Roadside service facilities are generally designed to provide various services and opportunities depending on their functionality. These facilities fall into two general types: parking facilities (such as rest areas, parking and rides, and truck stops) as well as enforcement facilities (commonly recognized as fixed weigh stations or inception stations) [1, 2]. However, the rest area facility is one of the most common roadside service facilities designated for parking and resting purposes [3]. Essentially, this facility has been established to provide road users with various services to rest, sleep, eat, use the restroom, or check vehicles and goods [4, 5].

Drowsy and fatigued drivers use rest areas as safeguarded parking spots to pull over and rest before continuing their journey [6]. Although commercialized facilities, such as fuel stations, truck stops, and fast-food restaurants may provide more services than rest areas, accessibility to rest areas in limited-access roadways is more convenient and comfortable than these alternative facilities [7]. Rest areas also possess incomparable characteristics that are not found at other facilities, for instance, the ability to walk and enjoy the natural surroundings, accommodating features for children and disabled users, and available parking lots for heavy vehicles [7]. The locations of rest areas are designed to provide stopping opportunities in rural routes, between towns, and at state borders or large metropolitan regions where road users are in great need of services [8]. Rest area facilities generally help improve traffic safety, promote travelers' comfort and convenience, and readily provide relevant information to road users [6].

This paper focuses on the safety aspect of the presence and utilization of rest area facilities due to the extraordinary growth of freight transport, which reflects on the increase in the implementation of such facilities alongside transport channels. Research synthesis methodology is specifically applied to collect, summarize, and review the literature by concentrating on specific objectives in the topic of rest areas (i.e., potential safety impacts). Other researchers had focused on different aspects regarding this topic, such as parking demands, designs, landscapes, and operational cost-efficiency. The novelty of this paper is that according to our best knowledge, this is the first study that reviews the characteristics and potential impacts of rest areas.

The primary objective of this review is to extensively summarize and document information regarding the planning, benefits, and potential impacts of rest areas to enhance the understanding and evaluation of their features and essential influences. This will aid decision-makers and safety engineers in implementing effective policies that will improve the safety level in public transportation. This review further provides insight into the planning and essential benefits of rest area facilities. The potential adverse effects, challenges, and recommendations for implementing such facilities are also discussed.

The review is organized as follows: Section 2 provides the strategies and sources of literature in the paper. Section 3 presents the features of rest area planning. Section 4 discusses the three essential benefits of rest areas: enhanced traffic safety, comfort and convenience, and reduced excess travel and diversion. Section 5 highlights the potential adverse effects of rest areas. Section 6 examines the literature related to the challenges and recommendations of developing and implementing rest areas. Finally, Section 7 concludes the findings of this paper.

A literature search of major databases has been conducted to better understand the attributes and features of rest area systems. This includes the planning, benefits, possible adverse effects, as well as challenges and recommendations. Electronic database engines, such as Scopus, ProQuest, Transport Research International Documentation (TRID), Web of Science, and Google Scholar, have been intensively used during the literature search. The review consists of different research types such as international journals, conferences, as well as academic and technical reports.

The literature's scope focuses on four main subjects concerning rest area facilities: rest area planning, benefits of rest areas, potential adverse effects of rest areas, and the challenges and recommendations. The selection of studies was restricted to those published in English. The utilization, demand, and supply of rest area facilities were not within the scope of this paper. The scientific literature was retrieved using keywords and terms, such as “rest area,” “rest service area,” “truck parking,” “parking facility,” “commercial motor vehicle,” and “truck.” These terms were all separately combined with: “crash,” “accident,” “safety,” “fatigue,*” and “drowsing*.” The search then extracted a total of 74 references relevant to the aforementioned subjects. Table 1 illustrates the trend of references based on the publication year and subject concerned. Most studies were published between the period of 2011 and 2015 with 35 (47%), followed by the period after 2016 with 17 (23%). Each research paper or report was critically examined, summarized, and reviewed.

The planning process of the rest area system along a roadway should begin with a thorough development program. However, the remoteness of certain regions can further complicate the functional safety of rest areas [9]. Suggestions have been made for rest area planning components in Montana, specifically regarding location, development, operation, maintenance, and design [10, 11]. In terms of rest area planning, 16% and 25% of mainline peak traffic volume utilize rest points of arterial and interstate highways, respectively [7]. In 2012, the aspect of fatigued road users, in terms of public health and environmental issues, was investigated by Munala and Maina (2012) to suggest suitable planning engineering options along with road infrastructure to alleviate fatigue [12].

