R91 Lidar Annex Technology Description
Across the Road Laser (ATRL) Speedmeter
1.0
Technology overview
An across the road laser (ATRL) speedmeter is a system comprising at least two laser sensors
set up perpendicular to the longitudinal axis of the roadway. The laser sensors are spaced
apart by a known distance allowing for the calculation of vehicle speed by precise
measurement of time. As such, when a passing vehicle intersects the beam emitted from the
first laser sensor a timer is started. When the vehicle then intersects the beam of the second
laser sensor, the timer is stopped and the speed of the vehicle is calculated. An ATRL
speedmeter may be used on a tripod, mounted in a vehicle or installed in a fixed installation.
In addition to speed calculation, an ATRL speedmeter can be configured to calculate the
length of the vehicle for purposes of vehicle type classification, determining the time between
vehicles for collision detection, and the range to the vehicle for lane determination.
2.0
Technology details
2.1
Principles of operation
The operation of an ATRL Speedmeter is based on the provision of at least two laser
sensors set alongside the roadway, each of which transmit laser energy across the road toward
passing vehicles. When a vehicle traveling the roadway interrupts each of the laser beams, a
detection event for the particular laser sensor is recorded. Precision timing circuitry then
measures the elapsed time between the beam breaks. Using the timing results obtained, a
microprocessor can then calculate the vehicle speed, vehicle length, vehicle lane location and
the time (and hence, the spacing) between successive vehicles.
2.1.1 Speed detection
A vehicle traveling along a roadway monitored with an ATRL Speedmeter set up
perpendicular to the roadway will initially interrupt the laser beam of the first laser sensor
encountered. The same vehicle will then interrupt the laser beam of the second laser sensor.
The speed of the vehicle can be calculated using the elapsed time between the laser beam
interruptions from laser sensor one and laser sensor two. Since the distance between the laser
sensors is a known distance, the change in distance over the elapsed time between the laser
sensors is calculated and can be converted to kilometers per hour (km/h) or miles per hour
(mph) as required.
𝑠𝑝𝑒𝑒𝑑 (𝑖𝑛𝑐𝑜𝑚𝑚𝑖𝑛𝑔) = 𝑑⁄∆𝑡
Where:
d = distance between laser sensors
∆t = elapsed time between laser sensor interruptions
1
Basic speed measurement
A second speed measurement may be carried out when the vehicle exits the
measurement field of the laser sensors as shown at numbers 3 and 4 above. The two
measurements can then be compared with one another and considered a valid speed
calculation if there is a minimum difference in vehicle speed determined between the two
calculations. This verification method serves to prevent calculation errors due to internal
noise in the ATRL system or from other external factors. In addition, if two or more vehicles
were to simultaneously transit side-by-side in different lanes through the sensor detection
zone, the second speed measurement will prevent the calculated speed measurement from
being attributed to the wrong vehicle.
2
Invalid measurement
2.1.2 Vehicle length
Vehicle length can be calculated from the beam interruption time using the
established speed calculation. Knowledge of the vehicle length allows for the possible
enforcement of differing speed limits for transport trucks and passenger cars. The duration of
the beam interruption from each laser sensor is proportional to the length of the vehicle.
𝑡3
𝑡1
𝑡2
Beam interruption time
In the diagram above, t1 is the incoming vehicle beam interruptions between laser 1
and laser 2, or the incoming vehicle speed. T2 is the outgoing vehicle speed, and t3 is the
duration the beam has been interrupted. Vehicle length is then calculated using the following
formula:
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𝑣𝑒ℎ𝑖𝑐𝑙𝑒 𝑙𝑒𝑛𝑔𝑡ℎ = 𝑣1 × 𝑡3
Where:
v1 = incoming vehicle speed
t3 = beam interruption time
In the case of a large transport truck (e.g. a tractor trailer combination), the gap
between the truck and the trailer could possibly be seen as the vehicle exiting the beam. In
such a case, the ATRL Speedmeter would consider the end of the vehicle had been found
only after, for example, a 2 meter distance had been determined without another detection.
2.1.3 Lane determination / distance to the vehicle
Distance to the vehicle from the laser sensors can be used to determine which lane of
a multilane roadway the vehicle is traveling in. Lane discrimination is a key factor if the time
between vehicles is to be calculated. There are two possible methods of determining vehicle
distance; 1) adding a third laser beam at the position of the first laser beam, but angled across
the roadway by a predetermined amount such as ten degrees, and 2) measuring distance to the
vehicle using time-of-flight (TOF) technology.
