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Section 2: 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 traffic volume, traffic speed, and percentage of trucks or other large vehicles.

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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 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 (TPP) 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 TPP 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:

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  • Lane Width
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  • Lateral Clearances
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  • Free-flow Speed
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  • Terrain
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  • 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:

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  • Selection of geometric design for an intersection
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  • Determining the appropriate type of facility and number of lanes warranted
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  • Performing ramp merge/diverge analysis, and
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  • 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 TRB’s 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. The speed of vehicles on a road depends, in addition to capabilities of the drivers and their vehicles, 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 either 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 these 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 manner with efficient traffic operations and with low crash frequency and severity. The facility should, therefore, accommodate nearly all demands with reasonable adequacy and also should only fail under severe or extreme traffic demands. Because only a small percentage of drivers travel at extremely high speed, it is not economically practical to design for them. They can use the roadway, of course, but will be constrained to travel at speeds less than they consider desirable. On the other hand, the speed chosen for design should not be that used by drivers 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- and high-speed designs. For design purposes, the following definitions apply:

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. The selected design speed should be a logical one with respect to the anticipated operating speed, topography, the adjacent land use, modal mix, and the 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 be consistent with the speeds that drivers are likely to travel on a given roadway. A roadway of higher functional classification may justify a higher design speed than a lesser classified facility in similar topography. A low design speed, however, should not be selected where the topography is such that drivers are likely to travel at high speeds.

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.

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, and pedestrian and bicycle activity.

Posted Speed. Posted speed refers to the maximum speed limit posted on a section of highway. TxDOT’s Procedure 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. The running speed is the length of the highway section divided 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, it should be clearly stated whether this speed represents peak hours, off-peak hours, or an average for the day. Peak and off-peak running speeds are used in design and operation; average running speeds for an entire day are used in economic analyses. The effect of traffic volume on average running speed can be determined using the procedures of the Highway Capacity Manual.

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Terrain

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. Whenever mountainous conditions are encountered, refer to AASHTO's A Policy on Geometric Design for Highways and Streets for appropriate design criteria and design considerations.

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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 turning templates for various design vehicles are shown in Chapter 7, Section 7, Minimum Designs for Truck and Bus Turns.

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Safety

TxDOT is developing additional strategies to incorporate safety into its system, contributing to the goal of eliminating traffic deaths statewide by 2050 (Vision Zero). Towards this end, 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 purpose of the SHSP is to identify and analyze highway safety problems and correction opportunities, include projects or strategies to address them, evaluate the accuracy of data, and prioritize the proposed improvements. The SHSP establishes five target areas including number and rate of fatalities, serious injuries, and non-motorized fatalities & serious injuries.

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

The historical crash data analysis involves review of 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. There are various spreadsheets and software 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, compare the relative safety effectiveness of multiple strategies. The Crash Modification Factor Clearinghouse (www.cmfclearinghouse.org) offers a repository of CMFs.

The Design Division has started a new section "Traffic Simulation and Safety Analysis". The purpose of the new section is to provide guidance and support for safety analysis. The Design Division is also developing a 'System Safety' tool which can be used to estimate a safety score of a particular roadway segment by selecting various design elements. The Rural Highways Tools (Two-Lane and Multi-Lane Rural) are available for use, with development currently underway for future Intersections, and Urban Highways Tools. Use of the applicable tools should begin during project scoping to evaluate the safety impacts of design decisions, as well as applicable projects in-progress to be let April 2020 and beyond. Please refer to the Design Division webpage for additional information and guidance on the tools.

Safety Analysis Data. TxDOT's Statewide Traffic Analysis and Reporting System (STARS) is a good resource for traffic data, while the Crash Records Information System (CRIS) is utilized for crash data. ( http://txdot.ms2soft.com/tcds/tsearch.asp?loc=Txdot&mod) ( https://cris.dot.state.tx.us/public/Query/app/public/welcome)

Historical crash data is analyzed to identify the potential safety problems that might be corrected. CRIS generates detailed crash data used to determine unsafe 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. Summary reports of crash data are available at the following link: https://www.txdot.gov/government/enforcement/annual-summary.html

TxDOT uses “crashes per year”, level of crash severity, and crash type. Severity is defined on the KABCO scale as follows:

K – Fatal Injury

A – Incapacitating Injury

B – Non-incapacitating injury

C – Possible injury

O – No Injury

Where accident frequencies include a wildlife-vehicle collision as a contributing factor in the CRIS records, consult with the District Environmental Coordinator or with Environmental Affairs Division to determine if a wildlife crossing structure could improve safety at these hot spot areas. 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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