traffic on a highway

Traffic Forecasting & Analysis

Coordinating MnDOT's traffic monitoring and vehicle classification programs

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Glossary

 

Acceptable Percent Change Criteria – When screening incoming short count data the adjusted count is compared to the most recent official AADT for the same location.  If the percent difference between the current adjusted count and the most recent official AADT falls within the Acceptable Percent Change Criteria (+ or - percent thresholds) then the count is determined to be valid and the official AADT will be estimated using the new data.  If the adjusted count falls outside the Acceptable Percent Change Criteria, a recount is requested.  For more details visit the Data Screening and Recounts section of the "Collection Methods" page.

 

Adjusted Count – a raw traffic count that is adjusted by the specific month and day of the week Seasonal Adjustment Factor (SAF) from the “cluster group” to which it is assigned.  If applicable, it may be adjusted by an axle correction factor (ACF).

 

Annual Average Daily Traffic (AADT) – the theoretical estimate of the total number of vehicles using a specific segment of roadway (in both directions) on any given day of the year. This estimate represents the total number of cars per year divided by 365 and is developed using factors to adjust for season, day of the week, and vehicle type.

 

Automatic Traffic Recorder (ATR) – permanent device in the pavement surface that continuously and automatically collects traffic data. 

 

Average Daily Load (ADL) - the estimate of a daily load on a roadway segment; calculated from the daily total vehicle type multiplied by their appropriate ESAL factors.

 

Average Daily Traffic (ADT) - a 24-hour traffic volume that should be qualified by stating a time period, (e.g., MADT – monthly average daily traffic, or ADT for the period 6/21/2011-6/23/2011)

 

Axle Correction Factor (ACF) - corrects for vehicles with 3 or more axles that contribute “extra” axle hits to the raw 48 hour traffic count. These factors are calculated from vehicle classification counts and are used on the Trunk Highway system only.

 

Axle Load – the total load transmitted by all wheels in a single, tandem, or tridem axle configuration.

 

Base Year – in a traffic forecast, the year construction will be completed and the roadway will be open to traffic.

 

Bituminous ESAL (BESAL) – the equivalent single axle load for a bituminous or flexible pavement.

 

Cluster Group – set of ATR locations with similar traffic pattern characteristics; each group has a set of Seasonal Adjustment Factors (SAF) for each month of the year and “middle day” of the 48 hour count. These factors are applied to 48 hour short counts with a “middle day” of Tuesday, Wednesday, or Thursday.

 

Concrete ESAL (CESAL) – the equivalent single axle load for a concrete or rigid pavement.

 

Continuous Counts – ATR and WIM device counts taken by sensors permanently installed in the pavement of roads; continuously reported by hour throughout the year.

 

Control Section - a segment of the state highway road system that is divided into shorter, more manageable parts for record keeping within MnDOT.

 

Count Cycle – how often a location is counted. There are varying count cycle lengths (counted every 1, 2, 4, 6, or 12 years) depending on the type of roadway and where it is located.  For more information visit the Traffic Counting Schedule section of the "Collection Methods" page.

 

County Road (CR) - roads locally maintained by county highway departments in Minnesota; span a wide variety of road types, varying from A-minor arterials that carry large volumes of traffic to an improved road.

 

County State Aid Highway (CSAH) - specialized form of county road that is part of the state aid system.  County State Aid routes are eligible for funding from the County State Aid Highway Fund.

 

Data Screening – process where incoming traffic counts are factored and the resulting adjusted (24 hour) count is then compared to the most recent official AADT for the same location.  If the adjusted count falls within the “Acceptable Percent Change” criteria then it is determined to be valid and the official AADT will be estimated using this new data.  If the adjusted count falls outside of the “Acceptable Percent Change” criteria, the count is examined further.  If a recount is necessary, it is assigned a priority based on the “Recount Decision Tree.”  For more details visit the Data Screening and Recounts section of the "Collection Methods" page.

 

D factor – the percentage of traffic moving in the peak travel direction during the 30th highest hourly volume of the year; calculated by dividing the higher directional volume occurring in the 30th highest hour by the total roadway volume for that hour.  

 

Design Hour Volume (DHV) - the traffic for a selected hour of the day; usually the 30th highest hour of the year for Greater Minnesota and the peak hour for the Metro Area.

 

Design Lane Factor (DLF) - factor to estimate traffic volume and truck components on heaviest traveled lanes for the purposes of ESAL estimation.

 

Directional Distribution (DD) or Directional Split - the split of traffic by direction for a selected period of time, usually the design hour; shown in “50/50” format, where north or east direction is first number and south or west is the second number.

 

District – MnDOT has divided Minnesota into eight operating districts based on geographic location and resources. For more information visit http://www.dot.state.mn.us/information/districts.html.