2.1.3.1 Third laser beam method
In this method, a third laser beam is installed in the same location as the first laser
beam but is angled away from the first laser beam at an angle of 0.17453 radians or 10
degrees toward oncoming traffic. The angle established between the first and third laser
beams results in the distance between the two beams increasing with distance. Consequently,
the distance between the third laser beam and the first laser beam can be calculated and used
to determine in which lane of the roadway the vehicle is traveling. As with a speed
measurement determination, when the vehicle interrupts the third laser beam, a timer is
started. The timer stops when the vehicle then interrupts the first laser beam. When the
measurement is completed, the vehicle’s speed is calculated and used with the time between
the third laser beam interruption and the first laser beam interruption to calculate the distance,
which will ultimately allow a determination as to the lane of the roadway in which the vehicle
was traveling.
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Interruption of third laser beam
Interruption of first laser beam
First calculate the distance between the third laser beam and the first laser beam:
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑐𝑜𝑣𝑒𝑟𝑒𝑑 = 𝑣 × 𝑡𝑑
Where:
v = speed of the vehicle calculated by the system
td = elapsed time between the third laser beam interruption and the first laser
beam interruption
Next calculate the distance to the vehicle:
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑜 𝑣𝑒ℎ𝑖𝑐𝑙𝑒 = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑐𝑜𝑣𝑒𝑟𝑒𝑑⁄𝑡𝑎𝑛𝑔𝑒𝑛𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑎𝑛𝑔𝑙𝑒
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2.1.3.2 Time-of-flight method
The time-of-flight (TOF) measurement system is based on the principles of precise
time measurement. In this case, TOF is a measurement of the time it takes a laser pulse to
travel to an object and its reflection to return. Since the speed of light is a constant, the time
it takes the laser pulse to travel to the target is proportional to the distance to the target. The
TOF range measurement technique can be done without using an additional (third) laser
sensor.
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 =
𝑐 × 𝑡𝑟𝑡
2
Where:
c = speed of light
𝑡𝑟𝑡 = TOF (round trip)
NOTE: round trip distance is divided by 2 for single trip distance to the vehicle
2.1.4 Space and time between vehicles
It is possible to calculate the space and time between two vehicles traveling in the
same direction and in the same lane of the roadway, wherein lane determination may be made
by either of the two methods previously described. Direction is determined by the order the
laser beams are broken. The distance between vehicles calculation will be discarded if the
same direction or same lane determination is not true. The process of measuring and
calculating the distance between vehicles is as follows:
𝑡𝐴
𝑡𝐵
Space and time measurements between vehicles
Time between vehicles: 𝑇𝑏𝑣 = 𝑇𝐴 − 𝑇𝐵
Space between vehicles: 𝑆𝑏𝑣 = 𝑉𝐴 × 𝑇𝑏𝑣
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Because of the lane discrimination capability provided, no determination of a vehicle
following too closely would be made if the scenario below were to occur.
2.2
Basic components
In its most basic form, an ATRL system comprises a computer with data storage, two
or more laser sensors, and a camera to record the number plate of the vehicle measured by the
system. The computer controls the operation of the system, triggers the camera, and stores
the measurement data and camera image if the speed measurement is greater than the speed
threshold set by the operator.
Digital Camera
Laser Sensors
CPU
data storage
ATRL Basic Components
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2.3
Typical laser characteristics
Characteristic
Wavelength
Modulation
Pulse repetition frequency
Laser eye safety Class to IEC 60825-1
3.0
Nominal value
905 nm (near infrared)
Discrete pulses
10 kHz or greater
Class 1
Laboratory vehicle speed simulator
An ATRL speedmeter vehicle speed simulator is composed of three parts:
Laser Interface
The laser interface receives the electrical pulses generated by the programmable
timer (described below) and converts them to a laser pulse which is compatible
with laser sensors.
Programmable Timer
The programmable timer generates a series of electrical pulses, one for each
of t he ATRL unit lasers, with a programmable length and delay from one
to the other. These signals are adapted by the laser interface and transmitted
to the laser sensors. Circuitry in the ATRL unit interrupts the laser pulse
emissions, thereby simulating the equivalent effect of a vehicle passage for the
receivers and speed measurement circuit. The ATRL unit can then calculate
the speed of the vehicle and transmit the speed data on a serial port (RS-232).
The programmable timer shall be certified by the cognizant metrological
laboratory, and certified by a primary metrological laboratory.
Computer
The computer executes an application program that enables a user to select the
vehicle speed to be simulated and programs the programmable timer (described
above) with this information.
Programmable timer /
pulse generator
Laser interface
ATRL UUT
computer
Block Diagram of the ATRL laboratory vehicle speed simulator
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