Memorandum
Date: November 27, 2023
To: Turner Lott, SGVCOG
Steph Wong, SGVCOG
Roy Choi, SGVCOG
From: Katy Cole, Jolene Hayes, and Biling Liu
Subject: City of Diamond Bar
Analysis of the City’s VMT Thresholds (Deliverable 2.3) OC20- 0715.02
This technical memorandum summarizes the vehicle miles traveled (VMT) differences between the SCAG 2016 and 2020 Regional Transportation Plan/Sustainable Communities (RTP/SCS) models and how these differences may affect the thresholds for vehicle miles of travel (VMT) impact analyses in the City of Diamond Bar (the City). The current SGVCOG VMT Tool is based on VMT outputs from the 2016 Regional Transportation Plan/Sustainable Communities Strategy (RTP/SCS) trip-based model (TBM) and needs to be updated to be consistent with the 2020 RTP/SCS activity-based model (ABM). The change of both the model simulation platform (trip-based model to activity-based model) and the underlying data, such as traffic counts and socioeconomic data, results in changes to the VMT threshold numbers and low VMT areas. The following memorandum describes the changes in VMT, and more specifically, the change in the number of low VMT areas that meet the low VMT screening criterion.
VMT Calculations
For the purpose of updating the SGVCOG VMT Evaluation Tool, we first considered how the change in model platforms (trip-based to activity-based) would change the low VMT areas in the City. In addition to the model platform, the change in the underlying inputs to the model (e.g., population, employment, traffic counts, etc.) also resulted in changes to the low VMT areas as illustrated by the differences between the 2016 and 2020 Total VMT per total population and employment (OD VMT/SP). This calculation is the same for both model versions, therefore, the change in low VMT areas shown in Table 1 for OD VMT/SP reflects the effect of the new data inputs only, whereas the changes to home-based VMT per capita and work-based VMT per SGVCOG VMT Tool Update
November 27, 2023
employee are combinations of data updates and the change in model methodologies (trips versus tours).
Model Methodology Comparisons
The SCAG 2016 RTP/SCS trip-based model (TBM) VMT methodology includes all trips within the SCAG model for each of the following variable formats:
1. Origin-Destination (OD) VMT per Service Population (all vehicles and all trip purposes): The total VMT to and from all traffic analysis zones (TAZ) in the geographic area are divided by the total service population to calculate the efficiency metric of VMT per service population. Service population is defined as all residents and employees of the zones being evaluated. The image shows a visual representation of OD VMT per Service Population for a single project. To calculate it for a set geography, an analyst determines the total VMT produced to or from the geography divided by the total population plus employment of that same geography.
2. Home-based VMT per capita (automobile only): All home-based trips are traced back to the residence of the trip-maker. Trips without an origin or destination from home are excluded. For example, if you leave home and stop for coffee, then for gas, and then at the store before returning home, the trip from coffee to the gas station and from the gas station to the store would be deemed an “other to other” trip and omitted from HB VMT. The image shows a visual
representation of Home-Based VMT per capita for a single home.
3. Home-based work VMT per employee: Captures only the segment of the trip to work that directly connects home to work. All stops along the way to or from work are defined as work to/from other and are not part of the HBW VMT calculation. The image shows a visual representation of Home-Based work VMT per employee for a single home.
OD VMT Per Service Population
Home-Based VMT per Capita
Home-Based Work VMT per
Employee
SGVCOG VMT Tool Update
November 27, 2023
The SCAG 2020/RTP/SCS model, an activity-based model (ABM), is quite different than the TBM because it considers the entire linked trip, or tour, that a person makes when leaving home or commuting to work. Whereas the calculation for total VMT per service population is the same under both models, the home-based and home-based work calculations are different.
1. OD VMT per Service Population (all vehicles and all trip purposes): The total VMT to and from all zones in the geographic area are divided by the total service population to calculate the efficiency metric of VMT per service population. This metric is the same as described above for the TBM.
2. VMT per capita (automobile only): Considers VMT for all tours embarked upon by residents, encompassing both Single Occupancy Vehicle (SOV) and High-Occupancy Vehicle (HOV) drivers (drivers only). The VMT is divided by the population within the geographic area. This method includes all trips made by the population of the geographic area, including those that do not directly start or end at home. For example, if you leave home and stop for coffee, then for gas, and then at the store before returning home, all of these legs of the trip together would be a “tour” and they are all included in the calculation. The image shows a visual representation of tour-based VMT per capita for a single home.
