## 3 Results and Discussion
### Keywords and Trending Topics of the Research on Pavement Management
#### 3.2.1 Pavement Data Collection  
As all pavement roads deteriorate with time, mainly due to traffic loading and weather actions [56], consistent and repeatable data collection is an indispensable element of a PMS. Traditionally, manual surveys were conducted for data collection, which were time consuming and biased. Nonetheless, \begin{table}
\begin{tabular}{c l c} \hline \hline
**Ranking** & **Keywords (Times Used Each Expression)** & **Times Used** \\ \hline
1 & Pavements (123) + Pavement (38) & 161 \\
2 & Pavement management systems (84) + Pavement management system (21) + PMS (4) & 109 \\
3 & Pavement management (70) + Management (30) & 100 \\
4 & Maintenance (43) + Pavement maintenance (20) & 63 \\
5 & Pavement condition (30) + Pavement condition indices (7) + Condition index (3) & 40 \\
6 & Optimization (28) + Optimization models (6) + optimizations (3) & 37 \\
7 & Pavement performance (30) + Deterministic pavement performance models (4) + & 36 \\  & Performance model (3) + Performance prediction (3) & 34 \\
8 & Deterioration (16) + Pavement deterioration (9) + Deterioration models (6) + & 34 \\  & Deterioration modeling (3) & 29 \\
9 & Decision making (14) + Decision making process (6) + Decision support systems (6) + & 28 \\  & Decision trees (3) & 29 \\
10 & Highway administration & 28 \\
11 & Life cycle (11) + Lifecycle costs (8) + Life-cycle costs (4) + Life-cycle cost analysis (3) & 26 \\
12 & Information management & 26 \\
13 & Mathematical models (11) + Numerical model (11) & 22 \\
14 & Highway planning & 18 \\
15 & Design (14) + Pavement design (4) & 18 \\
16 & Flexible pavements & 16 \\
17 & Genetic algorithms (9) + Genetic algorithm (6) & 15 \\
18 & Transportation & 15 \\
19 & Maintenance and rehabilitations (6) + Pavement maintenance and rehabilitation (5) & 15 \\  & + Pavement maintenance and rehabilitation (4) & 15 \\
20 & International roughness index (11) + IRI (3) & 14 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The top 20 most frequently used keywords during the 2012–2013 period. automated technologies for data collection were developed [57], being still an essential research area. Pavement condition can be divided in two groups: functional and structural. The functional condition of a pavement refers to the effectiveness in fulfilling its function of serviceability, shown by smooth and safe ride, sufficient capacity for demand, without bumps and high noise levels. By contrast, the structural condition assesses the load carrying capacity of the pavement during its life [58]. Pavement texture, defined as \"the deviation of the pavement from a true planar surface\" [59], is a key measure of pavement surface characteristics, as it is directly related to safety and comfort [60]. Macrotexture measurement attracted great research attention [61]. New macrotexture measurement systems, based on 3D technologies, have been tested, and provide accurate and repeatable data at usual highway speeds [61; 62]. Similarly, a 3D pavement surface measurement tool was also introduced to collect rut data, the longitudinal depression that occurs in the wheel path, without interrupting the traffic [63]. When referring to road safety, the skid resistance is said to be a vital feature [60]. Recent research deals with finding a correlation between the skid number and the mean profile depth without interrupting traffic circulation [64], and to develop a 3D behavior model integrating skid resistance, travel speed, and wheel slip ratio [65]. Research also focused on skid resistance measurements on chip seals provided by three different tests [66], and the factors that best indicate the flushing occurrence, an area with low skid resistance that must be avoided [67]. Pavement distresses and cracking are evident signs of pavement deterioration. Measuring and mapping them has been challenging, due to their nature, location, and size. Traditionally, inspectors walking along the roadway section carried out detailed manual inspection surveys.