TASK 3.2.2 TECHNICAL MEMORANDUM SUPPLEMENTAL
TRAVEL DEMAND FORECASTS
TO: Carolyn Mamaradlo (San Mateo County), Douglas Kim (Town of Atherton) FROM: Kay Cheng (WSP), George Naylor (WSP)
SUBJECT: Technical Memorandum - Travel Demand Forecasts DATE: March 5, 2025
INTRODUCTION
The travel demand forecasts prepared for the El Camino Real Complete Streets Corridor study provide key outputs that are used to support the transportation analysis of the study, quantifying the traffic impact of the various design alternatives under consideration. These key outputs include the peak hour traffic volumes at the study intersections and along roadways in the project corridor, peak hour bicycle volumes along the corridor and the level of traffic volumes that could divert onto local streets as a result of capacity changes on El Camino Real.
The C/CAG-VTA Countywide models were used to prepare the travel demand forecasts to support the traffic impact analysis for the El Camino Real Complete Streets Corridor study.
The C/CAG-VTA model is a traditional four-step model used to support countywide transportation system analysis and detailed project corridor analysis for projects located within San Mateo County. The C/CAG-VTA models provide a comprehensive analytical framework for estimating base year and future travel demand at varying levels-of-detail, from larger system corridor impacts to smaller level intersection turn movements. The C/CAG-VTA models provide the ability to forecast detailed trips by mode (auto vehicle by occupancy, transit and non-motorized trips) by time of day and can reflect changes in regional growth patterns and adjacent roadways that interact with the countywide expressway system. Three model horizons were available from the C/CAG-VTA model data files: 2015, 2025 and 2040. For purposes of this study, the year 2025 model data was used as the basis for the travel demand forecasts, as it was closest to the existing year 2024 and represents a close alignment to the more short-range timeline of 1 to 2 years when the project could be constructed and in operation.
The remainder of this memorandum summarizes the base year model calibration and the near-term year 2025 traffic forecasts for the project alternatives. Three network alternatives were modeled for the year 2025: a No Project Concept, a 6-lane road with bicycle lane and sidewalks, a 4-lane road diet with buffered/protected bicycle lanes and sidewalks and a 6/5- lane road diet with buffered/protected bicycle lanes and sidewalks.
BASE YEAR VALIDATION
For this project, the C/CAG-VTA models were validated to local traffic conditions for the base year using intersection turn movement counts collected for the corridor in spring 2024.
Model validation is the process by which the traffic volumes from the travel demand models are compared to observed traffic count data, and adjusted to ensure the models can reproduce observed conditions within an acceptable level of tolerance.
The starting point with the base year model validation was the C/CAG-VTA model. The C/CAG-VTA model provided three horizon years available for the project: 2015, 2025 and 2040.
For this study, the approach was to use the 2025 model as the base year and review and refine the model inputs to achieve the model validation in the project corridor. The initial comparison of observed model volumes to the counts showed that the year 2025 C/CAG-VTA model overestimated traffic between 30 % to 140 % on El Camino Real, depending on the direction of travel. Much of the overestimation of traffic was due to the relatively high free- flow observed speed of 40 mph along El Camino Real; however, in addition to the input free- flow speeds, various input parameters were reviewed and modified to improve the model validation results. These model changes consisted of the following:
Review of the base networks to ensure number of lanes for roadways in the project corridor and surrounding areas was up to date (using Google Maps as the review medium),
• Recoding of the free-flow speeds on El Camino Real from 40 miles per hour to the posted speed limit of 35 miles per hour,
• Review of households and the subsequent updating of the base year households in Atherton to be consistent with year 2020 census block number of households, if the C/CAG-VTA base year values were too low,
• Updated the public and private elementary and high school enrollment in the Town of Atherton based on publicly available data,
• Updated student enrollment and total employment at Menlo College with publicly available data, and
• Adjusted peak hour traffic factors to improve comparison of model volumes to counts Table 1 summarizes the final comparison of model volumes to observed count data for traffic volumes along El Camino Real for the AM and PM peak time periods after modification of the above model inputs. The volumes summarized in Table 1 consist of an accumulation of the approaching and departing traffic volumes at each of the twelve project intersections. These intersections included the following:
• Oakwood Drive/Dumbarton Avenue (signalized)
• Selby Lane
• Fifth Avenue (signalized)
• Stockbridge Avenue
• Almendral Avenue
• Tuscaloosa Avenue
• Atherton Avenue/Fair Oaks Lane (signalized)
• Isabella Avenue
• Watkins Avenue
• Alejandra Avenue
• Encinal Avenue (signalized), and
• Valparaiso Avenue/Glenwood Avenue (signalized)
A typical rule of thumb is that the model volumes should be within plus or minus 15%, as that is typically how much traffic varies on a day-to-day basis. In general, for the peak directions of travel, which is southbound in the AM and northbound in the PM, the model volumes are very close to the observed count data at 2% difference of observed in the AM and essentially matching the PM northbound direction. The off-peak directions for both the AM and PM peak hours are overestimated by the model, by 27% in the AM and 13% in the PM. While this is outside the acceptable range of plus or minus 15% difference for the AM off-peak direction, this overestimation occurs in the off-peak which would likely not influence the level-of- service analysis and peak hour traffic impacts. It should be noted that the volumes estimated by the model are not used directly from the model, rather, the volumes are adjusted using the base year observed volumes and the relative differences from the forecast model volumes to minimize volume over or under estimation in the base year validation. This correction process is described in more detail in a separate technical memorandum for the traffic impact analysis.
