Traffic Monitoring Guide

An Overview of

Department of Transportation Engineering & ManagementUniversity of Engineering & Technology, Lahore

Contents

In this Part we will have knowledge about

Data Collection DesignFactoring traffic countsFreeway-expressway ramp

counting procedures

What is Traffic Monitoring Guide?

A manual to monitor traffic for various uses given by FHWA

USES

Why do we need to monitor traffic?

Traffic varies over a number of different time scales, including:

o time of dayo day of weeko season (month) of the year.o Directional variationo Geographic variation Research has shown that truck volumes

vary over time and space differently than car volumes (Hallenbeck et al 1997)

Variation

Data Collection Design

ACCOUNTING FOR VARIABILITY

Data Collection

• Recommended Plan

Continuous

Integration of data collection efforts

The ability to simultaneously collect all three types of traditional traffic monitoring data is

called “nesting” traffic counts. Traffic data collected by other agencies within the State. tracking of HOV lane usage traffic surveillance centers The Intelligent Transportation

Systems

Nesting

Advantages of Integration

Reduce the # of continuous data collection sites

Increase the # of data available

Less Cost

Caution! Care must be taken while using this data

Continuous Data Collection

Not being used in proper way

As input for traffic management

Used in real time and then discarded

If used properly, quality of TMI can improve.

Continuous traffic monitoring data collection programs

ATRs AVC WIM

Automatic Traffic Recorders

Recorded on site as hourly volumes by lane Downloaded periodically to a central

location Summary volume statistics : AADT, AAWDT,

seasonal adjustment factors, day-of-week adjustment factors, 30th and 100th highest annual hourly volume as a fraction of AADT, lane distribution factors, growth trends

Data from different ATRs is averaged for getting representative factors and short counts are adjusted for annual conditions.

Locations for ATRs

Locations selected to measure specific trends:o Monitoring movement accurately on a road of

particular importanceo Traffic activity on a larger group of roads by

monitoring on a single location. Historic locations Semi-random selection of locations within

specific categories of roads (e.g. rural interstates E-W)

Availability of power and/or telecommunications access to locate the counter

Continuous Vehicle Classification

Truck volume and load information Other information obtained: the size of

seasonal commodity movements, the seasonal fluctuations in truck travel on roads, trends in annual truck volumes on specific roadways, day-of-week traffic patterns for trucks as opposed to cars, the lane distribution patterns of trucks.

Axle and length classification for 13 classes

Aggregate vehicle categories

Locations for AVCs

Length & axle classifiers can be placed at different locations e.g. on freeway & rural area respectively and then combined with care.

For specific pavement sections (LTPP sites)

For creating truck factors

Weigh-In-Motion Sites

WIM takes all three types of data Located upstream of enforcement

scales Semi random selection within the

area because equipment only works accurately on level ground, with good pavement, and with little or no roadway curvature and are expensive.

These sites point to the most important truck routes

Steps for determining Continuous Count Locations

Determine Objectives• # &

dist. Of locations

• For developing factors

What is needed and present• For

specific projects

• From traffic management & enforcement scales

Determine available Funding• Traditi

onal funds

• Funds from outside divisions

Prioritize and place • Place

there for which funding exists

Determine how these can help• can

counter locations be used for factor creation?

Determine # of additional locations needed• Using

existed & desired specific count locations

Prioritize and Allocate• Alloca

te counters to statewide needs locations

Place additional Counters• If

funding Permits

Short Duration Counts

Largely revised each year Frequent and Occasional counts “Project Counts” for site specific

studies Statewide counts and project counts

combined give accurate and cost-effective data

This data is not the “design data”, it requires day-of-week, seasonal and other factors for adjustment, obtained from Continuous data.

Short Count Program Design

Defining and overlaying the short duration counting programs.

Making the separate counts in an area as “general coverage “ counts

Collecting the data for most precise needs only In general, these are taken under

consideration:o counts taken to provide system coverageo counts taken to meet the HPMS needso counts for special needs studies. Statistical sampling is done before special

needs sample

Coverage Count Programs

These are data collection efforts that are undertaken to ensure that “at least some” data exist for all roads maintained by the agency.

