Size: px
Start display at page:



1 CHAPTER 2: MODELING METHODOLOGY 2.1 PROCESS OVERVIEW The methodology used to forecast future conditions consisted of traditional traffic engineering practices and tools with enhancements to more accurately reflect local features and issues specific to West Berkeley. A review of computer software applications was conducted to ensure that the current practices in transportation planning and traffic engineering practices were used. Figure 2-1 shows the modeling process and the various tools used during the process. The modeling process used the existing conditions analysis as the baseline. Traffic operations from this analysis were calibrated and validated to the baseline 2007 conditions. The future conditions forecast analysis then incorporated expected changes in land use and transportation infrastructure and overlays those onto the existing network to show how the system would perform in the future. This process was simplified into the following four steps: Step 1: Define future land use changes and transportation improvements; Step 2: Estimate the number of new trips generated by the change in land use; Step 3: Assign future trips to the future transportation network; and Step 4: Evaluate the performance of the network. Step 1: Define Future Land Use Changes and Transportation Improvements Existing land use information was obtained through a geocoded assessor database (August 2007) with parcel by parcel information. This was used as the baseline for the existing land use. This land use information was then updated to reflect approved, pending, and potential 2030 development projects as identified by the City of Berkeley Planning Department. Based on their location, these projects and assumptions were assigned to each of the 44 study area TAZs within West Berkeley. Future West Berkeley specific transportation improvements were identified and coded into the existing network to represent both 2015 and 2030 conditions. These changes include geometric changes and adjustments to intersection control including signalization. 2-1


3 Step 2: Estimate the Number of Trips Generated by the Change in Land Use Accurately estimating the trips generated by the specific land use changes is a rather complex process. Industry standards often defer to using rates published in the Institute of Transportation Engineers (ITE) Trip Generation Manual. These rates are developed using observed driveway counts from various developments across the country. These rates are often times reflective of suburban conditions and taken from small sample sizes. General travel behavior in West Berkeley differs from locations where these samples were conducted and these differences are addressed in this step of the process. To account for the difference in travel behavior, the ITE trip generation rates were adjusted to more accurately predict trips in West Berkeley. This adjustment process incorporated travel survey data from the US Census and Bay Area Transportation Survey (BATS) to determine accurate mode splits of travel for the various types of trips made to and from West Berkeley. A detailed summary of this adjustment process can be found in Appendix B. Step 3: Assign Future Trips to the Future Transportation Network The assignment of future trips to the future transportation network was done using the TRAFFIX software application. This application takes the existing traffic network and layers on future traffic generated by land use changes in the study area. To do this, regional travel behavior from the ACCMA regional demand model and local travel behavior specific to West Berkeley was entered along with annual estimates of regional through traffic. This was performed for both year 2015 and 2030 conditions. A detailed summary of the TRAFFIX modeling process can be found in Appendix C. Step 4: Evaluate the Performance of the Network Intersection level traffic volumes were imported to the Synchro/SimTraffic microsimulation package for the network evaluation. Along with the future traffic volumes, estimates of future pedestrian and bicycle volumes were also entered into the program. Transportation improvements were incorporated with the existing roadway geometries and traffic control devices to create a complete network which was used as an input for the SimTraffic microsimulation portion of the package for analysis. Ten runs of each scenario were carried out and averaged to create a report summarizing the measures of effectiveness (MOEs). 2-3

4 2.2 MEASURES OF EFFECTIVENESS (MOE) Measures of effectiveness are quantitative measures that summarize the operating performance of a traffic environment. In terms of the traffic network, these often include: Level of Service (LOS) and delay, Travel speed and time, and Queuing. LOS is a qualitative description of the performance of an intersection based on the average delay per vehicle. Intersection levels of service range from LOS A, which indicates free flow or excellent conditions with short delays, to LOS F, which indicates congested or overloaded conditions with extremely long delays. Urban and suburban arterials are characterized by platoon flows or traffic flow operations were vehicles tend to be clustered together. Operational quality is controlled primarily by the efficiency of signal coordination and is affected by how individual signalized intersections operate along the arterial. Level of service is primarily a function of average travel speed along segments, and is calibrated from field data. Travel time runs were conducted in the field in order to calibrate the SimTraffic models to ensure compliance with existing conditions. It was also applied as a measure of effectiveness for identifying impacts along the major arterials within the project area LEVEL OF SERVICE (LOS) Levels of Service for signalized intersections were calculated in Synchro using the Highway Capacity Manual 2000 (HCM 2000) methodology. The LOS is based on the average delay (in seconds per vehicle) for the various movements within the intersection. A combined weighted average delay and LOS are presented for each of the signalized intersections. The average delay for signalized intersections was calculated using the Synchro analysis software and is correlated to the level of service designation as shown in Table

5 Table 2-1: Level of Service Criteria Signalized Intersections Level of Description of Operations Service Operations with very low delay occurring with favorable progression and/or short A cycle length. Operations with low delay occurring with good progression and/or short cycle B lengths. Operations with average delays resulting from fair progression and/or longer cycle C lengths. Individual cycle failures begin to appear. Operations with longer delays due to a combination of unfavorable progression, D long cycle lengths, or high V/C ratios. Many vehicles stop and individual cycle failures are noticeable. Operations with high delay values indicating poor progression, long cycle lengths, E and high V/C ratios. Individual cycle failures are frequent occurrences. This is considered to be the limit of acceptable delay. Operation with delays unacceptable to most drivers occurring due to over F saturation, poor progression, or very long cycle lengths. Average Delay* * Delay presented in seconds per vehicle. Source: Highway Capacity Manual, Transportation Research Board, 2000 Unsignalized intersections were evaluated using the Highway Capacity Manual 2000 methodology. The LOS rating is based on the weighted average control delay expressed in seconds per vehicle as illustrated in Table 2-2. Control delay includes initial deceleration delay, queue move-up time, stopped delay, and final acceleration. At two-way controlled intersections, LOS is calculated for each controlled movement, as opposed to the intersection as a whole and the highest delay along any of the approaches is used to calculate the LOS for that intersection. For all-way stop controlled locations, LOS is computed for the intersection as a whole. 2-5

