2.2.3 Traffic Counts

One of the fundamental data types needed to perform traffic operational analysis is traffic count data. There are several types of counts that may be needed based on the level of detail necessary for the analysis. It is recommended that traffic count data be collected on typical weekdays that represent a typical day’s travel patterns. Avoid days with atypical traffic conditions, which can be caused by the following situations:
  • On or around holidays;
    • Especially for areas that are retail-heavy, unless the study is specifically to analyze impacts during such time periods
  • During inclement weather;
  • During crashes or other major incidents affecting travel patterns;
  • During construction or lane closures within the project limits;
  • During construction or lane closures in the vicinity of the project that result in detours within the project limits;
  • During summer break or other times when school is not in session; or
  • Any other situations that may present atypical travel patterns (e.g., special events)
It is recommended that traffic counts be collected for atypical travel patterns based on project specifics. For example, if the purpose of the project is to study summer traffic due to an increase in tourism, game-day traffic at stadiums, holiday or event traffic, or other unique situations, an atypical traffic count could be necessary.
In general, collect traffic counts in 15-minute time intervals for the defined time period(s). It is suggested that traffic counts begin before the start of the congestion and end after congestion dissipates.

2.2.3.1 Turning Movement Counts (TMCs)

TMCs count vehicles, pedestrians, and bicyclists making a specific movement or turn at an intersection for all approaches during a defined period. Collect TMCs on a typical weekday and for a minimum of two consecutive weekdays, based on coordination with the TxDOT project manager. A typical weekday count is defined as a count occurring on a Tuesday, Wednesday, or Thursday. It is recommended that the traffic count data be collected in 15-minute increments and during the same period as the remainder of the data collection. TMCs may include vehicular, pedestrian, and bicyclist modes of travel.
Recommended TMC collection method:
  • 15-minute increments;
  • Typical weekday peak period (Tuesday, Wednesday, Thursday);
  • Two consecutive weekdays; and
  • Minimum of 2-hour period, which includes the peak hour
For atypical locations, it is suggested that the period of collection is based on the circumstances. Examples include corridors or roadways near malls, stadiums, or other land uses that generate large volumes of traffic at non-standard times of day.

2.2.3.2 Segment Traffic Counts

Segment traffic counts are conducted along a roadway during a typical week for a consecutive seven-to-14-day period for Interstate IAJR projects with microsimulation and one to two days for other projects. This is based on coordination with the TxDOT project manager and collected in 15-minute increments. Segment traffic counts are often separated by direction to analyze directional split. For analyses that use microsimulation as part of an IAJR, segment traffic counts are recommended to be conducted for a period of at least 7-to-14 consecutive days. is an
example
of a generic segment traffic count location.
Example Segment Traffic Count Corridor Profile
Figure 2-1: Example Segment Traffic Count Corridor Profile
Suggested segment traffic data collection method:
  • 15-minute increments;
  • Typical week; and
  • Seven to 14 consecutive days for projects needing microsimulation on IAJR projects; one to two days for other projects

2.2.3.3 Classification Counts

Classification counts determine the breakdown of the 13 FHWA vehicle categories, per the TxDOT Traffic Recorder Instruction Manual. Classification counts are similar to segment traffic counts and include two-way traffic volumes on a roadway for a specified period of time greater than 24 hours but less than a year. Classification counts help determine the percentage of different vehicle categories using a roadway, which is important in performing traffic operational analysis and pavement design. It is suggested that classification counts be conducted during a typical week for a period of seven consecutive days for projects needing microsimulation on an IAJR project. Classification counts are to be conducted for a minimum of 48-hours for other projects, based on coordination with the TxDOT project manager. For locations where classification counts are performed in place of segment traffic counts, it is suggested the segment traffic collection method be used.
Suggested classification counts collection method:
  • 15-minute increments;
  • Typical week; and
  • Minimum seven consecutive days for microsimulation on IAJR projects; one to two days for other projects

2.2.3.4 Oversaturated Counts

Oversaturated conditions occur when the demand for the facility exceeds the capacity. These conditions result in a significant drop in speeds and flow rate through a point or segment, depending on the severity of saturation. The HCM defines conditions as oversaturated when any of the following conditions are satisfied:
“(a) the arrival flow rate exceeds the capacity of a point or segment, (b) a queue created from a prior breakdown of a facility has not yet dissipated, or (c) traffic flow is affected by downstream conditions.”
For uninterrupted flow, oversaturated conditions result from a bottleneck on the facility. During oversaturated conditions, queues extend upstream from the point of the bottleneck.
It is recommended that traffic counts collected for oversaturated conditions span a time interval greater than just the peak hour or peak period. It is suggested that field observations are conducted, along with segment traffic counts, to determine the overall peak period and the shouldering effect of traffic or queue build up and dissipation. A shouldering effect occurs during a peak period when a facility is oversaturated and cannot process any additional demand, causing the peak period to extend longer than an hour. The effect can occur on both ends of the peak hour during a peak period, where the demand for the facility is higher than the capacity, allowing only a portion of the traffic volumes to process through the facility until the demand falls below capacity.
For oversaturated and bottleneck conditions, it is suggested that counts are started at least one hour before congestion starts and end one hour after congestion has been cleared. Similar to TMCs, oversaturated counts are collected on a typical weekday and for a minimum of two consecutive days, based on coordination with the TxDOT project manager. Depending on how congested the study area is, collect counts at least one intersection or interchange upstream of the study area.
In addition to traffic counts, it is recommended that queue data is collected for oversaturated conditions, depending on the type of analysis being performed. See
Section 2.2.4 - Queue Data
 for calculating demand in oversaturated conditions.
Recommended oversaturated counts collection method:
  • 15-minute increments;
  • Typical weekday peak period (Tuesday, Wednesday, Thursday);
    • Include the shouldering effect of traffic
    • Begin counts at a minimum one hour before congestion starts and end at a minimum one hour after congestion dissipates
  • Collected at least one intersection or interchange upstream of the congested study area or other logical terminus determined through engineering judgement; and
  • Collected for two consecutive weekdays