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Section 3: Traffic Characteristics

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Overview

Information on traffic characteristics is vital in selecting the appropriate geometric features of a roadway. Necessary traffic data includes:

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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 (ADT) 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.

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 are not justified.

Traffic Forecast

Project level daily travel forecasts are developed and approved by the Transportation Planning and Programming Division (TPP). Generally, projected traffic volume is expressed as ADT with peak (K) and directional (D) factors provided. For high-volume facilities, a tabulation showing traffic converted to DHV or directional design hourly volume (DDHV) will be provided by TPP if specifically requested.

There are generally three approaches to developing daily traffic forecasts:

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  • Metropolitan Planning Organization (MPO) Travel Demand Model: MPO's travel demand model is used to estimate traffic on the project for opening and design year.
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  • Pivot/Trend Line/Growth Method: A growth rate is developed using the historical average annual daily traffic data for 20 years and projected for the next 20 years (pivot year). Growth factors are used to convert existing year traffic to opening year traffic and opening year traffic to design year traffic.
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  • Hybrid Approach: This approach uses a combination of the first and second methods. MPO's travel demand model is used for developing traffic projections and adjustments are made using growth factors developed by historical or trend line analysis.

There are three options for obtaining approval of daily travel forecasts:

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  • Option A: TPP develops the traffic forecasts, signs and seals the data, and provides the data to the Districts and project consultants.
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  • Option B: Districts and project consultants are responsible for developing the traffic forecasts. TPP reviews and approves the methodology prior to development, reviews and approves the traffic forecasts, and signs and seals the data.
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  • Option C: Districts and project consultants are responsible for developing traffic forecasts. Districts are also responsible for developing the methodology, and developing, reviewing, approving, signing and sealing the traffic forecasts.

Design Hourly Volume

The peak DHV is usually the 30th 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 ADT, some adjustment of DHV may be appropriate.

Computation of DHV and DDHV

For one-way facilities, the ADT 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 ADTNDIR is the total traffic in both directions of travel (i.e. non-directional).

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

DHV = (ADTNDIR)(K)

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 Design Division's (Traffic Simulation and Safety Analysis Section) webpage 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 ADT.

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

DDHV = (ADTNDIR)(K)(D)

The percentage of ADT 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 ADT to DHV as follows:

DHV = (ADT)(K)(D)(2)

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, the directional distribution is frequently close to 60 to 70 percent.

K Factors (K)

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 lower, ranging from 8 to 12 percent.

Service Flow Rate (SF)

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 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 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:

All roadway design should reflect proper consideration of capacity and level of service procedures as detailed in the Transportation Research Board's (TRB) Highway Capacity Manual.

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Speed

Speed is one of the most important factors considered by travelers in selecting alternative routes or transportation modes. In addition to capabilities of the drivers and their vehicles, the speed of vehicles on a road depends upon five general conditions: the physical characteristics of the roadway, the amount of roadside interference, the weather, the presence of other vehicles, and speed limitations established by law or by traffic control devices. Although any one of these factors may govern travel speed, the actual travel speed on a facility usually reflects a combination of factors.

The objective in design of any engineered facility used by the public is to satisfy the public’s demand for service in an economical, efficient, and safe manner with low crash frequency and severity. The facility should accommodate nearly all demands with reasonable adequacy and should only fail under severe or extreme traffic demands. Because only a small percentage of drivers travel at extremely high speed, it is not practical to design facilities for these speeds. On the other hand, the speed chosen for design should not be based on the speeds drivers use under unfavorable conditions (such as inclement weather), because the roadway would then be inefficient, might result in additional crashes under favorable conditions, and would not satisfy reasonable public expectations for the facility.

There are important differences between design criteria applicable to low-speed and high-speed designs. For design purposes, the following definitions apply:

Design Speed

Design speed is a selected speed used to determine the various geometric design features of the roadway. The selected design speed should be logical with respect to the anticipated operating speed, topography, adjacent land use, modal mix, and functional classification of the roadway. In selection of design speed, every effort should be made to attain a desired combination of safety, mobility, and efficiency within the constraints of environmental quality, economics, aesthetics, and social or political impacts.