6 Table 2-2: Level of Service Criteria Unsignalized Intersections Level of Average Description of Operations Service Delay* A No Delay for stop-controlled approaches B Operations with minor delays C Operations with moderate delays D Operations with some delays E Operations with high delays, and long queues F Operations with extreme congestion, with very high delays and long queues unacceptable to most drivers * Delay presented in seconds per vehicle. Source: Highway Capacity Manual, Transportation Research Board, 2000 Major arterials were evaluated using the Highway Capacity Manual 2000 methodology. The LOS rating is based on the average speed obtained from the SimTraffic outputs expressed in miles per hour as illustrated in Table 2-3. Table 2-3: Level of Service Criteria Arterials Urban Street Class I II III IV Range of Free Flow Speeds (mph) 55 to to to to 25 Typical Free Flow Speed (mph) Level of Service Average Travel Speed (mph) A > 42 > 35 > 30 > 25 B > > > > C > > > > D > > > > 9-13 E > > > > 7-9 F Source: Highway Capacity Manual, Transportation Research Board, SPEED AND TRAVEL TIME Travel times along the major arterials were obtained as an output of the SimTraffic model. The resulting travel speed creates the measure of the LOS of arterials. The average travel speed is computed from the running times on the urban street and the control delay of through movements at signalized intersections. 2-6

7 95 th Percentile Queuing The length at which 95 percent of vehicles are queuing at or below during a given time period is known as 95 th percentile queuing. The 95 th percentile queue length for each intersection was obtained from SimTraffic simulation analysis. Ten simulation runs were performed and the 95th percentile queue lengths based on the average value of these runs. When compared to the actual storage bay lengths in the field, these queue lengths will give us an estimate of the capacity constraints due to queue spillback as well as inadequacies in storage bay lengths. 2.3 CITY OF BERKELEY TRAFFIC IMPACT ANALYSIS GUIDELINES To identify intersections with significant transportation impacts, the City of Berkeley s Office of Transportation has established the following significance criteria guidelines. City of Berkeley Level of Service Thresholds LOS D is the level of service standard within the City of Berkeley. It applies to all signalized intersections for operational planning and for major non-freeway segments for long-range planning. As long as a minimum threshold of project trips is met, impacts requiring adequate mitigation are those assumed to have occurred if the LOS goes from D to E or F or is already at E or F. The manner in which level of service is calculated and assessed depends upon the type of traffic control involved, as follows: Signalized intersections: The lower threshold as defined in the HCM for LOS E is 55 sec/veh and for LOS F is 80 sec/veh. The average delay can be significantly affected by signal timing at a signalized intersection. In general, traffic impact analyses should retain cycle lengths, phase minimums, and phasing that occur for existing conditions. Phase lengths can be adjusted but should not adversely affect signal coordination. Any major changes need to be documented and fully justified. The City has established significance thresholds based on the fact that for a given level of traffic on critical movements, the delay increases at a greater rate as LOS F is approached. The following average delay thresholds have been established: LOS D to E = 2 seconds; and LOS E to F = 3 seconds. The volume-to-capacity ratio (v/c) is also an important indicator of capacity and should be included as part of all Level of Service tables. It can indicate the extent to which the signal timing is optimal and provides a useful indicator for over-saturated conditions. However, v/c s are not utilized for identifying level of service. As the delay can increase dramatically with small increases of traffic 2-7

8 after LOS F has been reached, a threshold of an increase of 0.01 in the volume-to-capacity ratio will be used. Intersection level of service is dependent on a variety of factors. In general, existing timing and phasing should be retained for scenarios with and without the project. In this way, the only variable is the traffic volume, which ensures a valid comparison of project impacts. Nevertheless, with the approval of City staff, mitigations can include changes in signal timing but care must be taken to ensure that these changes do not affect operations at adjacent signals. Finally, where closely spaced signals exist, estimated queue lengths should be provided to demonstrate whether or not there are potential impacts on upstream intersections or on access to turn lanes. Unsignalized intersections: The lower level of service thresholds for LOS E and F, respectively, are 35 and 50 seconds, for unsignalized intersections. For all-way stop intersections, the results of the level of service analysis provide a meaningful overall delay that can be presented similar to that for a signalized intersection. However, for two-way stop intersections, levels of service are established separately for each movement with conflicting movements that pass through the intersection. As a result, an unfavorable level of service can occur for a small number of vehicles, and a large increase in delay can occur for a small increase in traffic volume. Unlike for signalized intersections, it is difficult to establish fixed significance thresholds for unsignalized intersections, particularly those with only side-street stop control. In general, mitigations are required if a movement is at LOS F, the peak hour signal warrant is met, and a minimum of 10 vehicles are added to the critical movement. Nevertheless, as delays increase dramatically once LOS F is reached, consideration should be given to the number of new trips added by a project and other factors, such as the feasibility of alternative routes and the proximity to adjacent traffic signals. 2-8