Merge pull request #246 from SANDAG/external_models_docs_update

(wiki) introduce documentation on external aggregate models

docs/design/demand/external.md

@@ -1,7 +1,42 @@
 # External Models
+The external aggregate travel models predict characteristics of US-SD and SD-US/MX travel behavior for all non-commercial, non-visitor vehicle trips and selected transit trips. Note that non-commercial MX-SD trips are forecast in the crossborder model, and non-commercial SD-US and SD-MX trips are forecast in the resident model.
 
-Details aggregate external models.
+![](../../images/design/external_external_county_cordons.png)
 
-## External Internal Model
+## External Model Estimation of Trip Counts by Type
+The total count of trips by production and attraction location was estimated in a series of steps:
 
-## External External Model
+1. The number of crossborder trips made by Mexican residents to attractions in San Diego was previously determined.
+2. The trips in the resident travel survey were expanded to estimate the total number of trips made by San Diego residents to attractions in Mexico.
+3. To derive an estimate of the number of US-MX trips, the number of MX-SD (1) and SD-MX (2) trips was subtracted from the total number of border-crossings.
+    * The distribution of US-MX trips among external stations on the US-side of San Diego County will be assumed to be proportional to the total volume at each external station, regardless of the point of entry at the Mexican border.
+4. The number of US-MX trips was then subtracted from the total number of trips in the SCAG cordon survey to arrive at an estimate of the combined total of US-US, US-SD, and SD-US trips with routes through San Diego County.
+5. Finally, the actual amounts of US-US, US-SD, and SD-US trips at each external station were estimated from the remaining trips (4) according to their proportions in the successfully geocoded responses in the SCAG cordon survey.
+
+## External Model Design Overview
+The behavioral characteristics of the different types of external trip were derived from the various data sources available as follows:
+
+* US-US trips: a fixed external station OD trip matrix was estimated from the SCAG cordon survey.
+    * */src/main/emme/toolbox/model/external_external.py*
+* US-MX trips: a fixed external station OD trip matrix was estimated from the SCAG cordon survey, Customs and Border Protection vehicle counts, and Mexican resident border-crossing survey as described in the previous section.
+    * */src/main/emme/toolbox/model/external_external.py*
+* US-SD trips: rates of vehicle trips per household for each external county were developed from the SCAG cordon survey, and the trips were distributed to locations in San Diego County according to a destination choice model estimated from the interregional survey.
+    * Intermediate stops and transit trips are not modeled in this segment due to the small contribution of these events to the total demand in the segment.
+    * */src/main/emme/toolbox/model/external_internal.py*
+
+### External-Internal Destination Choice Model
+The external-internal destination choice model distributes the EI trips to destinations within San Diego County. The EI destination choice model explanatory variables are:
+
+* Distance
+* The size of each sampled MGRA
+
+Diurnal and vehicle occupancy factors are then applied to the total daily trip tables to distribute the trips among shared ride modes and different times of day.
+
+### External-Internal Toll Choice Model
+The trips are then split among toll and non-toll paths according to a simplified toll choice model. The toll choice model included the following explanatory variables:
+
+* In-vehicle-time
+* Toll cost
+
+## Sources
+information primarily taken from this SANDAG document: [link to pdf](https://www.sandag.org/-/media/SANDAG/Documents/PDF/data-and-research/applied-research-and-performance-monitoring/surveys/appendix-t-SANDAG-travel-demand-model-documentation.pdf)