Intelligent Roadway Information System
Vehicle Detection Systems
There are several types of vehicle detection systems (VDS). The earliest of these is the inductive loop, which is a wire looped under the road surface. Some systems use radar mounted on the side of the road. Video detection uses a camera and computer vision software. Collectively, these systems are simply called detectors.
To create a detector, first select the r_node at the proper location. Then
select the Detectors tab for that
r_node. Enter the detector Name
and press the Create button.
After selecting a detector in the
r_node detector table, its properties, such
as lane type and lane # can be changed. Lanes are numbered from
right-to-left, starting with the right lane as 1. A label will be created
from this information, including abbreviations of the roads associated with
The field length of a detector determines how density and speed are estimated from counts and occupancy. It is in units of feet.
If a detector is no longer used, it can be marked abandoned.
|Auxiliary||Mainline auxiliary (ends within a mile)|
|CD Lane||Collector / Distributor|
|Merge||Freeway on-ramp (counts all merging traffic)|
|Queue||Ramp metering queue|
|Bypass||Ramp meter bypass|
|Passage||Ramp meter passage|
|Velocity||Mainine speed loop|
|Green||Ramp meter displayed green count|
|Wrong Way||Exit-ramp wrong way detector|
|HOV||High occupancy vehicles only|
|HOT||High occupancy or tolling only|
|Parking||Parking space presence detector|
It is possible to move a detector to another
r_node. Select the target
r_node and enter the detector Name. The current label for that detector
will appear on the right. To move it to the current
r_node, press the
The preferred method of collecting traffic data, supported by newer detectors, is to log every vehicle that passes by the detection area. Older detectors use fixed time intervals and put data into bins.
Every 30 seconds, an XML file is generated containing the most recent sample
data from all online detectors. The file is called
det_sample.xml.gz, and it
is written to the XML output directory.
IRIS supports several different protocols for communicating with vehicle detection systems. The protocol used depends on the comm link of the controller to which it is assigned. The following table summarizes features of each protocol.
|NTCIP||0-255 sec||Count, Occupancy|
|MnDOT-170||30 sec||Count, Scans|
|SS105||5 sec to 1 month||Count, Occupancy, Speed, Class|
|SS125||5 sec to 1 month||vlog, Count, Occupancy, Speed, Class|
|G4||5 sec to 1 hour||vlog, Count, Occupancy, Speed, Class|
|DXM||N/A (presence)||Magnetic Field|
For protocols which allow the binning intereval to be adjusted, it will be set to the poll period of the comm link.
Traffic data is continuously checked for five common failure conditions. When
one of these first occurs and every hour that it persists, an event is logged in
detector_event database table. The
detector_auto_fail_view can be used
to check recent events.
detector_auto_fail_enable system attribute is
true, the auto
fail flag for each detector will be set and cleared automatically whenever
these conditions change.
This failure condition occurs if no vehicles are counted for a duration determined by the lane type. It clears immediately when a vehicle is counted.
|Mainline, CD Lane, Velocity||4 hours|
|Exit, Wrong Way, HOV||8 hours|
|Queue, Passage, Merge||12 hours|
|Bypass, Green, Omnibus, HOT, Reversible, Shoulder||72 hours|
If a detector reports an unreasonably high count of 38 vehicles or more in a 30 second period, this condition will be triggered. It will be cleared if 24 hours pass with all counts below that threshold.
This condition occurs if the detector reports 100% occupancy for a duration determined by lane type. It is also sustained if the occupancy drops to zero with no intervening values. The condition will be cleared after 24 hours of good occupancy data.
|Mainline, Auxiliary, CD Lane, Reversible, Velocity, HOV, HOT, Shoulder||2 minutes|
|Merge, Queue, Exit, Bypass, Passage, Omnibus, Green, Wrong Way||30 minutes|
If occupancy is greater than zero and does not change for 24 hours, this condition will be triggered. It will clear immediately if the occupancy changes.
A spike timer is kept for each detector. For every 25% change in occupancy between two consecutive data samples, 30 seconds are added to the timer. If its value ever exceeds 60 seconds, the condidtion is triggered. After every poll, 30 seconds are removed from the timer. The condition will be cleared after 24 hours of no spikes.
If a detector has a fault which is not handled automatically, it can be force failed. This flag is only set manually, so it must be cleared once the failure is corrected.
When a detector is failed (auto fail or force fail), its data will not be used for travel time, ramp metering, etc. In that case, fake detection can be used — this field can contain one or more other detector names, separated by spaces. The average density or speed of those detectors (which are not also failed) will be used instead.