The TMG recommends, as a general rule, that each roadway segment be counted at least once every six years

The HPMS Sample

Highway Performance Monitoring System is a combination of complete coverage for the NHS and other principal arterials, and a structured sample of roadway sections for the remaining functional systems excluding the rural minor collectors and local.

A primary goal of the HPMS traffic data collection effort is to provide a statistically valid estimate of total annual vehicle distance traveled (VDT).

Other statistical samples data are also collected

Special Needs Counts

Includes the data that are not part of the HPMS or any other existing State-specific sampling study

Project counts are also done which normally include sections with poor pavement that require repair or rehabilitation, locations with high accident rates, sections that experience heavy congestion, and roadways with other significant deficiencies.

FACTORING TRAFFIC COUNTS

Common Necessary Adjustments

Time-of-day adjustments for counts that consist of less than 24 consecutive hours (the TMG recommends 48-hour counting periods)

Day-of-week adjustments for counts that do not measure traffic conditions for all days of the week

Seasonal adjustments for counts that do not cover periods

long enough to account for variation from month to month or season to season

Axle correction adjustments for axle counts (such as counts

taken with a single road tube sensor) that do not directly convert the number of axle pulses into vehicle counts by vehicle classification.

Recommendations for Factoring

There is not a single best method. Depends upon number of continuous

counters a State can afford to operate and the extent of the roadway system for which factors must be developed and applied.

Factors must be applied to short counts Factors should be developed to best utilize

available data collection resources Factors should be developed separately for

total volume and for estimates of volume for individual truck classifications.

Creation Of Factor Groups

Assumption: temporal characteristics affect all roads Note: Analyses need to be performed separately for

total volume factors and for factors that are applied to volumes by vehicle classification

Factor groups are used to create temporal variation factors to statistically convert short counts to annual averages.

Set of roads as a “group”, all group members are similar in characteristics

Data from sample of locations on roads are collected and mean is taken

The procedure assumes something so it has errors which are then minimized using correction factors

The Factoring Process(Assumptions & Errors)

Defining the Groups

Measured Characteristic

Selecting a representative sample

Computation Of Factors

Defining the Groups

It is difficult to define groups of roads that “are similar with respect to traffic variation,” and the more “mathematically alike” the factoring groups created from the data, the more difficult it is to define the attributes that determine which roads belong to a given group.

Example: Group is Rural interstate Highway but the travel pattern is not same within this group

Measured Characteristic

The grouping process is made more difficult and error prone because the appropriate definition of a “group” changes depending on the characteristic being measured.

Example: Volume factor groups of trucks VS computation of axle correction factors

Selecting a representative sample

It is very difficult to select a representative sample of roads from which to collect data for calculating the mean values used as factors.

The primary reason for this is the location of continuous data collection sites.

Computation Of Factors

The last source of error discussed in this section occurs in the computation of factors because the datasets used to compute those factors are not complete.

This is mainly due to FAILURE of collection devices

HOW TO CREATE FACTOR GROUPS

The three techniques are:o cluster analysiso geographic/functional assignment of

roads to groupso same road factor application. Each of these techniques starts from

existing permanent counter data. The first step is to compute the

adjustment factors that will be used in the group selection process

Cluster Analysis

Using least squares method most similar sets of factors are determined.

Most similar stations according to factors are grouped and process is repeated

Where to stop clustering process depends on analyst e.g. not more than 5 factor groups

Next step is to assign each cluster the best continuous counter data for which the group fits.

Geographic/Functional Classification of Roads Factor

Groups Allocation of roads into alternative factor groups on

the basis of available knowledge about traffic patterns.

Available knowledge is usually obtained from a combination of existing data summaries and professional experience with traffic patterns.

The characterization of roadways using functional class makes it easy to assign individual road sections to factor groups and also allows the creation of factor groups that are intuitively logical.

For each initial factor group, continuous counter data is allocated, mean and SD calculated

Example urban Interstates and expressways other urban roads rural Interstates other rural roads in the eastern

portion of the state other rural roads in the western

portion of the state recreation routes.

Geographic/Functional Classification of Roads Factor

Groups

Same Road Application of Factors

This process assigns the factor from a single continuous counter to all road segments within the influence of that counter site

The boundary of that influence zone is defined as a road junction that causes the nature of the traffic volume to change significantly.