Table 1 Peak Hour Model Validation Summary - El Camino Real Corridor Model Volumes compared to Counts
Direction Model Count Model/Count
AM Southbound* 47,223 46,509 1.02
27,452 21,607 1.27
AM Northbound
74,675 68,116 1.10
AM Peak Hour All
PM Southbound 34,114 30,244 1.13
PM Northbound* 40,705 40,639 1.00
PM Peak Hour All 74,819 70,883 1.06
* Indicates peak direction of travel
TRAVEL DEMAND FORECASTS
With the completion of the base year model validation, the models were applied to develop travel demand and traffic volumes for the three project alternatives. Since the model horizon for the alternatives was the same as the base validation (year 2025), minimal changes to the model inputs were required, such as inputs that would vary when applying a different horizon year of analysis. The project alternatives for the El Camino Real facility studied in the transportation analysis include the following:
• No Project that assumes the lane geometry of existing conditions on El Camino Real as of 2025: 6 lanes total, with 3 lanes in each direction from Oakwood to Valparaiso, with the exception of a 4-lane section in the northbound direction of El Camino Real from Valparaiso to Spruce Avenue, with no bicycle facilities nor continuous sidewalks, • 6-Lane Configuration with striped Class II bike lane (Concept #1): This alternative maintains six travel lanes and incorporates a dedicated striped bike lane and continuous sidewalks.
• 4-Lane Configuration with separated bike lane (Concept #2): This alternative reduces the travel lanes to four, allowing additional space for a separated bike lane and wider continuous sidewalks.
• 6 and 5-Lane Configuration with separated/striped bike lane (Concept #3): This alternative provides a middle ground, maintaining five travel lanes and adding a separated or striped bike lane and continuous sidewalks.
Implementation of the travel demand forecasts for the various project concepts was straightforward and only required the coding of the various network alternatives lane configurations and bicycle facilities, as the socioeconomic data and all other inputs remained unchanged from the existing year validated models. Once the alternative lane configurations and bicycle infrastructure features were coded into the model networks, the C/CAG-VTA model was run to produce the revised travel demand associated with each alternative. Key outputs generated by the models included traffic volumes for the AM peak hour, PM peak hour and off-peak time periods as well as bicycle volumes on the bicycle lanes and adjacent roadways. The outputs of the travel demand model were used to develop the traffic operations and level-of-service analysis for the project, which is reported in a separate technical memorandum.
Table 2 provides a summary of the volumes on El Camino Real, calculated by summing the approach and departure volumes at each of the 12 project intersections, similar to the volumes shown in Table 1. The No Project and Concept 1 volumes are the same, as the roadway configurations for those alternatives are equal, and the improvements for Concept 1 do not alter the roadway volumes on El Camino Real. The results of Table 2 show that the Concept 2 (the 4-lane cross section), traffic volumes along El Camino Real reduce the most relative to the No Project/ Concept 1, particularly for the peak direction of travel in the southbound AM. Traffic volumes also reduce for the non-peak direction of travel, but to a lesser extent. As expected, with the increased lane capacity for Concept 3, traffic volumes are not reduced as significantly relative to the No Project/ Concept 1 baselines, but they still are less along El Camino Real. The same relative traffic reductions also occur in the PM peak hour, however, the volume reductions are less than in the AM, when traffic volumes are at their highest in the corridor. The results shown in Table 2 indicate that with the reduction of capacity along El Camino Real, there is a corresponding reduction in traffic volumes, likely caused by the diversion of traffic onto parallel streets. For the AM peak hour, the reduction is highest for Concept 2. Since El Camino Real is a major regional facility that carries both local and through traffic, the reduction in capacity would cause through vehicle trips to divert to parallel arterials and freeways if the travel time increase is significant enough on El Camino Real. Based on the results in Table 2, for the most restrictive lane configuration in Concept 2,
14.5% of the traffic on El Camino Real is reduced for the AM southbound which is the peak direction of travel. Approximately 30% of the traffic on El Camino Real was determined to be through traffic (based on the Existing Conditions analysis using Streetlight cell phone-based origin-destination data) that did not have an origin or destination in Atherton, which indicated that there was a significant travel market that could divert off El Camino Real. The result of the model simulation predicted approximately half of what would be considered the maximum amount of regional traffic that would divert.
