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.