GEOG 855
Spatial Data Analytics for Transportation

8.1 Traffic


There are two types of traffic data that state DOTs use. The first is live traffic data, and it is generally used to measure and address congestion. Often, live traffic data is measured by monitoring vehicle speeds. There are a number of companies that collect speed data by enlisting fleets of vehicles to share their location data. These companies sell the data to state DOTs and others interested in real-time traffic. One such company is INRIX.

The other type of traffic data that state DOTs use is traffic count data. There are a variety of devices which are commonly employed to obtain traffic counts. A few of the more common devices are described below:

  • Automatic Traffic Recorder (ATR)
    ATRs are permanent traffic counting devices which operate 24/7/365 and are installed below the road surface. ATRs capture traffic volume, but do not differentiate between vehicle types.
  • Continuous Automatic Vehicle Classification (CAVC) 
    CAVCs are also permanent traffic counting devices which operate 24/7/365 and are installed below the road surface. In addition to counting vehicles, these devices classify vehicles into the 13 categories which FHWA requires (see Figure 1).
  • Weigh-in-Motion (WIM)
    Like ATRs and CAVCs, WIMs are also permanent traffic counting devices which operate 24/7/365 and are installed below the road surface. In addition to counting and classifying vehicles, WIMs also capture truck weights.
  • Short-Term In-Pavement (STIP)
    STIPs are semi-permanent traffic counting devices much like ATRs. These devices are typically only used to collect counts once a year for a 24-hour period.
  • Portable Short-Term Counter
    These are temporary traffic counting devices which make use of pneumatic tubing which is laid across the road. Different configurations can be used to achieve simple traffic counts or vehicle classification counts. The majority of traffic counts are performed using these devices.

The computers which capture, store, and transmit ATR, CAVC and WIM data back to a central location are typically housed in secure enclosures which are positioned alongside the roadway and are equipped with a permanent power source. STIP sites are not equipped with permanent computers and power. Instead, portable units are used to complete the period counts at these locations.

13 vehicle classifications. More information in text description below.
Figure 8.1 - FHWA's 13 Vehicle Classifications
Click for a text description of Figure 8.1

Class 1: Motorcycles

Class 2: Passenger cars

Class 3: Four tire, single unit

Class 4: Buses

Class 5: Two axle, six tire, single unit

Class 6: Three axle, single unit

Class 7: Four or more axle, single unit

Class 8: Four or less axle, single trailer

Class 9: 5-Axle tractor semitrailer

Class 10: Six or more axle, single trailer

Class 11: Five or less axle, multi trailer

Class 12: Six axle, multi-trailler

Class 13: Seven or more axle multi-trailer

Credit: FHWA

Watch This

Watch the first 4 minutes of the Traffic Counting Training Video below which was developed by PennDOT to train traffic count technicians.

Click for a transcript of first four minutes.

Welcome to an overview of PennDOT's traffic counter training. As a traffic field technician, you'll be playing an important role for the Pennsylvania Department of Transportation. PennDOT's traffic monitoring system collects, analyzes, and maintains highway traffic data on over 40,000 miles of roads in Pennsylvania.

Specifically, you will help us collect information on traffic volume and vehicle classification throughout the state. With your assistance, over 10,000 traffic counts are taken annually in Pennsylvania. After all traffic data is collected in the field, it is submitted to PennDOT's central office where it is processed and used to make important decisions about highway design; safety and planning; for research to improve congestion management; to help design and install traffic control systems; to track statistics to help make air quality conformity determinations; to provide information for economic development; and for required federal reporting.

To assist with the collection of this data, traffic field technicians use five types of counting devices. Of these five types, three of them are permanent traffic sites that record information 24 hours a day, 7 days a week, 365 days a year.

The most sophisticated of these permanent devices are Weigh In Motion sensors or WIMs. These sites collect axle weight information as well as vehicle classification information.

The second type of device is the Continuous Automatic Vehicle Classifier or CAVC. These sites collect vehicle classification information.

The third type is the Automatic Traffic Recorder or ATR which collects traffic volume data. The permanent sites have a power source and collect traffic data through magnetic loop sensors, piezos, and/or Kistler sensors that are embedded in the road surface. A modem automatically transfers the data directly to the PennDOT central office daily.

The fourth type of counting device is a semi-permanent counting device commonly referred to as a STIP or Short-Term In Pavement site. The STIP is much like the ATR except that there is no counter in the roadside cabinet. A field technician installs the counter on a predetermined schedule, usually one time a year for a 24-hour period. The counter is then retrieved and the data downloaded and processed.

The fifth type of counting device is a Portable Short-Term Counter. A portable short-term counter installation is a count that is set for a 24-hour period. This type of traffic data collection device provides the majority of the traffic counts taken as part of our statewide count program. A portable short-term traffic counting device collects the data using pneumatic rubber tubing laid across a roadway and attached to a portable counter that has been secured along the side of the road. Tires passing over the road tube register axle counts. The installation of one road tube registers volume counts while a 2 road tube installation produces a vehicle classification count.

The following diagram shows typical setup configurations for portable short-term traffic volume and classification counts: center turn lane class count; center turn lane volume count; divided highway class count; divided highway volume count; two-lane class count; two-lane volume count.

The locations of PennDOT’s permanent traffic counters are presented in a series of district and county maps. Below each map is a table identifying the county, route, segment, and offset of each device. You’ll use this information in Assignment 8-1.

The Value of Traffic Data

Traffic data has a wide variety of uses, a few of the most significant of which are listed here:

HPMS Reporting

One of the most significant uses is to comply with federal HPMS reporting requirements. As was covered last week, states need to report HPMS data to the FHWA each year. HPMS reporting is particularly important to state DOTs since the apportionment of federal funding is based on it.

Highway Safety Improvement Programs (HSIP)

HSIP is a federal aid program aimed at reducing traffic fatalities and major injuries. FHWA requires the project selection process to be data-driven. Traffic data is required to determine crash rates and, consequently, it plays an important role in selecting eligible HSIP projects.

Highway Design

Traffic count data is considered in the design of new roads and the rehabilitation and enhancements of existing roads.

Congestion Management

While real-time congestion management approaches typically use live traffic data based on vehicle speeds, traffic count data can be used to assess congestion and the prospective benefits of ITS technologies.

Roadway Video Logging

Many state DOTs acquire roadway video from specially equipped vehicles which drive all the state-owned roadways. The video data is then made available via a software application which allows users to go on a “virtual drive.” A few examples are Oregon Department of Transportation’s (ODOT) Digital Video Log (DVL) system and PennDOT’s Video Log application. There are many uses of roadway video systems, not the least of which is that they can serve as a means to monitor and maintain roadways and roadway assets, which is both safer and less expensive than an actual visit. The traffic group in PennDOT, for example, uses Video Log to scope out new locations for conducting counts.

A screenshot of PennDOT's Video Log Application. Shows point on a map, the street view of the area, and basic info.
Figure 8.2 - PennDOT's Video Log Application
Credit: PennDOT