Traffic Characteristics in Roadway Design

Overview

Information on traffic characteristics is vital in selecting the appropriate geometric features of a roadway. Necessary traffic data includes Traffic Volume, Traffic Speed, and percentage of trucks or other large vehicles.

Traffic Volume

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 average daily traffic or design hourly volumes. These volumes may be used to calculate the service flow rate, which is typically used for evaluations of geometric design alternatives.

Average Daily Traffic

Average daily traffic (ADT) represents the total traffic for a year divided by 365, or the average traffic volume per day. Due to seasonal, weekly, daily, or hourly variations, ADT is generally undesirable as a basis for design, particularly for high-volume facilities. ADT should only be used as a design basis for low and moderate volume facilities, where more than two lanes unquestionably are not justified.

Design Hourly Volume

The design hourly volume (DHV) is usually the 30th highest hourly volume for the design year, commonly 20 years from the time of construction completion. For situations involving high seasonal fluctuations in ADT, some adjustment of DHV may be appropriate.

For two-lane rural highways, the DHV is the total traffic in both directions of travel. On highways with more than two lanes (or on two-lane roads where important intersections are encountered or where additional lanes are to be provided later), knowledge of the directional distribution of traffic during the design hour (DDHV) is essential for design. DHV and DDHV may be determined by the application of conversion factors to ADT.

Computation of DHV and DDHV. The percent of ADT occurring in the design hour (K) may be used to convert ADT to DHV as follows:

DHV = (ADT)(K)

The percentage of the design hourly volume that is in the predominant direction of travel (D) and K are both considered in converting ADT to DDHV as shown in the following equation:

DDHV = (ADT)(K)(D)

Directional Distribution (D). Traffic tends to be more equally divided by direction near the center of an urban area or on loop facilities. For other facilities, D factors of 60 to 70 percent frequently occur.

K Factors

K is the percentage of ADT representing the 30th 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 somewhat lower, ranging from 8 to 12 percent.

Projected Traffic Volumes

Projected traffic volumes are provided by the Transportation Planning and Programming Division upon request and serve as a basis for design of proposed improvements. For high-volume facilities, a tabulation showing traffic converted to DHV or DDHV will be provided by that Division if specifically requested. Generally, however, projected traffic volume is expressed as ADT with K and D factors provided.

NOTE: If the directional ADT is known for only one direction, total ADT may be computed by multiplying the directional ADT by two for most cases.

Service Flow Rate

A facility should be designed to provide sufficient capacity to accommodate the design traffic volumes (ADT, 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 termed 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:

• lane width
• lateral clearances
• free-flow speed
• terrain
• distribution of vehicle type.

Service flow rate is the traffic parameter most commonly used in capacity and level-of-service (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:

• selection of geometric design for an intersection
• determining the appropriate type of facility and number of lanes warranted
• performing ramp merge/diverge analysis
• performing weaving analysis and subsequent determination of weaving section lengths

All roadway design should reflect proper consideration of capacity and level of service procedures as detailed in the Highway Capacity Manual.

Traffic Speed

Traffic speed is influenced by volume, capacity, design, weather, traffic control devices, posted speed limit, and individual driver preference. For design purposes, the following definitions apply:

• Low-speed is 45 mph [70 km/h] and below
• High-speed is 50 mph [80 km/h] and above

Several tables and figures for high-speed conditions will show values for 45 mph [70 km/h] to provide information for transitional roadway sections.

Design Speed

Design speed is a selected speed used to determine the various geometric design features of the roadway. It is important to design facilities with all elements in balance, consistent with an appropriate design speed. Design elements such as sight distance, vertical and horizontal alignment, lane and shoulder widths, roadside clearances, superelevation, etc., are influenced by design speed.

Selection of design speed for a given functionally classified roadway is influenced primarily by the character of terrain, economic considerations, extent of roadside development (i.e., urban or rural), and highway type. For example, the design speed chosen would usually be less for rough terrain, or for an urban facility with frequent points of access, as opposed to a rural highway on level terrain.

Choice should be influenced by the expectations of drivers, which are closely related to traffic volume conditions, potential traffic conflicts, and topographic features.

Appropriate design speed values for the various highway classes are presented in subsequent sections. Whenever mountainous conditions are encountered, refer to AASHTO’s A Policy on Geometric Design for Streets and Highways.

Posted Speed

Posted speed refers to the maximum speed limit posted on a section of highway. TxDOT’s Procedure for Establishing Speed Zones states that the posted speed should be based primarily upon the 85th percentile speed when adequate speed samples can be secured. Speed zoning guidelines permit consideration of other factors such as roadside development, road and shoulder surface characteristics, public input, and pedestrian and bicycle activity.