## 3 Results and Discussion
### Keywords and Trending Topics of the Research on Pavement Management
#### Development of New Pavement Management Systems and Sustainability  
A PMS is defined as a set of techniques and methods that provide support to road agencies in order to allocate limited resources optimally while maintaining the road networks in the level of service demanded by users. As a consequence, use of optimization techniques for pavement management has increased in the last few decades, due to more limited budgets, increasing demands, and stricter responsibility in infrastructure allocation [139]. In general, optimization tools applied in pavement management can be classified into single-objective or multi-objective optimization. Single-objective optimization follows different targets, such as to minimize cost or maximize the treatment benefits in terms of pavement's condition or life [140]. Nonetheless, it is not recommended to make a decision according to a sole criterion, especially in road management, where more than one goal must be taken into account. On the contrary, multi-objective optimization attempts to find an optimal solution satisfying various restrictions and optimizing some objective functions. Therefore, the research on pavement management become an appropriate area for developing the multi-objective optimization methods, confirmed by the keywords \"optimization\" and related words (6 th position in Table 6), \"decision making\" (9 th), or \"genetic algorithm(s)\" (17 th). In the pavement management field, diverse methods have been applied in the current century to solve multi-objective optimization problems with varying success. Examples of applied techniques in the period 2012-2013 are the weighting sum method [141], compromising programming [142], analytic hierarchy process [143], the parametric method [144], and genetic or evolutionary algorithms [145; 146; 147]. Sometimes, two methods are combined to solve an optimization, such as the generic algorithm and the weighting sum method [104; 105], or the genetic algorithm and goal programming [148]. Optimization has been also utilized to allocate resources between competing elements of highway asset management too, such as pavement and bridge deck maintenance [1], heterogeneous infrastructure systems [149], or between various programs, like pavement improvement and capacity expansion of the road [150]. Moreover, some researchers have exported the idea of PMSs to other infrastructures, usually not analyzed, like median and sidewalks [151], road segments in landfills [152], and parking [153]. The World Bank, as explained in Section 3.2.2, has developed several evaluation modules to combine technical and economic appraisals of road projects, in order to prepare road investment programs and to analyze road network strategies. The last version, HDM-4 (Highway Development & Management) still provided advanced highway investment evaluation tools [154], which were calibrated in Malaysia [155] and in Taiwan [156]. At the project level of PMSs, decisions are targeted to define the projects with the best cost-benefit ratio, so the limited budgets are used to construct, maintain, or rehabilitate longer lasting roads, considering all the road-life [157]. Accordingly, the concept of lifecycle cost analysis (LCCA) is highlighted during the project-level analysis. LCCA, born in the United States, is utilized to evaluate competing alternatives that take into account all related significant costs, e.g., construction, maintenance, and operations, over the life analysis period, and has become the most successful technique worldwide [2; 158]. LCCA received increasing attention as a methodology to help road agencies in order to make economical investment decisions, (11 th position in Table 6). Researchers developed advanced models to conveniently assist the decision-making process.