 

Draft AADT - traffic volume values that are currently estimated but have not yet been made official.

 

Equivalent Single Axle Load (ESAL) factor – numeric factor based on the equivalent load concept that represents the average effect of each vehicle type on the pavement.  The equivalent load concept relates the effect that axles in different configurations and weights have on pavement performance as compared to the effect of a single 18,000-pound axle.  These ESAL factors can vary depending on commodities being hauled.

 

Federal Highway Administration (FHWA) http://www.fhwa.dot.gov/ – federal agency that administers federal funds and issues policy and procedure timetables for implementation of federal legislative directives; however, they do not have a direct role in the development of urban transportation plans or their development.  The FHWA use TSM’s continuous count data, annual AADT, and VMT estimates in federal-level travel analysis and determination of funds.

 

Functional Classification - the process by which all roads are grouped into classes or systems according to the character of service they are intended to provide.  Functional class defines the part that any particular road or street should play in serving the flow of trips through a highway network.

 

 

close-up view of glossary page

 

 

Greater Minnesota – the area of Minnesota that lies outside the seven-county Metro Area.

 

Half Tolerance Guidelines – The Half Tolerance Guidelines are half (inside) the Acceptable Percent Change Criteria or double (outside) the Acceptable Percent Change Criteria.  When screening incoming short count data or estimating AADT the Half Tolerance Guidelines can help to determine if a count should be reviewed more closely (outside half tolerance) or used for the official AADT (inside half tolerance).  These guidelines can also help make decisions regarding recount priority.  For more details visit the Data Screening and Recounts or AADT Estimation Procedures section of the "Collection Methods" page.

 

Heavy Commercial Annual Average Daily Traffic (HCAADT) – theoretical estimate of the total number of heavy commercial vehicles using a specific segment of roadway (in both directions) on any given day of the year.  This estimate represents the total number of heavy commercial vehicles per year divided by 365 and is developed using factors to adjust for season.

 

Heavy Commercial Average Daily Traffic (HCADT) – a 24-hour heavy commercial traffic volume that should be qualified by stating a time period (e.g., MHCADT – monthly average daily heavy commercial traffic, or HCADT for the period 6/21/2011-6/23/2011).

 

Heavy Commercial Traffic – traffic from all trucks with at least 2 axles and 6 tires.

 

Heavy Commercial Vehicle Miles Traveled (HCVMT) – derived from HCAADT, and important because the number of heavy vehicles on a road affects traffic operations, safety, and pavement performance.

 

Highway Performance Monitoring System (HPMS) http://www.fhwa.dot.gov/policyinformation/hpms.cfm – national level highway information system that includes data on the extent, condition, performance, use and operating characteristics of the nation's highways.  The TSM office submits state-level traffic data to HPMS on a monthly and yearly basis.

 

Induction Loop – a coil of wire embedded in the pavement that acts as an electrical circuit; when a vehicle passes over the loop, the inductance decreases and allows the detection device to identify the presence of a vehicle.

 

Intelligent Transportation Systems (ITS) data - encompasses a broad range of wireless and wire line communications-based information and electronics technologies. When integrated into the transportation system's infrastructure and in vehicles themselves, these technologies relieve congestion, improve safety, and enhance productivity in the U.S.

 

K factor - the ratio of the 30th Highest Combined Direction Hourly Volume to the AADT from continuous count location data.

 

Local system roads - any road not on the Interstate or Trunk Highway system; can be designated as a CSAH (County State Aid Highway), CR (County Road), MSAS (Municipal State Aid Street), township, or municipal road.

 

Manual Count – a classification count that is taken manually for 4-16 hours at one time.  Data is collected by hour and direction, and body types are noted.

 

Maximum Loaded Vehicle – heavy commercial vehicle type that is usually loaded to the legal gross weight limit (gravel trucks, grain trucks, tank trucks, etc). The presence of these body types in the traffic mix can indicate the need to use ESAL factors higher than the default values.

 

Metro Area – the seven-county Twin Cities Metropolitan Area comprised of Anoka, Carver, Dakota, Hennepin, Ramsey, Scott, and Washington counties.

 

Metro District – comprised of eight counties-- the seven-county Twin Cities Metropolitan Area plus Chisago County.

 

Monthly Average Daily Traffic (MADT) – average daily traffic for a given month; calculated by summing traffic volume for all days in month, then dividing by number of days in month.

 

MnESAL – Excel program that uses AADT/HCAADT and vehicle class data to forecast 20-year BESALs and 35-year CESALs.  This information is then used to help develop pavement design options.

 

Municipal State Aid Street (MSAS) - similar to the County State Aid system, this is a system of designated municipal streets in cities above 5,000 in population that are not already on the state highway or CSAH systems.  Municipal streets on the MSAS system are eligible for funding from the Municipal State Aid Highway Fund.

