PCs). For highways with a speed limit of 90 km/h for PCs, the speed limit of large trucks is 60 km/h, while for highways with a speed limit of 100 km/h for PCs, the speed limit for large trucks is 70 km/h.  
Internationally, in Canada, the provinces of Ontario and Quebec are the only provinces to apply DSL to restrict the large truck speed limit [2] using mandated speed devices to limit large trucks' speed to 105 km/h for highways with a speed limit of 100 km/h [2; 3]. As of June 2016, seven of the fifty-two states in the United States of America (USA) operate a DSL for PCs and heavy vehicles on selected highways [4], while in June 2014 there were eight states [5]. The main idea of using the DSL strategy is to reduce the severity of collisions involving heavy vehicles [6; 7].  
Large trucks require a much greater stopping distance than light vehicles. When a large truck is involved in a collision, the outcome tends to be notable for its severity when compared to average motor vehicle collisions [3]. On average, approximately one in five Ontario traffic fatalities result from a collision involving a large truck [3]. Although, large truck drivers are far less likely to be guilty of a fatal accident than other drivers involved in the same accident [3]. Heavy vehicle collisions on high-speed highways are largely due to human errors such as speeding, drunk driving and fatigued driving. Speed is considered an important contributing factor to heavy vehicle collisions on high-speed highways [8].  
The DSL has two effects: the positive effect which results from the improvement in vehicle dynamics (brakes and maneuvers) for trucks at lower speeds; and the negative effect of the increase in speed fluctuations and the number of interactions between vehicles. These two effects act in opposite directions, and ultimately lead to an observable effect on safety data on motorways [9]. Based on the opinions of speed deviations from truck drivers, DSL increases interactions between vehicles and increases the probability of accidents [9].  
[PERSON] et al. (2003 and 2006) [10; 11] compared USL safety effects for all vehicles and DSLs for cars and heavy trucks for rural highways for the period 1991 to 2000 from nine states in the United States, and came to the conclusion that the speed limit directive does not have a uniform influence on safety [10; 11]. In addition, the results of [4] support the transition from DSL to USL on two-lane rural highways in Montana.