Traffic volume is an important basis for determining what improvements, if any, are required on a highway or street facility. Traffic volumes may be expressed in terms of projected annual average daily traffic (AADT) for the design year, or design hourly volumes (DHV). These volumes may be used to calculate the service flow rate, which is typically used for evaluations of geometric design alternatives and safety analysis.

AADT represents the total traffic for a year divided by 365, or the annual average traffic volume per day. Due to seasonal, weekly, daily, or hourly variations,

AADT by itself lacks the level of precision to adequately predict the anticipated vehicular operations. Design Hourly Volume (DHV) is the preferred method for determining the anticipated vehicular operations of the roadway.

Historically, project-level traffic forecasts were developed and approved by TPP. However, at the time of the writing of this document, TPP is in the process of decentralizing these efforts.

Please consult the for additional details.

There are generally three approaches to developing daily traffic forecasts:

Guidance on each of these methods or a combination of the methods at the project level for traffic forecasts can be found in the .

The peak DHV is usually the 30

th

highest hourly volume for the design year, which is commonly 20 years from the time of expected construction completion. For situations involving high seasonal fluctuations in AADT, some adjustment of DHV may be appropriate.

For one-way facilities, the AADT is the total traffic volume. For two-lane, two-way, rural highways without major intersections (i.e., intersections where two arterial roads cross) or where additional lanes are not anticipated for the foreseeable future, the volumes are relatively balanced in both directions. Therefore, the AADT

NDIR

is the total traffic in both directions of travel (i.e., non-directional).

The percent of AADT occurring in the design hour (K) may be used to convert non-directional AADT to DHV as follows:

For urban and metropolitan areas, traffic volumes often show significantly different directional distribution, especially at the interchanges/intersections during AM and PM peak durations. In some cases, significant traffic occurs during mid-day and weekends. Traffic volumes for peak hour or peak period are vital in developing existing and future design transportation needs. Review of 24-hour traffic volume profiles at key locations will determine the peak hour/period. Estimating future traffic volumes in AM and PM peak periods can be a complex process. Refer to and the for additional information and guidance.

On two-way facilities with more than two lanes (or on two-lane, two-way facilities where major intersections are encountered or where additional lanes are to be provided later), knowledge of the directional distribution of traffic during the design hour, Directional Design Hourly Volume (DDHV), is essential for design. DHV and DDHV may be determined by the application of conversion factors to AADT.

The percentage of AADT occurring in the design hour (K) and the design volume that is in the predominant direction of travel (D) are both considered, and doubled, in converting directional AADT to DHV as follows:

The K-factor and the percent of directional distribution (D) are both considered in converting non-directional AADT to DDHV, as follows:

Traffic tends to be more equally divided by direction near the center of an urban area or on loop facilities. For other facilities, the directional distribution is frequently close to 60 to 70 percent.

K is the percentage of AADT representing the 30

th

highest hourly volume in the design year. For typical main rural highways, K-factors generally range from 12 to 18 percent. For urban facilities, K-factors are typically lower, ranging from 8 to 12 percent.

A facility should be designed to provide sufficient capacity to accommodate the design traffic volumes (AADT, DHV, DDHV). The necessary capacity of a roadway is initially based on a set of “ideal conditions.” These conditions are then adjusted for the “actual conditions” that are predicted to exist on the roadway section. This adjusted capacity is referred to as the service flow rate (SF) and is defined as a measure of the maximum flow rate under prevailing conditions.

Adjusting for prevailing conditions involves adjusting for variations in the following factors:

Service flow rate is the traffic parameter most commonly used in capacity and LOS evaluations. Knowledge of highway capacity and LOS is essential to properly fit a planned highway or street to the requirements of traffic demand. Both capacity and LOS should be evaluated in the following analyses:

All roadway design should reflect proper consideration of capacity and LOS procedures as detailed in the HCM.