California Department of Transportation

Project Development Workflow Tasks (PDWT)

Part 3 - Identify Project Need and Project Initiation Document

II. Project Initiation Document

B. Develop Initial Alternatives (WBS 150.10) - Develop Concept Alternatives (WBS 150.10.15)

P37. Request Traffic Information

Engineering decisions made by the project engineer will require a thorough traffic study of the proposed project’s existing facility. Virtually all highway improvement projects will require the engineer to collect and interpret traffic data. Average Daily Traffic, Peak Hourly Volume, turn movements, truck usage, expected traffic growth, etc. are basic factors that the project engineer will need to consider in order to apply the scientific principles of Traffic Engineering and to make correct and justifiable engineering decisions. Civil Engineers within the Design Offices may collect field traffic data, however, the Traffic Operations Office and Planning Office of Forecast and Modelings provide functional support to the project engineer and can provide the most common data that is needed upon memo request. Request must contain the construction year and the design year.

Traffic Counts/Volumes

The Department collects and publishes traffic volume data for State highways and ramps. The Traffic and Vehicle Data Systems Unit collects traffic count information from automated count stations that are located at key locations on highways and freeways.


Annual reports for AADT, Peak Hour Volume, Truck Traffic, and Ramp Volumes website

Traffic Forecasts and Modeling

Forecasting of future traffic volumes and development of traffic models is most likely a function of the District Planning Division. District Planning staff work with local agencies in the development of traffic models that are used for the planning of transportation systems and future projects. Using growth factors and existing traffic volumes, future traffic volumes are projected.

The use of any local or regional traffic model should reflect the most current land use and planned improvements (i.e., where programming or funding is secured). When a general plan build out model is not available, the closest forecast model year to build-out should be used. If a traffic model is not available, historical growth rates and current trends with the coordination of local transportation and land use agencies can be used to project future traffic volumes. Any traffic impact study should clearly describe any changes made in the model to accommodate the analysis of a proposed project.


Caltrans Guide for the Preparation of Traffic Impact Studies

Traffic Capacity and Level of Service Analysis

The project engineer, if competent to do so, may perform traffic capacity and Level of Service (LOS) analysis. The Department’s basic design policy is covered under Topic 102 Highway Capacity of the HDM. Detailed data on design capacity are available in the "Highway Capacity Manual", published by the Transportation Research Board. The Highway Capacity Manual is not a Department publication, thus copies must be purchased. The 1994 HCM is the approved version for Caltrans use.

The District Planning Branch is typically responsible for performing traffic forecasting and modeling, and may also provide assistance to the project engineer with capacity and LOS analysis. Some units within the Traffic Operations Division may also provide capacity and LOS analysis.

Time Constraint

The project engineer must use correct traffic information because the application of design standards and subsequent engineering decisions are premised upon this information. The project engineer should anticipate the following time requirements for complex projects, but may confer with the various traffic units for anticipated wait times. Design Designation and Traffic Index are completed after requests for Traffic Counts/Volumes or Forecasting and Modeling

Traffic Counts/Volumes 1 - 2 months
Traffic Forecasts and Modeling 2 - 4 months
Traffic Capacity and Level of Service Analysis 1 - 2 months
Micro Simulation
Design Designation 1 month
Traffic Index 1 month


Transportation Research Board Internet Website

HDM, (Chapter 100, Topic 102, “Highway Capacity”)

Design Designation

The design designation is a simple, concise expression of the basic factors controlling the design of a given highway. The following is an example of this expression:

ADT (2000) = 9800 D = 60%
ADT (2020) = 20 000 T = 12%
DHV = 3000 V = 110 km/h

Where ADT (xxxx) = average daily traffic in year xxxx, DHV = design hour volume, D = directional split, T = percent trucks in design hour, and V = design hour speed.

The project engineer will typically request the Design Designation number from the Planning Division. The project engineer will need to provide the design life period (typically either 10 years for rehabilitation projects, or 20 years for new construction), design speed, and the proposed completion of construction year.

The Planning Division may not provide the design speed value (V), and the selection of the appropriate design speed for project alternatives may be left up to the project engineer. The topic of Design Speed is covered in Chapter 101 of the Highway Design Manual. Project alternatives may have multiple Design Designations for different locations within the project.

The Design Designation should be expressed in the Project Initiation Document.


HDM, (Chapter 100, Topic 103.1, “Design Designation, Relationship to Design”)

Traffic Index

The Traffic Index number is required for the design of roadway structural section alternatives sufficient for the intended design period of the project. Design Period is covered in HDM Topic 603.2. The Traffic Index or TI is a measure of the number of Equivalent Single Axle Loads (ESAL's) expected in the design lane over the design period. The TI does not vary directly with the ESAL's but rather according to an exponential formula. The Design Engineer must first obtain a Traffic Index number before requesting a Materials Report and structural section recommendations from the Materials Branch. The project engineer will typically request the Traffic Index number from the Planning Division, and will need to provide the design life period (typically either 10 years for rehabilitation projects, or 20 years for new construction), and the proposed completion of construction year.

Multiple Traffic Index numbers may be required for a project. If the truck traffic in multiple-laned facilities varies significantly, then different Traffic Index numbers are required. Separate Traffic Index numbers will be required for mainline, shoulders, ramps, and temporary roadways.


HDM, (Chapter 600, Topic 603.2, “Design Period”)

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This page last updated July 20, 2010