A number of speed measuring systems measure the time it takes a vehicle to travel a known distance to calculate speed. Distance traveled equals speed multiplied by time. Speed equals distance divided by time.
v = Vehicle Speed
d = Distance Traveled
t = Time
All timing and distance measuring instruments should be calibrated for accuracy on a periodic basis. Testing data should include at the least; dates tested, test facility, test methodology and data, and due date for next test.
|Automatic Timing Systems|
An inductive loop uses 2 sensors spaced typically ten's of feet apart. Each sensor is a conductive wire, cable, pipe or metal strip in the shape of a relatively long narrow rectangular loop buried in and across the road. Some portable systems consist of two rubber type strips placed on top of and perpendicular to the road. Each sensor is electrically charged and generates a magnetic field directly above the loop. When a vehicle enters the magnetic field the vehicle changes the magnetic field which is detected by the sensor. The time it takes a vehicle to pass each sensor is used to calculate speed.
Magneto resistive sensor systems use 2 sensors spaced typically ten's of feet apart buried into the road. The time it takes a vehicle to pass each sensor is used to calculate speed. Each sensor is a cylindrical shape with a 4 inch diameter and 2 inches deep. The sensors are battery powered and communicate to a processor using a wireless link.
Piezo Ceramic Sensors
Piezo ceramic sensor systems use 2 sensors spaced typically ten's of feet apart buried in or flush with the road. Each sensor covers an entire lane and is roughly several inches wide. The sensor detects pressure from a vehicle's weight. The time it takes a vehicle to pass each sensor is used to calculate speed. The sensor only measures the vehicle front tires because the ceramic sensor cannot recover to a steady state, zero pressure, fast enough to measure rear tires.
Road cables are portable systems that consist of 2 or more pneumatic cables placed across the road typically ten's of feet apart. The time it takes a vehicle to pass each cable is used to calculate speed. Each pneumatic cable MUST be the same length, different lengths result in different trigger times introducing an error. Shorter cables trigger faster than longer cables.
Across the Road Laser
Across the road laser uses two independently pulsed laser beams that cross the road at a 90 degree angle. The system measures the time it takes the front of a vehicle to travel from the first beam to the second beam. Additionally a second measurement tracks when the rear of the vehicle clears both beams. The two measurements must match closely or the data is rejected. Two measurements are taken in the event multiple vehicles cross the beams.
|Manual Timing Systems|
Aerial Clocking is probability the best system for police in terms of catching large numbers of speeders, but one of the worst in terms of personnel, equipment and expense. A police officer observes traffic from an airplane or helicopter and times targets as they pass between two points, typically lines painted across the road spaced 1/10 mile apart. The airborne officer times targets using a timing computer or a stop-watch. The officer radios to patrols on the ground which vehicle to ticket when a violation is calculated.
Distance / Time Computer
Distance / Time computers can be hand held or built into a patrol vehicle such as VASCAR. Some microwave and laser radars have an optional timing mode to measure speed without transmitting. The operator inputs distance between points and uses the computer to time targets by pushing a button when a target passes each point. Most lidars have a range only mode to allow the operator to measure distance from lidar to an object for use in manual timing mode.
VASCAR (Visual Average Speed Computer And Recorder) is a distance / time computer built-in to the patrol vehicle. Distance is manually entered into the computer, or by measuring distance by driving pass two points. A button is pushed at each point so distance can be derived from patrol vehicle odometer. The operator inputs the time parameter by pushing another button when the target passes each point.
A stop-watch can be used to calculate speed by measuring the time it takes a target to travel between 2 points of known distance. The greater the distance the more accurate the measurement.
MPH = (15 / 22) x Distance / Time
Distance in feet, Time in seconds
KPH = 3.6 x Distance / Time
Distance in meters, Time in seconds
|Timing Systems Errors|
Timing systems are vulnerable to any errors in measured time or distance. Timing vehicles over longer distances and longer times tends to reduce errors. Plus or minus a fraction of a second error over a long distance introduces a smaller error than for a short distance.
Errors in both time and distance can add or subtract from actual speed. If time short and distance long, or time long and distance short, the combination error will partially or fully cancel out.
The faster a target the greater the time and distance required to calculate an accurate speed measurement. For most cases a small distance error produces a small speed error, however small time errors can produce significant speed errors in many situations.
|vm = (do ± derr) / (to ± terr)|
vm = Measured Speed
vo = Speed
to = vo / do
to = Time vehicle travels Distance do
± terr = Time Error
do = Distance vehicle travels in Time to
± derr = Distance Error
Insure all unit dimensions work out; if distances in feet (or meters), speeds in feet per second (or meters per second), etc.