The selected design speed should generally be greater than or equal to the anticipated operating speed of the roadway. A roadway of higher functional classification may justify a higher design speed than a lower classified facility in similar terrain. A low design speed should not be selected where the topography is such that drivers are likely to travel at high speeds. Factors to consider when choosing a design speed include the expectations of drivers which are closely related to traffic volume conditions, potential traffic conflicts, and terrain features.

Appropriate design speed values for various highway classes are presented in subsequent sections.

Operating Speed

Operating speed is the speed at which drivers are observed operating their vehicles during free-flow conditions. The 85th percentile of the distribution of observed speeds is the most frequently used measure of the operating speed associated with a particular location or geometric feature. The following geometric design and traffic demand features may have direct impacts on operating speed: horizontal curve radius, grade, access density, median treatments, on-street parking, signal density, vehicular traffic volume, lane widths, sight distance, and pedestrian and bicycle activity.

Posted Speed

Posted speed refers to the maximum speed limit posted on a section of highway. The Procedures for Establishing Speed Zones Manual 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.

Running Speed

The speed at which an individual vehicle travels over a highway section is known as its running speed. Running speed is calculated by dividing the length of the highway section by the time for a typical vehicle to travel through the section. For extended sections of roadway that include multiple roadway types, the average running speed is the most appropriate measure for evaluating level of service and road user costs. The average running speed is the sum of the distances traveled by vehicles on a highway section during a specified period of time divided by the sum of the travel times.

The average running speed on a given roadway varies during the day, depending primarily on the traffic volume. Therefore, when reference is made to a running speed, clearly state whether this speed represents peak hours, off-peak hours, or an average for the day. It is most appropriate to use peak and off-peak running speeds in design and operation. Average running speeds for an entire day should be reserved for economic analyses. The effect of traffic volume on average running speed can be determined using the procedures of TRB’s Highway Capacity Manual.

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Terrain

Terrain classifications pertain to the general character of a specific route corridor. The terrain classification determines the maximum allowable grades in relation to design speed. Selection of classification should be chosen on the surrounding terrain of the corridor and not on the roadway profile grade and embankment slopes.

Level or rolling are the two types of terrain often presented when choosing appropriate design criteria since these are the predominate terrains in Texas. Some areas of the El Paso District and some areas of other western Districts may be considered mountainous. When mountainous conditions are encountered, refer to AASHTO’s A Policy on Geometric Design for Highways and Streets for appropriate design criteria and design considerations.

Level Terrain

Level terrain is where highway sight distances, as governed by both horizontal and vertical restrictions, are generally long or can be designed to be so without construction difficulties or major expense. In level terrain, the surrounding terrain is considered to range from 0% to 8%.

Rolling Terrain

Rolling terrain is where the natural slopes consistently rise above and fall below the road grade and where occasional steep slopes offer some restrictions to horizontal and vertical roadway alignment. In rolling terrain, the surrounding terrain is generally considered to range from 8.1% to 15%.

Mountainous Terrain

Mountainous terrain is where longitudinal and transverse changes in the elevation of the terrain with respect to the road are abrupt and where benching and side hill excavations are frequently required to obtain acceptable horizontal and vertical alignment. In mountainous terrain, the surrounding terrain is considered to range over 15%.

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Safety

TxDOT continues to develop additional strategies to incorporate safety into its system, contributing to the goal of eliminating traffic deaths statewide by 2050 (Road to Zero). The Department uses a number of initiatives related to developing and operating a safer highway system.

Highway Safety Improvement Program (HSIP)

HSIP is a federally funded program administered by TxDOT's Traffic Safety Division that allows highway safety improvements through strategic safety planning and performance measures. The HSIP requires states to develop and implement a Strategic Highway Safety Plan (SHSP). The SHSP identifies and analyzes highway safety problems and correction opportunities, including projects or strategies to evaluate the accuracy of data and prioritize the proposed improvements. The SHSP establishes target levels for five areas of fatal and serious injuries including the number and rate of fatalities, the number and rate of serious injuries, and trends for non-motorized fatalities and serious injuries. See Highway Safety Improvement Project Manual for more information and specific design criteria for HSIP projects.