It is essential to consider the objective of rest areas and the needs of road users [13]. The three major groups of road users include general road users, truck drivers, and tourists. Each group has a distinct motivation for stopping [14]. General road users include village and town residents traveling for personal matters, such as shopping or work. Truck drivers must stop for a particular period following HOS regulations. Since tourists usually travel in diverse groups, they can choose when they would like to stop.

The planning of rest areas is based on the classification and amenities provided at a particular rest area location. However, classification and services are influenced by several parameters, such as the volume of traffic, highway classification, local factors, and economic analysis outcomes. Table 2 presents the different classes of rest areas and an overview of the necessary amenities and safety features [15].

Road users are encouraged to report their duration of stay at rest areas. Field observations can be simultaneously conducted to determine the user’s length of stay. This will help determine the type of services required at rest areas in the planning stage, including the number of toilets, phone communication, picnic tables, and parking bay sizes required [16, 17].

It is generally recommended to divide the area into functional zones to separate the traffic of specific vehicles on the site and maintain the safety of facility users (Fig. 1) [14].

3.1. Considerations for Spacing Distance Between Rest Areas

Rest areas are mostly designed for travelers on limited-access roadways who travel long distances. Drivers and passengers can rest, eat, or refuel without exiting onto secondary roads. Nonetheless, the past four decades have seen growing interest regarding the development, usage, and sufficiency of rest areas. According to rest area policies of development and maintenance in the US, under the AASHTO guidelines, the spacing distance of rest areas must be 100 km or 60 minutes of driving, as illustrated in Fig. (2). Long-term parking spaces for trucks should also be allocated at rest areas in a routine manner. In this case, suitable separate facilities within the rest site must be planned to meet all physical needs [18].

Fig. (1). Typical functional zones of rest area divisions [14].

Fig. (2). Example of heavy vehicle rest area (HVRA) spacing.

Table 2. Classification of different rest areas for heavy vehicles [15].

Key Features	HVRA Classification
	1 and 2	3 and 4	5
The flow of unidirectional freeways	√	√	√
No reverse movements (pull-through (clearway) capability)	√	√	√
Safe vehicle movement and access, including accommodating dimensions reflecting on the likely maximum truck size	√	√	√
Minimized chance for conflict between vehicles and pedestrians	√	√	-
Separation of light and heavy vehicles	√	-	-
Separation of vehicles carrying noisy freight	√	-	-
Separation for short term/long term visitors	√	-	-

Table 3. Various Types and Distances of Rest Areas by Countries.

Country	Type	Standard Spacing (Km)
U.S.	Service Area Safety Rest Area	100 40–50
Japan	Service Area Parking Area	50 15
U.K.	Service Area Service Area	48 100
Australia	Simplified Service Area Parking Area	50 30
China	Service Area	60
France	Service Area	20
Finland	Rest area	20
Spain	Rest Area Service Area	25 70
Poland	Rest Area Service Area	40 80
Korea	Service Area Safety Rest Area	25 42
Malaysia	Service Area Safety Rest Area	25 80
Nigeria	Rest Area	30
Denmark	Service Area Rest Area	30 70
Germany	Service Area Parking Area	35 10

The use of a rest area can be affected by facility characteristics, including capacity, accessibility, and visibility. The capacity of the rest area should accommodate the current parking demands. Moreover, to provide road users with a sense of safety, at least part of the rest area must be visible from the roadway. The other two main issues regarding public travel are cleanliness and safety [12]. In Finland, the local municipality is responsible for providing local maps and sanitary services. In terms of commercial activities, shops are in charge of the area's cleanliness [19]. According to the leading nations' road safety figures, the average rest area's spacing is roughly 15 km [20, 21]. Table 3 presents the standard spacing distance between consecutive rest areas in some countries. In Japan, rest areas are spaced every 50 km, and relatively small rest areas are set up every 15 km.

It was suggested that insufficient parking spaces are considered to be a factor of rest area planning [22-26]. The number of parking spaces provided should be given thorough consideration since distinct locations will have diverse traffic demand fluctuations. The combination of parking spaces allocated for light and heavy vehicles will vary between rest areas.

The rest area is a vital and integral element of primary road networks as it provides distinct benefits to road users, stakeholders, and external entities [6]. In general, the rest area offers quick access from the mainline and provides basic amenities, such as parking, restrooms, food, picnic tables, and travel information [7]. Three groups have been identified to benefit from the presence of rest areas on the mainline: road users, roadway stakeholders and other authorities, and outer entities (e.g., the tourism industry, local commercial businesses, etc.) [6]. The literature exclusively covers benefits associated with road users. Other beneficiary groups are not within the scope of this paper. Essentially, there are different groups of road users: passenger vehicle occupants, heavy vehicle operators, motorcyclists, vacationers, recreational vehicle drivers, etc [5]. The benefits of rest areas can be divided into three main categories: enhanced safety in public transportation, improved comfort and convenience of a roadway, and reduced excess travel and diversion [4, 6, 7].