The short count in question must be taken on the same road as the continuous counter.

Limitations: More counters or less roads are required

These techniques can be used in combination

ALTERNATIVES TO FACTORING

Appropriate where factor groups are not readily known and the annual traffic estimate must be very accurate.

Taking week-long counts removes the day-of-week variation. Counting at the same location four times at equally spaced intervals removes the majority of seasonal bias.

TYPES OF FACTORS

For seasonal adjustments, some techniques use monthly factors, whereas othersuse weekly factors. Both of these techniques can be successful.

Computation of Factors (monthly factors)

There are two basic steps in computing the factors to be used: computing the numerator and the denominator.

The numerator is assumed to be AADT. The denominator is dependent on the factoring approach taken.

Computing AADT

A simple average of all days: simple average of all 365 days in a given year. Missing data can cause biases

An average of averages (the AASHTO method): The AASHTO approach first computes average monthly days of the week. These 84 values (12 months by 7 days) are then averaged to yield the seven average annual days of the week. These seven values are then averaged to yield the AADT. This method explicitly accounts for missing data by weighting each day of the week the same, and each month the same, regardless of how many days are actually present within that category.

Continued

where: VOL = daily traffic for day k, of day-of-week i, and month ji = day of the week j = month of the year k = 1 when the day is the first occurrence of that day of the week in a month, 4 when it is the fourth day of the week. n = the number of days of that day of the week during that month (usually between 1 and 5, depending on the number of missing data).

Computing the Denominator for Monthly Factors

An adjustment factor that converts any weekday ADT for a given month into AADT. This would convert monthly average weekday traffic to annual average daily traffic.

Definition of weekday Example :(Monday to Friday), then the

denominator is the sum of all weekdays (Monday to Friday) divided by the number of days of data present.

Continued

If the State chooses to compute an average monthly day-of-week factor (i.e., combining the monthly variation and the individual day-of-week variation), then the denominator is the simple average of available daily volumes for that day of the week for that month.

If the State decides to use a weekly factor, the denominator is simply the average of the seven days for the appropriate week.

Adjustments to Short Duration Volume Counts

In general, a 24-hour, axle count, is converted to AADT with the following formula:

AADThi = VOLhi * Mh * Dh * Ai * Ghwhere

AADThi = the annual average daily travel at location i of factor group h

VOlhi = the 24-hour axle volume at location i of factor group h

Mh = the applicable seasonal (monthly) factor for factor group h

Dh = the applicable day-of-week factor for factor group h (if needed)

Ai = the applicable axle-correction factor for location i (if needed)

Gh = the applicable growth factor for factor group h (if needed).

Determining the Appropriate Number of Continuous ATR

Locations

The basic assumption made in the procedure is that the existing locations are equivalent to a simple random sample selection. Once this assumption is made, the normal distribution theory provides the appropriate methodology. The standard equation for estimating the confidence intervals for a simple random sample is: where

B = upper and lower boundaries of the confidence interval X = mean factor T = value of Student's T distribution with 1-d/2 level of confidence and n-1 degrees of freedom n = number of locations d = significance level s = standard deviation of the factors.

FREEWAY-EXPRESSWAY RAMP COUNTING PROCEDURES

Problems Involved

Portable counters are impossible to install because of very less safety

Two methods for counts can be applied:

o Permanent Counterso Counters on ramps

Procedure

Mainline volumes are known at two points and all input/outputs are measured between those two points. The two boundary points are normally ATRs or other instrumented mainline locations that provide a highly accurate measurement of annual traffic volumes. These points are used to control the counting and adjustment process and are referred to as “anchor points.”

Establishing Anchor Points and count duration

Each State will have to make its own determination regarding the appropriate number of anchor points. As a general rule-of-thumb, the recommended number of interchanges between anchor points is five.

The minimum period recommended for collecting ramp volume data is 24 hours. Ideally, all ramps between two anchor points should be counted for the same 24-hour period.

In the next Episode…

Vehicle Classification Monitoring

Truck Weight Monitoring

Format and supplement

Coming Soon!!

End of Session one

A variety of traffic monitoring activities, including vehicle speed monitoring, traffic management activities, toll collection devices, and incident detection sensors, can provide traffic volume information.