3. VMT per employee: Encompasses VMT for all work-related tours, encompassing both work and work-related business. The VMT is divided by the employees within the geographic area. For example, if a person commutes to work, then has a lunch meeting and runs a work errand mid-day, and then commutes home from work stopping at the gym on the way, all of these legs are summed together as work-related tours and are included in the calculation.
Baseline Data Differences (2016 vs. 2020 SCAG RTP/SCS)
The SCAG 2016 RTP/SCS was calibrated and validated based on traffic counts collected in 2012, the 2012 roadway network, 2012 transit network and ridership information, land use information, and other socio-economic inputs. The 2020 RTP/SCS was calibrated and validated based on the datapoints collected in 2016. The changes in the underlying data affected the low VMT areas within the SGVCOG region. To conduct comparative VMT results, the VMT from the SCAG 2016 RTP/SCS and 2020 RTP/SCS were both interpolated to 2023 baseline in the next VMT Comparison section.
VMT per Capita (TourBased)
VMT per Employee (TourBased)
VMT Comparison
Diamond Bar is located in the Southeast Subarea of the SGVCOG region. According to the City of Diamond Bar Transportation Study Guidelines for Vehicle Miles Traveled and Level of Service Assessment (August 2020), the City adopted the City of Diamond Bar (City) baseline VMT as the benchmark to determine CEQA transportation impacts for all project types (residential, employment, and mixed-use/regional retail). A comparison of the number of low VMT areas under the 2016 versus the 2020 RTP using the City’s adopted threshold (15% below the City’s baseline) is shown in Table 1.
Table 1: Comparison of Low VMT Areas Using Diamond Bar’s Adopted VMT Baseline (2016 vs. 2020 RTP/SCS) Pursuant to the OPR Guidance, cities can adopt different baselines based on project types and states that employmentbased VMT thresholds should use subregional or regional baselines. The City’s current threshold (City) is appropriate for Total and home-based VMT types, but the City should consider using the Southeast as the baseline for employment-based VMT (VMT/Employee) consistent with OPR’s guidance. The threshold for Total VMT, since it is a mix of both residential and employment, can use any baseline.
Table 2 shows the comparison of the thresholds for the different baseline options (SGVCOG, Southeast Subarea, and City) using the 2020 RTP model. The higher thresholds result in the ability to screen out more projects, whereby more transportation analysis zones (TAZs) generate less miles of travel per person than the baseline. For example, for Total VMT per service population (OD VMT/SP), a project that generates 31.7 miles of travel per person would exceed the thresholds for the SGVCOG and the Southeast Subarea of 28.93 and 31.67 miles per person, respectively but would fall below the City threshold (31.85 miles). The numbers in bold in Table 2 are the basis for the comparisons in Table 3 and serve as the thresholds that would achieve the lowest VMT thresholds and that are consistent with OPR’s guidelines. As previously stated, the OPR Guidelines recommend a subregional 2023
VMT Type
TAZs More than
15% below
Benchmark
TAZs 0 to 15%
below
Benchmark
TAZs Above
Benchmark
TAZs More than
15% below
Benchmark
TAZs 0 to
15% below
Benchmark
TAZs Above
Benchmark
TAZs More than
15% below
Benchmark
TAZs 0 to
15% below
Benchmark
TAZs Above
Benchmark
OD VMT/SP 3 5 6 1 9 4 -2 4 -2
VMT/Capita 0 14 12 0 12 15 0 -2 3
VMT/Employee 2 18 7 6 15 6 4 -3 -1
Adopted Threshold (2016 RTP) Adopted Threshold (2020 RTP) Changes SGVCOG VMT Tool Update
November 27, 2023
or regional VMT threshold for analyzing employment-generating projects. The City currently uses the City of Diamond Bar’s employment VMT rate as the threshold. Changing the threshold from City to Southeast Subarea would result in the conversion of three low and four moderate VMT zones to seven high VMT zones.
Table 2: Baseline VMT Rates
In Table 3 below, based on the 2020 RTP, the Adopted Threshold (City) is compared to an alternative threshold. The City threshold continues to maximize low VMT areas for all project types, however, the OPR Guidance recommends a regional baseline for employment based VMT. The table below reflects the Southeast rather than the City threshold for employment generating projects, which results in seven more red areas, three fewer blue areas, and four fewer yellow areas as reflected in Table 3 and in the maps.