Safety Analysis

Safety analysis uses crash data, traffic volume, and roadway geometrics. Various analytical tools and methods are available for analyzing potential safety impacts of potential improvements, including historical crash data analysis, AASHTO’s Highway Safety Manual (HSM) Predictive Method, and a Crash Modification Factor (CMF) evaluation.

The historical crash data analysis involves 3 to 5 full calendar years of crash data with respect to characteristics such as severity, crash types, frequency, rates, patterns, clusters, and contributing factors. Crash diagrams such as heat maps, bar charts, and other maps graphically showing the crash emphasis locations are used to help interpret the data. A crash rate is the number of crashes that occur at a given location during a specified time period divided by measure of exposure. Crash rate is calculated per 100 million VMT using the following formula:

Crash Rate = 100,000,000 * A/(365T * V * L)

Where:

A = Number of reported crashes (in section or at location)

T= Time frame of the analysis, years

V = AADT, vehicles/day

L = Length of section, miles

The HSM Predictive Method provides procedures to analyze safety performance in terms of crash severity levels and collision types. Various spreadsheets and software have been developed to automate predictive analyses.

CMFs are used to estimate the anticipated impact of a countermeasure or mitigation on safety performance. A CMF is an index of the expected change in safety performance following a modification in traffic control strategy or design element. It can be used to estimate the safety effectiveness of a given strategy and compare the relative safety effectiveness of multiple strategies. The Crash Modification Factor Clearinghouse ( www.cmfclearinghouse.org) offers a repository of CMFs.

The Design Division (DES) has a “Traffic Simulation and Safety Analysis” section whose purpose is to provide guidance and support for safety analysis. DES has developed multiple ‘System Safety’ tools which are used to estimate a safety score of a particular roadway segment by selecting various design elements. Use of the applicable tools should begin during project scoping to evaluate the safety impacts of design decisions. Refer to the Design Division webpage for additional information and guidance on all available tools.

Safety Analysis Data

TxDOT’s Statewide Traffic Analysis and Reporting System ( STARS) is the preferred resource for traffic data, while the Crash Records Information System ( CRIS) is utilized for crash data.

Historical crash data is analyzed to identify potential safety problems that might be corrected. CRIS generates detailed crash data used to determine high crash locations, crash types, and contributing factors.

Statewide average crash rates are used in the crash rate analysis method and are useful to compare against the crash rates of a particular highway segment/intersection. TxDOT maintains ten years of crash data, available in summary reports.

TxDOT uses “crashes per year”, level of crash severity, and crash type. Crash severity is classified as follows:

K – Fatal Injury

A – Suspected Serious Injury

B – Non-incapacitating injury

C – Possible injury

N – Not Injured

99 – Unknown

Where crash frequencies include wildlife-vehicle collision(s) as a contributing factor in the CRIS records, consult with the District Environmental Coordinator or with Environmental Affairs Division (ENV) to determine if a wildlife crossing structure could improve safety. The ENV Natural Resource Management Section and District Environmental Coordinators can provide information to conduct hot spot analysis and details on types of crossings, including schematics used within TxDOT and other states.

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Additional Considerations

Turning Roadways and Intersection Corner Radii

Traffic volume and vehicle type influence the width and curvature of turning roadways and intersection corner radii. Minimum designs for turning roadways and various design vehicles are shown in Chapter 7, Section 7, Minimum Designs for Truck and Bus Turns.

Older Drivers and Older Pedestrians

Older drivers are a significant and growing segment of the road user population and should be accommodated in the design of road facilities to the extent practical. Research has shown that enhancements to the highway system to improve usability for older drivers and pedestrians can also improve the system for everyone.

The FHWA's Handbook for Designing Roadways for the Aging Population and AASHTO's A Policy on Geometric Design of Highways and Streets provide additional information for modifying geometric design elements and traffic control devices to better meet the needs and capabilities of older road users.

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