Traffic Data Archiving
Sample data is archived only if the
sample_archive_enable system attribute
true. Files are stored in
/var/lib/iris/traffic, in a directory with the
district name. Within that directory a new subdirectory is created for each
year, with a 4-digit name (
As data is collected, a new subdirectory is created every day — the name is
12 and day-of-month
At 10 PM, all traffic data from the previous day is moved into a single ZIP file
with the 8-digit base name and a
.vlog format is a comma-separated text log. Each vehicle event is
recorded as a single line of values, ending with a newline
|1||Duration||How long vehicle occupied detector (ms)|
|2||Headway||Time since previous vehicle (ms)|
|4||Speed||Vehicle speed (mph)|
|5||Length||Vehicle length (ft)|
Duration is the time a vehicle occupied the detector area, between 1 and
60000 ms. An invalid or missing value is represented by a
Headway is the difference in arrival time from the previous vehicle to the
current one. It is a positive integer between 1 and 3600000 ms (1 hour). An
invalid or missing value is represented by a
Time is when the vehicle left the detection area. It is included only when the headway is invalid or missing, otherwise it is left empty.
Speed is the measured vehicle speed. It is a positive integer value from 5 to 120 mph. An invalid or missing value is left empty.
Length is the measured vehicle length. It is a positive integer value from 1 to 255 ft. An invalid or missing value is left empty.
All trailing commas at the end of a line are removed. This means that an event with only duration and headway would only contain the two values, separated by one comma.
A gap in sampling data due to communication errors is represented by
(U+002A) on a line by itself.
.vlog data for 11 vehicles:
IRIS can collect these types of binned traffic data:
|Sample Type||Description||Code||Sample Size|
|Vehicle Count||Count of vehicles detected||v||8 bits|
|Occupancy||Percent occupancy count (0 to 100.00)||op||16 bits|
|Scans||Scan occupancy count (0 to 1800)||c||16 bits|
|Speed||Average speed (mph) of detected vehicles||s||8 bits|
A binned sample file consists of some number of periods of equal duration. The first period begins (and the last period ends) at midnight. The binning interval determines the number of samples collected per day — a shorter interval results in more samples. If the period is longer than 30 seconds, the samples are allocated evenly into 30-second bins for storage.
|Period||Binning Interval||Samples||Stored Bins|
|5||5 seconds||17280||5 seconds|
|6||6 seconds||14400||6 seconds|
|10||10 seconds||8640||10 seconds|
|15||15 seconds||5760||15 seconds|
|20||20 seconds||4320||20 seconds|
|30||30 seconds||2880||30 seconds|
|60||60 seconds||1440||30 seconds|
|90||90 seconds||960||30 seconds|
|120||2 minutes||720||30 seconds|
|240||4 minutes||360||30 seconds|
|300||5 minutes||288||30 seconds|
|600||10 minutes||144||30 seconds|
|900||15 minutes||96||30 seconds|
|1200||20 minutes||72||30 seconds|
|1800||30 minutes||48||30 seconds|
|3600||60 minutes||24||30 seconds|
|7200||2 hours||12||30 seconds|
|14400||4 hours||6||30 seconds|
|28800||8 hours||3||30 seconds|
|43200||12 hours||2||30 seconds|
|86400||24 hours||1||30 seconds|
For each detector, a binned sample file is created for each
traffic data sample type. The base file name is the
detector name. The extension is the traffic data code followed by the
period (in seconds). For example, 60-second vehicle count samples collected
from detector 100 would be stored in a file called
100.v60, containing 2880
Each data sample is either an 8- or 16-bit signed integer, depending on the sample type. 16-bit samples are in high-byte first order. A negative value (-1) indicates a missing sample. Any data outside the valid ranges should be considered bad.
The IRIS client user interface includes a traffic map layer which is created automatically from the road topology. By default, this layer uses traffic density to determine the color of each segment. Other themes are available for speed and flow. The Legend menu at the top of the map can be used to view the thresholds used for each color in a theme.
Every 30 seconds, the client will make an HTTP request for the current
traffic data. The URL to locate that file is declared as a
property in the
/etc/iris/iris-client.properties file (on the IRIS server).
The property is
tdxml.detector.url, and it should point to the
det_sample.xml.gz XML file, as made available by nginx on the IRIS server.
The appearance of the traffic map layer changes depending on the current zoom level. If the zoom level is below 10, the layer will not be visible. At zoom levels 10 through 13, the layer will display segments as aggregate of all detectors in each mainline station. At zoom level 14 or above, each mainline detector will be displayed as a separate segment.
The maximum distance between adjacent stations to draw segments on the map is
specified by the
map_segment_max_meters system attribute. It is also the
maximum downstream distance for associating station data with a segment.