## 4 Methodological Procedure to Estimate Emissions in Road Transport Corridors  
The methodology uses the Social and Environmental Effects (SEA) sub-model of the Highway Development and Management System, known as HDM-4 [36]. The HDM-4 is useful because this tool was specifically designed for roads, and the main objective of this research work is to estimate emissions from road transport in major freight corridors of the country.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**No.** & **Climate Zone** & **Description** \\ \hline
1 & Af & Tropical with year-round rainfall \\ \hline
2 & Am & Tropical with monsoon rains (short dry) \\ \hline
3 & Aw & Tropical with summer rains (winter dry) \\ \hline
4 & Bs & Semi-arid (steppe climate) \\ \hline
5 & Bw & Arid (desert climate) \\ \hline
6 & Cf & Temperate with year-round rainfall \\ \hline
7 & Cw & Temperate with summer rain (dry winter) \\ \hline
8 & Cs & Temperate with winter rains (dry summers) \\ \hline \hline \end{tabular} Source: Own elaboration based on [PERSON] (1998) [45].  
\end{table}
Table 3: Climate classification in México.  
The sub-model SEA of HDM-4 calculates at a macro level, the quantity of emissions in the form of chemicals and noise generated by the operation of transport on a road network or road segment, and the energy balance of the life cycle of conservation strategies (see Figure 3).  
The HDM-4 emission model is based on the proposal by [PERSON] [48] and predicts emissions from vehicle exhaust in terms of fuel consumption and average speed. Relationships and model coefficients were adjusted in terms of grams per kilometer [49].  
Vehicle emission inventory based on a bottom-up approach requires the following information: emission factors according to the number of vehicles, vehicle age, and type of vehicle; the distances traveled by each type of vehicle; environmental data to characterize the study area, such as altitude and temperature; the quality state of the road network and comprising elements such as the pavement surface, operating speeds [32].  
The proposed methodology for estimating emissions for transport operations on roads consists of three stages, as shown in Figure 4. The main effort is focused on the first stage, which is necessary to collect traffic data from the transport corridors, as well as obtaining information about the road geometry (horizontal and vertical alignment), characterization of the vehicle fleet (classification, age, etc.), environmental data (altitude, climate zone, etc.), where each road section is located, and the quality of pavement surface thought the IRI (see Section 3 \"Methodology and Data Collection\"). Stage 2 consists of the data capture process within the HDM-4 to perform all analyses. In Stage 3, the output of the analysis derived from the emissions model is obtained. The following pollutant emissions are estimated: hydrocarbons, nitrogen oxides, sulfur oxides, carbon monoxide, carbon dioxide, and particulate matter [50].  
Figure 4: Methodology for estimating emissions of motor vehicles on road corridors. Source: Own elaboration. Notes: hydrocarbons (HC), nitrogen oxides (NOx), sulfur oxides (SO2), carbon monoxide (CO), carbon dioxide (CO2) and particulate matter (PM). The results are expressed in tons per kilometer per thousand vehicles.  
Figure 3: Structure of sub-model of Environmental Effects HDM-4. Source: Own elaboration based on World Road Association (PIARC) [36].