Table 3: Comparison of Low VMT Areas (Adopted VMT Baseline vs. Alternative Baseline Option) Region/City OD VMT/SP VMT/Capita VMT/Employment OD VMT/SP VMT/Capita VMT/Employment SGVCOG 34.03 24.10 39.23 28.93 20.49 33.34
Southeast 37.26 24.33 39.47 31.67 20.68 33.55
Diamond Bar 37.47 30.53 41.38 31.85 25.95 35.18
Baseline 15% Below Baseline (Threshold)
2023 Changes
VMT Type
TAZs More than
15% below
Benchmark
TAZs 0 to 15%
below
Benchmark
TAZs Above
Benchmark
TAZs More than
15% below
Benchmark
TAZs 0 to
15% below
Benchmark
TAZs Above
Benchmark
TAZs More than
15% below
Benchmark
TAZs 0 to
15% below
Benchmark
TAZs Above
Benchmark
OD VMT/SP 1 9 4 1 9 4 0 0 0
VMT/Capita 0 12 15 0 12 15 0 0 0
VMT/Employee 6 15 6 3 11 13 -3 -4 7
Adopted Threshold (2020 RTP) Alternative Threshold (2020 RTP) The 2023 baseline VMT maps reflecting the City’s adopted VMT baseline under both the 2016 and 2020 RPT, as well as the alternative thresholds for consideration are shown in Attachments A-C.
Conclusion
This analysis compares the VMT metric options for CEQA VMT analysis purposes between the 2016 RTP/SCS and 2020 RTP/SCS SCAG regional travel demand model. The following provides takeaways based on this analysis:
• The SCAG regional model changed formats from a “trip-based” model to a “tour-based” activity-based model. This change, along with changes in traffic volumes, transportation behavior and socioeconomic conditions between the 2016 and 2020 RTP/SCS planning documents, resulted in changes to the way average VMT metrics are calculated and the actual VMT results throughout the region.
• A thorough comparison of VMT metrics and changes in the VMT was conducted to understand the implications for CEQA VMT analysis.
• Retaining the City’s current thresholds of significance maximizes the number of “low VMT” screening areas; however, it is recommended that the City consider the use of Southeast for screening employment-based VMT consistent with OPR’s guidance.
Next Steps
The findings for each participating SGVCOG city will be evaluated by each city to determine if the thresholds should be changed. Changing the thresholds will require City Council approval.
Attachments
• Attachment A: 2023 Baseline OD/SP VMT Maps
• Attachment B: 2023 Baseline Home-Based Work VMT Maps
• Attachment C: 2023 Baseline Home-Based VMT Maps
SGVCOG VMT Tool Update
November 27, 2023
Attachment A
OD/SP VMT Maps
Chino
Hills OS
·142
∙60
∙71
∙57
2023 OD VMT per Service Population from
2016 RTP Model with Adopted Threshold
Diamond Bar
Less than 15% below regional average
0 to 15% below regional average
Higher than regional average
Diamond Bar
Chino
Hills OS
·142
∙60
∙71
∙57
2023 OD VMT per Service Population from
2020 RTP Model with Adopted Threshold
Diamond Bar
Less than 15% below regional average
0 to 15% below regional average
Higher than regional average
Diamond Bar
SGVCOG VMT Tool Update
November 13, 2023
Attachment B
Home-Based VMT Maps
Chino
Hills OS
·142
∙60
∙71
∙57
2023 Home-based VMT per Capita from
2016 RTP Model with Adopted Threshold
Diamond Bar
No Population
Less than 15% below regional average
0 to 15% below regional average
Higher than regional average
Diamond Bar
Chino
Hills OS
·142
∙60
∙71
∙57
2023 VMT per Capita from
2020 RTP Model with Adopted Threshold
Diamond Bar
No Population
Less than 15% below regional average
0 to 15% below regional average
Higher than regional average
Diamond Bar
SGVCOG VMT Tool Update
November 13, 2023
Attachment C
Home-Based Work VMT Maps
Chino
Hills OS
·142
∙60
∙71
∙57
2023 Home-based Work VMT per Employee from
2016 RTP Model with Adopted Threshold
Diamond Bar
Less than 15% below regional average
0 to 15% below regional average
Higher than regional average
Diamond Bar
Chino
Hills OS
·142
∙60
∙71
∙57
2023 VMT per Employee from
2020 RTP Model with Adopted Threshold
Diamond Bar
Less than 15% below regional average
0 to 15% below regional average
Higher than regional average
Diamond Bar
Chino
Hills OS
·142
∙60
∙71
∙57
2023 VMT per Employee from
2020 RTP Model with Alternative Threshold
Diamond Bar
Less than 15% below regional average
0 to 15% below regional average
Higher than regional average
Diamond Bar