Table 2 El Camino Real Corridor Traffic Volumes by Concept AM and PM Peak Hours
Direction AM Peak Hour No Project/Concept 1 Concept 2 Concept 3 Southbound 47,223 40,479 44,458
Change from No Project/Alt 1 N/A -6,744 (-14.3%) -2,765 (-5.9%) Northbound 27,452 26,969 27,358
Change from No Project/Alt 1 N/A -483 (-1.8%) -94 (-0.3%) Both Directions Total 74,675 67,448 71,816
Change from No Project/Alt 1 N/A - 7,227 (-9.7%) -2,859 (-3.8%) Direction PM Peak Hour No Project/Concept 1 Concept 2 Concept 3 Southbound 34,114 32,794 33,514
Change from No Project/Alt 1 N/A -1,320 (-3.9%) -600 (-1.8%) Northbound 40,705 38,564 40,229
Change from No Project/Alt 1 N/A -2,141 (-5.3%) -476 (-1.2%) Both Directions Total 74,819 71,358 73,743
Change from No Project/Alt 1 N/A -3,461 (-4.6%) -1,076 (-1.4%)
TRAFFIC DIVERSION
While the results provided in Table 2 show the total decrease in traffic using El Camino Real when capacity is constrained. There are also traffic impacts that can be associated with traffic that diverts to local streets when capacity along El Camino Real is reduced. Traffic diversion onto local streets can occur when there are capacity reductions due to road diets. Reduction in vehicle carrying capacity on roadways with through and local serving traffic may reroute through adjacent streets to minimize delay, intruding on local neighborhood streets not typically designed to handle high traffic volumes, which increases potential safety issues for vehicles, pedestrians and bicyclists. For the El Camino Real project, several alternative roadway configurations decreased the effective roadway capacity by decreasing the number of lanes on the facility from the baseline existing conditions, which essentially provides 6 lanes of capacity, the exception being a small segment between Valparaiso and Spruce in the northbound direction.
The evaluation of the traffic diversion impacts for each of the alternatives was performed by using the C/CAG-VTA travel demand model. The C/CAG-VTA travel demand model was used to estimated traffic volumes for the project alternatives, and is sensitive to changes in roadway capacity, estimates traffic flows for a large geographical area, and includes arterials and local streets in the simulation networks. The model thus provides an ideal means for quantifying the amount of traffic that occurs on each facility in the project corridor, with reporting ability to illustrate how local streets are impacted by changes in lane configurations for the project alternatives. For the traffic diversion analysis, the unit of measurement is vehicles. This metric is basically the number of vehicles traveling in the network for each time period. The model can be used to classify the type of roadway in the network in addition to the vehicles, allowing for the tabulation of vehicle travel occurring on El Camino Real and all other roads within the Town of Atherton limits.
The traffic diversion was quantified at two levels of analysis - a systemwide summary of vehicles by street for the entire Town of Atherton as described above and a more focused analysis using a technique known as a volume-difference analysis. The volume-difference analysis was used to identify local street diversion 'hot spots' to more specifically identify locations that could be influenced by the capacity reductions along El Camino Real for each alternative. The volume-difference analysis is straightforward and can be illustrated graphically by subtracting the traffic volumes of the build alternatives from the No Project.
The volume-differences are color coded based on whether there is an increase in traffic or a decrease in traffic. This allows for the quick identification of specific locations and streets that may experience traffic diversion.
Table 3 summarizes the vehicles for the No Project Conditions and the various corridor alternatives, which is the systematic analysis of local traffic diversion. Vehicles were summarized based on two categories: all local roads and arterials and the segments of El Camino Real located on the project corridor, all within the Atherton Town limits. Local roads in Table 3 would include all roads other than El Camino Real. This would include the cross streets at the 12 study intersections as well as streets and roads outside of the project corridor but still within the Atherton Town limits, such as Middlefield Road, Alameda de Las Pulgas and various local streets that could possibly use El Camino Real as an alternative. It should be noted for the diversion analysis, the No Project and Concept 1 vehicles are equal, as the 6-lane cross section is the same for both the No Project and Concept 1. The inclusion of bicycle facilities and sidewalks is assumed to not alter roadway capacity, yielding no difference in the traffic volumes as reported by the C/CAG-VTA model.
