# Impact of Lowering Speed Limit on Urban Transportation Network
## 3 Methodology
### MOVES Analysis  
As previously mentioned, this study utilized OpModeof MOVES, developed by the U.S. Environmental Protection Agency (EPA). The MOVES OpMode approach uses emission factors according to the vehicle operation mode to classify 23 vehicle operation modes according to the microscopic operating conditions of the vehicle: speed per second, acceleration per second, and vehicle-specific power (VSP) per second. Emissions are calculated by applying emission factors appropriate to the mode (EPA, 2010). As it takes a long time to calculate when multiple vehicle exhaust gases are analyzed using the MOVES tool directly, the MOVES-matrix method is used as an alternative. MOVES-matrix is a model developed by the Georgia Institute of Technology research team which gives the same results as MOVES; its calculation is 200 times faster than the MOVES model. MOVES-Matrix calculates emission factors per unit length and unit time according to road environment, vehicle type, and vehicle age through MOVES, and configures a MOVES-matrix to minimize excessive calculation time that occurs during macroscopic analysis of vehicle exhaust gas [31]. The equation for estimating the emission using the calculated emission factor is calculated as the product of the emission factor, traffic volume, and section length. The micro-emission calculation method proposed in this study is as follows. * step 1. Micro traffic simulation: extract individual driving trajectory data using VISSIM COM Interface
* step 2. Emission analysis using MOVES OP Mode approach: calculate individual vehicle VSP  
To calculate the emission factor using the MOVES OpMode for micro-analysis, it is necessary to calculate the velocity and acceleration of the vehicle trajectory data in seconds as well as the VSP. VSP is calculated as the formula below. \[\text{VSP (STP)}=\left(\frac{A}{M}\right)\cdot\text{v}+\left(\frac{B}{M} \right)\cdot\text{v}^{2}+\left(\frac{C}{M}\right)\cdot\text{v}^{3}+\left( \frac{m}{M}\right)\cdot\left(\text{a}+\text{g}\cdot\sin\theta\right)\cdot \text{v} \tag{1}\]  
Figure 1: Study procedure. where:  
\(A\) = rolling resistance coefficient (kW-s/m),  
\(B\) = rotational resistance coefficient (kW-s/m\({}^{2}\)),  
\(C\) = aerodynamic drag coefficient (kW-s\({}^{3}\)/m\({}^{3}\)),  
\(m\) = mass of individual test vehicle (metric tons),  
\(M\) = fixed mass factor (metric tons),  
v = instantaneous vehicle velocity at time t (m/s),  
a = instantaneous vehicle acceleration (m/s\({}^{2}\)),  
g = gravitational acceleration (9.8 m/s\({}^{2}\)), and  
u = fractional road grade in percent grade angle (in this study, u = 0)  
step 3. Calculate operation mode ratio by scenario  
The VSP (or STP) is assigned to its proper operating mode bins, and the operating mode bins are assigned to emissions and fuel consumption rates stored in MOVES-matrix. To speed up the processing time, the research team prepared a subset of MOVES-matrix for the predefined scenarios. The scenario set is summarized below  
1. Calendar year: 2018; Month: July,
2. Temperature: 80F (average summer temperature in Seoul),
3. Humidity: 80% (average summer humidity in Seoul),
4. Region: Fulton County, Atlanta (used as a substitute because there is no available MOVES-matrix for Seoul at the time of the analysis; summer weather conditions in Atlanta are similar to those in Seoul), and
5. Fuel: default fuel supply and fuel share from MOVES  
Descriptions of the operating modes are provided in Figure 2. A MOVES-matrix is prepared for all four running processes, in emission-per-hour by individual source type, model year and operating mode bin (0-40). For each process, VHT (vehicle hours travelled) is calculated from VMT (vehicle miles travelled) and average speed.Then, emissions are estimated by multiplying VHT by source type, model year, and OpMode Bin with corresponding emission rates and then summing emissions by link. step 4. Calculate micro-emissions  
The last step is to calculate micro-emissions.