 

Peak (or highest) hour volume – hourly volume with the highest value in a 24 hour period.  Higher traffic volumes occur in the evening and in the morning of weekdays because of work-related trips, and the evening peak is typically the highest since non-work related trips are also made in this period.

 

Piezos – sensors permanently installed in the pavement surface that collect volume, classification, and speed data; emit a digital signal when a vehicle passes over the sensor.

 

Raw Count – a non-adjusted traffic count.

 

Recount – an additional traffic count is requested if the adjusted volume exceeds the acceptable percent change margins and therefore does not pass TSM examination criteria.  The “Recount Decision Tree” is used to assign a priority to all recounts. For more details visit the Data Screening and Recounts section of the "Collection Methods" page.

 

Regional Transportation Management Center (RTMC) http://www.dot.state.mn.us/rtmc/ – MnDOT facility in Roseville that provides the communications and computer infrastructure necessary for coordinated transportation management on metro freeways.  More specifically, it is responsible for analyzing the effectiveness of traffic management tools, collecting traffic flow information, and developing new traveler information products.  Traffic data from the RTMC is used to estimate AADT at many locations.

 

Seasonal Adjustment Factors (SAF) – used to adjust incoming data to account for differences between weekday and weekend traffic, as well as fluctuations in traffic volumes from month to month to determine AADT and HCAADT.

 

Sequence Number (SEQNUM) – unique identifier (e.g. 2465, 28637) for all MnDOT traffic segments in the traffic monitoring program.

 

Transportation Information System (TIS) – integrated database system that stores roadway related traffic information such as AADT and HCAADT, and roadway physical characteristics such as number of lanes, presence of medians, etc.

 

Tractor Semi-Trailers (TST) - an articulated vehicle consisting of a towing engine and a semi-trailer that carries the freight.  A semi-trailer does not trail completely behind the towing vehicle, but is attached at a point that is just forward of the rear-most axle of the towing unit.

 

Traffic Forecasting – the estimation of future traffic volumes and loads on a specific roadway segment.  The projections are derived by trending historic total volume and truck data as well as considering the effects that future changes in the socioeconomic factors will have on particular segments.

 

Traffic Volume – count of motorized vehicles that travel past a certain location over a specific period of time.

 

Travel Behavior Inventory (TBI) - battery of surveys conducted by MetCouncil, MnDOT, and WisDOT that examines where people travel as well as when, why, and how.  Conducted roughly every ten years since 1949, the TBI is the most comprehensive source of travel data in the 19-county MN/WI extended metro region.

 

Travel Demand Modeling - forecasts the expected demand for transportation facilities using information such as roadway and transit networks, land use, population and employment data, and travel behavior data. The four steps in travel demand forecasting are trip generation, trip distribution, modal choice, and trip assignment.

 

Trunk Highway (TH) - major roadways such as Interstates, US Highways, and State Highways.

 

Tube Count - uses portable devices that span the width of a road to count axles and classify vehicles based on their axle spacing; generally taken for a period of 48 hours during weekdays and during the months of April-October.  Tube counts are used by district, county, city, and TSM personnel to gather data at more than 32,000 Minnesota locations over a four-year cycle.

 

20-year cumulative ESAL – a measure of pavement damage aggregated across a 20-year forecast period for the design lane.

 

Typical Traffic – traffic volume that is typical for the majority of the period of time that the Annual Average Daily Traffic (AADT) estimate will represent.  When scheduling traffic counts, we try to avoid effects due to construction, harvest, school vacations, or any other “unusual conditions.”  For more information visit the Construction Policy section of the "Collection Methods" page.

 

Vehicle Classification (VC) – categorization of traffic by 13 vehicle types (motorcycles, single unit trucks, semis with single or twin trailers, etc.); data is routinely collected on trunk highways and some local roads, and is used to produce HCAADT.

 

Vehicle Classification Scheme - the axle spacings of vehicles to classify traffic into 13 vehicle types.  For information on the specific vehicle types view the Vehicle Classification Scheme table.

 

Vehicle Classification Sites – locations where classification of vehicles in the traffic mix takes place.

 

Vehicle Miles Traveled (VMT) - commonly used to measure the demand on our transportation network; computed by multiplying the AADT by the centerline road miles. For yearly figures visit TSM's Roadway Data page.

 

Wavetronix – a device that uses radar to classify vehicle types by length at locations where the use of tube counters is impractical or unsafe.

 

Weigh-In-Motion (WIM) - permanent devices that continually collect and store vehicle data such as axle weight, axle spacing, length, speed, type, and count; emit an analog signal whose strength is directly proportional to the axle weight of the vehicle.