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Intersection Approach Counts and Turning Movement Information
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Introduction:


The basic measurement in traffic engineering is counting cars, trucks, and pedestrians, passing through a specific location or intersection during a fixed period of time. These counts, in conjunction with other measurements and conditions, are used to provide estimates of volume, speed, demand, and capacity.

General Measurement Information:


The following are some of the basic traffic measurements used in the analysis of traffic, which provides information to determine if a signal installation is justified:
  • Volume: is the number of vehicles passing a point during a specified time period. It can be expressed as the total number of vehicles per hour, day, week, month, or year.
  • Demand: is the number of vehicles that desire to travel past a point during a specified time period. Demand is higher than actual volumes where congestion exists.
  • Capacity: is the maximum number of vehicles that can pass a point during a specified period. This is a characteristic of the roadway.

Traffic Measurement Activities:


Turning Movement Count


The Turning Movement Count data shows the hourly turning movement of vehicles (usually from 6:00 am 9:00 am and 3:00 pm 6:00 pm). Currently, the most reliable means of collecting this data is through manual count. The resulting data is used in designing the signal timing plan (phases and cycle length).

Intersection Approach Count


The Intersection Approach Count data shows the averaged hourly traffic volume (24 consecutive hours) approach at an intersection. Various types of equipment are deployed to collect this information. The resulting data is used to justify the installation of a signal control system.

Annual Average Daily Traffic (AADT)


Average Annual Daily Traffic (AADT) reports traffic volume data for a specific roadway location (segment) in a standardized way that represents an estimate of the yearly volume of all motorized vehicles divided by 365 days. There are approximately 8,000 48-hour counts collected annually.


These counts are taken on weekdays only between April and November. They are adjusted to AADT by using factors derived from Automatic Traffic Recorders (ATR's). These factors take into account weekend and seasonal variations in traffic. Traffic counts on the Trunk Highway (TH) system also have axle correction factors applied to them to adjust for those trucks which have 3 or more axles. The counts are then analyzed to determine what will be used for AADT on each traffic segment. More specialized traffic data such as weekend traffic volumes and peak hours can be obtained from the (ATR) data. AADT is useful for planning, geometric design, analyzing patterns and trends, and performing traffic projections. These data are collected on a 2 year cycle (even years) for TH. Metro county roads (CR's) and County State Aid Highways (CSAH's) are also on a 2 year cycle. Beginning in 1999, 2/3 of the municipalities in the Metro area remain on a 2 year cycle for their Municipal State Aid Streets (MSAS) system while 1/3 have chosen a 4 year cycle. The county roads, CSAH's and MSAS's in greater Minnesota are collected every four years with the exception of 6 municipalities which have chosen a 2 year count cycle beginning in 1999. The data is available in the form of maps. Electronic traffic data is available from the Transportation Information System (TIS).


Average Weekday Daily Traffic (ADT)


Average Daily Traffic estimates can be obtained based upon a 48 hour traffic count by applying the appropriate weekday or weekly factor to the raw data.


Collection Technologies

Technology: Radar Detector (RTMS)
A radar detector is used to sense the presence or passage of vehicles typically providing presence, volume, occupancy, speed and classification information. There may be discrete user-defined detection zones up to a defined number of feet away. Output information is provided to existing controllers via contact pairs and to computer systems via a RS-232 serial communications port.

Technology: Pneumatic Tubes
Pneumatic Tube technology has been the mainstay of many traffic counting programs when more permanent installation measures are not justified. Pneumatic tubes use internal algorithms to derive vehicle class and speed data on-the-fly, as vehicles pass the survey site, incrementing class and speed "bins" for the current time interval. This information is downloaded after the survey and may be reformatted and presented in a few limited ways.

Why use pneumatic tubes? Detecting axles with rubber pneumatic tube is easily the most cost-effective traffic sensing method, especially for short-term surveys. Pneumatic tubes are cheap, accurate and reliable and will continue to operate effectively over a huge range of environmental conditions. Throughout the world, all detailed vehicle classification schemes are now based on axles. Many systems have tried to perfect inductive, optical or magnetic methods. Only axle detectors, however, give precise speeds and wheel positions over the entire vehicle spectrum, from motorcycles through to heavy articulated vehicles. By measuring intra-vehicular axle distributions, axle detectors also help you monitor road wear and manage pavements.

Technology: Hand Counters
 
Use of the electronic hand counter (Jamar) consists of depressing different buttons on the device to use for specific movements (left, through, right), for classification of vehicles (auto, truck, bus), or for other purposes.   The mechanical hand counter shown above was used before the electronic hand counter.