Pavement or road surfaces can be categorized into four general classes, i.e., asphalt, concrete, gravel, and brick and block [16]. Asphalt, also known as flexible pavement, is widely used to construct national, regional, or local roads across the road network and has different sub-categories depending on its construction. Over 90% of the total European road network has an asphalt surface. Concrete surfaces are usually used in urban environments and can be subdivided into joined cement concrete and continuously reinforced concrete surfaces [17]. Concrete pavements are expensive and time-consuming to construct, but they are typically more potent and durable than asphalt roadways. They are more common in the USA; for example, approximately 60 percent of the interstate system in the USA is concrete. Pavement condition assessment considers several pavement characteristics, i.e., roughness, surface condition (distress detection), surface skid resistance, and pavement strength. Surface condition plays a significant role in pavement assessment, which requires pavement distress detection and quantification. Pavement surface distresses that occur in different geographical regions can be divided into six groups, i.e., cracks, surface openings, surface deformation, surface defects, joint deficiencies, and miscellaneous distress [17,18] (see Table 1).

Most of these distresses can be detected generally through visual inspection (standard practice) of pavement surfaces, and their severity and quantity can be recorded using manual measurement tools [17]. Visual distresses appears on the surface due to wear and tear, which may indicate a fault in the construction. It may appear differently in rural and urban regions, depending on the surface type, the severity (low, medium, high) of the underlying problem, and other environmental conditions.

Measuring different pavement characteristics is essential in long-term pavement performance incorporating all or a subset of pavement characteristics to conduct pavement assessments. These condition rating systems vary from country to country (or within a state in the USA), considering local variations, the characteristics of the pavements, and economic conditions.

Pavement characteristics that are generally separately measured include pavement roughness; a vital pavement characteristic measured on a rating index known as the International Roughness Index (IRI) [19]. It is estimated in a moving vehicle from a longitudinal pavement profile with sensors capable of measuring vertical movement [20,21]. Another essential characteristic is transverse deflection, also known as rut depth, measured manually or using sensors that generate transverse pavement profiles [9]. Visual pavement condition assessment requires distress detection and quantification to measure pavement conditions and is more reliable than other methods are. Engineers and professionals have proposed several standards for visual surface assessment, such as Pavement Surface Evaluation Rating (PASER) [2], Pavement Condition Index (PCI) [19,22], Pavement Surface Condition Index (PSCI) [23], and the Road Condition Indicator (RCI) [24]. Table 2 lists different pavement condition ratings used around the world. The standard ratings of various regions differ in scale granularity, formula to estimate a value on the rating scale, and data acquisition procedure.

The earliest work in creating a standardized condition assessment scale dates from the 1960s in the United States [25]. The scale used two pavement characteristics-pavement roughness and visual surface distress identification, to determine the Present Serviceability Index (PSI) ranges from zero (very poor) to five (very good condition). A roughness index was carried out by 3–5 individual raters trained to qualitatively estimate pavement roughness by driving a vehicle on the pavement. It was followed by visual inspection for cracks, patches, and potholes. These two were then combined mathematically to calculate the PSI score (0–5) [25].

Over the years, data acquisition techniques have evolved; different pavement condition assessment ratings have been proposed that mainly focus on assessing the different types of pavement characteristics, their quantity, and their effect on the overall condition of the pavement. PASER is a direct rating on a scale of 10–1 (9–10 is excellent condition, while 2–1 is extremely poor). On the other hand, the ASTM standard for pavement is PCI, a rating on a scale of 100–0 (85–100 is a good condition, while 0–10 is completely deteriorated. It is mathematically based on distress occurrence and severity level. The Irish PSCI [18,23,26] rating is on a scale from 1–10, similar to PASER, where index-1 is the lowest (surface completely worn out or failed), and index-10 (no distress, new pavement) is the highest. It covers flexible urban pavements, urban concrete pavements, and flexible rural pavement separately. PSCI ratings are given to continuous stretches of pavements with similar conditions, with 200 m being the minimum length to have their distinct rating [26]. In the United States, the Federal Land Transportation program recommends visual distress detection based on PASER for direct pavement condition evaluation [27]. Some transportation departments (or road authorities) that use scales similar to PCI use a subset of the visual distresses and roughness index to calculate the PCI rating. For example, the New Zealand Road Assessment and Maintenance Management System (RAMM) assigns a CI (Condition Index) from 0–100 (0–Excellent—100–Failed); it includes a visual inspection of not only the pavement but the surface water channels along the pavement [28]. China uses the Chinese Pavement Condition Index (CPCI), a scale similar to PCI, and considers cracking, raveling, potholes, rutting, and roughness. Japan used the Maintenance Control Index (MCI) until 2005, a function of cracking, rutting, and roughness, on a scale of 10 to 0 [29]. After 2005, the Ministry of Transportation Japan has used RRI, which is a function of cracking ratio, rutting depth, and International Roughtness Index [29]. A similar index is used in Tajikistan under Japan International Cooperation Agency [30].

The RCI is a rating from 1–4 (with 1 meaning no physical deterioration, while 4 is severe deterioration), adopted in England, Wales, Scotland, and Northern Ireland, and fuses visual condition and gauging parameters of pavement condition [24]. In Germany, the RMA (Road Monitoring and Assessment) protocol rate the pavement into four categories based on visual distresses [31]. Some states use four classes in the USA, i.e., Good, Fair, Poor, Very-Poor, as a condition scale based on the original PSI rating. In some countries, such as India and Brazil, a visible pavement distress condition rating on a scale of 0 to 3 is used [32]. Ratings are based on cracking, rutting, raveling, patching, and potholes, while roughness is not considered [3]. Pavement condition surveys of national and local roads are commonly conducted annually, every two years, or every five years in different regions across the world (for example, in Ireland, they are conducted every two years, while in Florida, state highway surveys are completed annually [33]). Therefore, these survey methods should be quick, fast, reliable, and economical.

In summary, different regions have different ways of performing pavement condition rating; some take roughness and visual condition combined to assign a rating from a standard scale (e.g., China, Japan, and some states in the USA), while others rate only a subset of visual distress (e.g., UK, Ireland, Brazil, Germany, New Zealand, India, and some states in the USA). Some of these indices are very granular (1 to 100) such as PCI in some parts of USA versus that (0 to 3 scale) used in Brazil/India. The choice of scale has evolved with economic prosperity and maturity of the road network.