Existing traffic volumes are the basis for traffic projections. Existing traffic volumes are developed from traffic counts that are typically collected as part of the overall data collection effort.

Historical traffic data can be obtained from STARS. Growth rate is typically determined for a corridor based on 20-year historical growth rates, while considering the travel demand model outputs and planned developments. Depending on the type of corridor and the overall length, a corridor may have different growth rates along its segments. Prior to developing future traffic projections, a Traffic Projections Methodology Memorandum is typically prepared outlining the growth rate and approach to develop traffic projections. Include any assumptions regarding the development of growth rates in the memorandum. This memorandum is submitted to TPP or the reviewing District for approval prior to proceeding with traffic projections. An

example

outline for Traffic Projections Methodology Memorandum is provided in

Appendix D, Section 2 – Traffic Projections Methodology Memorandum Outline

.

Future traffic projections are developed based on traffic data factors and approved growth rates. Typical traffic forecasts include the project opening year, the design year (i.e., opening year plus 20 years), and the pavement design year (i.e., opening year plus 30 years). Once traffic projections are estimated, they need to be balanced throughout the corridor. More detailed information regarding the development of growth rates, volume balancing, and future traffic projections is provided in

Chapter 3

.

After traffic projections are approved, the Traffic Analysis for Highway Design (TAHD) table is developed in coordination with TPP. See

Chapter 3, Section 3.2

for more information on the traffic data factors and the TAHD table.