The results shown in Table 3 indicate that Concept 2 creates the most diversion onto local streets relative to Concepts 1 and 3 for the AM and PM peak hours as well as for daily traffic conditions. This is intuitive, as Concept 2 has the most reduction in lane capacity on El Camino Real. However, while Concept 2 diverted traffic onto local streets is highest, the percent increase is small and on the order of a 2.4% increase over Concept 1 for the AM peak hour, and a 1.6% increase in the PM and a 1.2% increase for daily traffic. This could indicate that local streets in Atherton serve a different market not significantly impacted by changes to lane capacity on El Camino Real (i.e., local traffic movements across El Camino Real, rather than using El Camino Real), and the fact that there are no significant local routes that offer a parallel alternative to El Camino Real. It should be noted that vehicle volumes on El Camino Real are lowest for Concept 2 and for all streets in Atherton in total under all time periods.
This indicates that the road diet in Concept 2 likely causes a higher portion of traffic to divert to other facilities not within the Atherton town limits, for more regional through-traffic movements beginning or ending outside of Atherton previously served by El Camino Real under 6 lane conditions, but also for trips using a combination of local streets and then a shorter portion of El Camino Real than the other concepts.
The volume-difference analysis to identify diversion 'hot spots' is illustrated on Figure 1 through Figure 4. Figures 1 and 2 are for Concept 2 versus the No Project/Concept 1 volumes, for the AM and PM peak hours respectively. Figures 3 and 4 show the same information but for Concept 3 versus the No Project/Concept 1 for the AM and PM peak hours respectively.
These volume-difference plots show alternative volume decreases in red and increases in green bandwidths, relative to the No Project/Concept 1. From the figures, it was determined that three local street traffic diversion 'hot'spots' appear, as follows:
• Oakwood Drive traffic diverting to Oakwood Boulevard via Selby for Concepts 2 and 3 in the AM and the PM peak hours,
• Eastbound Atherton Avenue traffic diverting to Isabella Avenue/Alejandra Avenue for Concept 2 in the AM, and
• Eastbound and westbound Watkins Avenue/Encinal Avenue to eastbound and westbound Fair Oaks Lane for Concepts 2 in the AM hour.
While these three areas indicate possible local street diversion, it should be noted that the Oakwood Drive/Oakwood Blvd and Atherton Avenue/Isabella Avenue/Alejandra Avenue locations have significant traffic calming infrastructure in place (speed bumps/speed tables), which the demand model would not consider in the route diversions, which implies that the demand model may be overestimating the impact of the local street traffic diversion and the traffic may actually stay on El Camino Real. Nonetheless, the results of the diversion analysis identifies areas that might need to be monitored to minimize diversion if Concepts 2 or 3 are the preferred alternative designs.
Table 3 Year 2025 Vehicle Summary - Streets within the Town of Atherton:
Local Roads and Arterials versus El Camino Real
AM Peak
Hour No Project and Concept 1 Concept 2 Concept 3
Local Local Local Roads El Camino
Facility Type Roads and Camino Roads and El Camino Real Total Total Arterials Real and Arterials Real
Arterials
Vehicles 1,146 3,168 4,314 1,174 2,843 4,018 1,165 3,044 4,209 Versus Base N/A N/A N/A 28 (2.4%) -325 (-10.3%) -297 (-6.9%) 19(1.7%) -124 (-3.9%) -105 (-2.4%) PM Peak
Hour No Project and Concept 1 Concept 2 Concept 3
Local Local
Facility Type Roads and Camino Roads and Local Roads El Camino Arterials Real Total Arterials El Camino Real Total and Arterials Real Total Vehicles 1,147 3,170 4,317 1,165 3,014 4,178 1,156 3,114 4,271 Versus Base N/A N/A N/A 18(1.6%) -156 (-4.9%) -138 (3.2%) 9(0.8%) -55 (-1.7%) -46 (-1.1%) Daily No Project and Concept 1 Concept 2 Concept 3
Local Local
Facility Type Roads and Camino Roads and Local Roads El Camino Arterials Real Arterials El Camino Real Total and Arterials Real Total Vehicles 10,157 35,615 45,771 10,276 34,333 44,610 10,202 35,175 45,377 Versus Base N/A N/A N/A 120 (1.2%) -1,281 (-3.6%) -1,161 (-2.5%) 45(0.4%) -440 (-1.2%) -395 (-0.9%) Figure 1 Volume Difference Concept 2 - No Project/Concept 1, AM Peak Hour Year 2025 Volume Difference 6 Lane Cross Section to 4 Lane Cross Section AM Peak Hour Volumes
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