The literature search has identified several studies that demonstrated diverse inferences concerning the safety impact of using rest area facilities. Numerous researchers have examined the safety effect of the presence of rest areas on roadways, further assessing the crash frequency and injury severity on their proximate segments. Several studies have found a positive relationship between fatigue-related crashes and the spacing distance between two consecutive rest areas [22, 30, 41-48].

Although the safety benefits of rest area facilities have been previously discussed, the design, location, and operation of rest areas and their proximate segments may pose safety concerns. Potential adverse effects of rest areas may result from the additional diverging and merging lanes and lane changing in the proximity of facility entrances and exits [1, 6, 63]. For instance, when truck drivers attempt to enter or exit a facility, a speed variation occurs on traffic stream, posing direct or indirect safety concerns at proximate segments [64]. Another example is that when trucks enter a rest area, they begin maneuvering and changing lanes to access the facility, thus causing safety risks at the mainline [1, 65].

Inadequate design or parking space shortage at rest areas may also contribute to road accidents along proximate segments [30, 44, 51, 54, 55]. Pigman et al. found that truck collision hot spots are directly related to parking facilities' proximity and utilization rate [23]. Banerjee et al. also reported a low crash frequency at rest area ramps and that most crashes related to rest area ramps occur in parking lots [30]. The authors suggested that inadequate design or parking space shortage may lead to this result. However, other researchers had found that crash severity decreased in the proximity of exit and entry ramp locations [66 – 68]. These studies have explained that drivers are usually attentive and reduce their speed while maneuvering to merge and diverge on ramps. Chiou & Fu, and Lang came to the same conclusion; however, they found that the presence of rest areas on roadways increases the likelihood of sustaining property damage only (PDO) during a crash [41, 69].

Essentially, several researchers have noted that the presence of rest areas in proximity to the mainline may generate road accidents (Table 8). McArthur et al. explained that proximate segments to entry ramps of rest areas experience high crash frequency due to access issues [44]. Banerjee et al. found that the patterns of non-fatigue-related crashes are significantly high within 30 miles downstream of rest areas and begin to decline after 30 miles [30]. However, there is no clear evidence or explanation for this phenomenon regarding these segments in particular. Alkhatni indicated that the presence of weigh stations and rest area facilities adjacent to the mainline are more likely to increase crash frequency along with their nearest adjacent segments [63]. Chiou & Fu found that the presence of rest areas increases the crash frequency, the percentage of injury, and PDO crashes on freeway segments [70]. Hernández & Anderson identified truck collision hotspots close to parking facilities and the segments between those facilities [71]. The researchers suggested that some locations that experience a high frequency of truck crashes may be related to parking facilities, yet this relationship is ambiguous. Manap et al. indicated that hotspots are concentrated near traffic flow disturbed by road facilities, such as interchange ramps, slip roads, rest areas, or lay-bys [72]. More recently, Hadi et al. have found that the rest area's presence increases the crash frequency on toll segments with dual two-lane and other toll segments [73]. Nevertheless, crash frequency reduces at toll segments with dual-three lanes and has no effect on toll segments with dual-four lanes.

Several issues and challenges have been encountered during the development and implementation of rest areas, including the spacing distance between rest areas, truck parking lot capacity, and the opportunities to decrease infrastructural costs while maintaining needed demand [7]. It is obvious from the literature that deficiency in rest area parking supplies poses safety and utilization concerns. Thus, it is highly recommended to embrace strategies to address the overcapacity issue and maintain an adequate parking supply [74]. In this regard, Fleger et al., Kimley Horn, and Pigman et al. suggested employing other roadside facilities such as park and ride lots, weigh stations or using abandoned facilities as supplement parking lots to mitigate the demand for parking facilities (i.e., rest areas and truck stops) [52, 59, 75]. Adopting public and private partnerships should be accomplished to cover the expenses associated with developing and constructing rest areas and road user services.

Numerous studies have investigated crash patterns around proximate segments of rest areas and parking facilities. Interestingly, this paper found several contradictory findings regarding the safety impacts of rest areas and parking facilities on road users. Some studies have concluded that these facilities are beneficial to improve the level of safety. Other research indicated that although these facilities may mitigate a particular type of crash (i.e., fatigue/sleep-related crashes), it is unclear how they affect other types of crashes.

Studies that explored fatigue-related crashes proximate to parking facilities further suggested that identifying the leading cause of such crashes is a difficult task and maybe inaccurate [6, 7, 30, 41, 44, 46, 51]. For instance, identifying a crash as fatigue-related may be biased and subjective from the aspect of police officers and/or other narrations. In 2011, fatigue-related crashes were 15%, and around 30% of collisions and fatalities from all crashes were in South Korea. This rate is very high compared to the US rate, which is 2.3% to 2.5% of all fatal crashes from 2011 to 2015, despite the vast difference in size between the two countries [45]. However, Wheaton et al. noted that between 15% to 33% of fatal crashes in the US are related to fatigue-related crashes [32]. Another study had stated that about 40% to 50% of severe injury crashes are related to fatigue driving accidents [22]. It can be noted that this variation in study outcomes may be attributed to inaccurate crash data and other factors. Fatigue-related crashes may be underreported for various reasons, such as insufficient report crash forms, biased police officer reports, and lack of information testimony [22, 34, 47, 76]. According to Stern et al., two primary reasons make it hard to define whether a crash is attributed to fatigue or driving while sleepy [34]. First, the absence of a credible method makes it difficult to determine whether fatigue or sleeping factors are the primary cause of crash incidences. Second, crash risks can be attributed to several factors, such as driver demographics and behavior, vehicle properties, and roadway environment. Overall, researchers recommend that further studies should be conducted to determine the appropriate approach for investigating fatigue driving data and fatigue crash estimations.

Researchers recommend several beneficial strategies to enhance the safety aspect of rest areas. Studies recommend educating road users on the hazards and consequences of fatigue and sleeping behind the wheel and their proper countermeasures. Real-time information regarding the locations of adequate rest areas should be provided to road users. The level of security and safety should also be enhanced at rest areas by equipping them with lighting, CCTV cameras, patrols, etc [23, 53, 74]. Implementing the DDA system between two consecutive rest areas, particularly those with long spacing distances and/or in rural areas, is considered an effective countermeasure for fatigue-related crashes [22, 47]. Electronic logging devices mandate strategy and stricter enforcement. These have been introduced to force truck drivers to comply with HOS regulations [77]. These devices further minimize shoulder parking incidents alongside roadway segments. Proper strategies should also be adopted such as expanding the current parking supply with the use of intelligent transportation systems (ITS) to improve parking capacity as well as constructing a new parking facility [57, 78]. Finally, introducing intelligent transport, such as vehicular automation, is expected to mitigate fatigue driving issues [79] which can help minimize the utilization and presence of rest areas alongside roadways in the future.

The review emphasizes the concern of Hernández and Anderson’s study, i.e., the presence of rest areas and their impact on injury severity, frequency of crashes (excluding fatigue-related crashes), and accurate estimation of crash patterns on proximate segments have not been thoroughly investigated [71]. Proximate segments of rest areas have also not been sufficiently identified.

The present paper provided an extensive review of the literature featuring the beneficial impacts of rest area facilities. The potential adverse effects, challenges, and recommendations were also addressed. This paper reviewed the current literature works addressing the characteristics of rest area planning. Three main categories of rest area benefits were discussed and summarized as follows: the enhancement of traffic safety, improvement in the comfort and convenience of roadways, and reduction of excess travel and diversion. The strategies and planning implemented to evaluate and measure the benefits of rest areas from the aspect of road users were briefly reviewed and discussed. The review found that although several studies have found that establishing rest area facilities proximate to roadway segments has positive effects on safety and operation, the presence of such facilities may also pose safety and operation risks along with their adjacent segments.

The study noticed that most research works focused on analyzing fatigue-related crashes at proximate segments of rest areas while neglecting other crash types. Therefore, this review also highlights the following research gap, i.e., investigating the relationship between roadway features and crashes occurring along proximate segments of rest areas to understand the distribution patterns of crashes that occur on prominent segments of such facilities. For instance, sensitivity comparison analysis should be performed on the collisions occurring near ramps of rest areas and similar geometric facilities (e.g., interchanges, slip roads, and weigh stations) to identify the contributing factors of crashes. Temporal and spatial analysis are other study areas that can be performed to determine crash patterns related to rest areas. Incorporating these study areas with a simulation approach to model the impact of rest areas on driving behavior would provide conclusive results.

Finally, the paper summarized and discussed the challenges and recommendations regarding the rest area's implementations and safety evaluation. This work also provided valuable information that can aid decision-makers and safety engineers in improving the benefits, safety, and planning of parking facilities in general.