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Reference Measurement Installations of PTB for the Testing of Speed Measurement Devices

The working group „Speed Measuring Instruments“ operates several reference measurement installations that are used in the conformity assessment tests of speed measurement devices. The devices under test must perform to high statistical significance within the error limits specified in the Verification Ordinance (±3 km/h for measured values up to 100 km/h, ±3 % of the measured value for values above 100 km/h). One of the reference installations is located next to PTB along a local highway (speed limit there is 70 km/h, one lane in each direction), another one along a federal autobahn (no speed limit, two lanes going in the same direction). Figure 1 shows photographs of both sites. Both sites can measure with full accuracy up to vehicle speeds of 300 km/h.

Figure 1: Reference measurement installations (left: local highway; right: federal autobahn, the installation is marked by a gray box)

The reference installations measure speed by a distance-vs.-time measurement:

In each lane four piezoelectric sensors have been installed into the surface, perpendicular to the direction of travel and parallel to each other with a separation of 6 m between sensors. Vehicle speeds can therefore be measured for the two lanes independently.
Figure 2 schematically shows the setup of the reference measurement installation. A sensor passed by a vehicle experiences a pressure pulse from the tire and delivers an electrical impulse to the recording units. From the time differences between the impulses and the known sensor separations three speed measurement values v1, v2, and v3 can be determined. A reference value v is considered valid only if the maximum deviation of the three individual values from their mean does not exceed a given maximum.

Figure 2: Setup of the reference measurement installations

 

The operation of the reference installations is described in the following, using a hand-held laser speed-measuring device as an example (Fig. 3). Each measurement value of the device under test (possibly annotated with lane number, direction of travel etc.) is transmitted via an interface (possibly by a radio link) to the reference installation where it can be compared with the simultaneously measured value delivered by the piezo sensors in the reference installation.In addition, both measurement values are also included into a video recording of the overall traffic situation at the time of measurement.

Figure 3: Setup of the test rig for the example of a hand-held laser speed-measuring device

The result of a comparison run with many vehicles is typically displayed as a histogram (Fig. 4). The horizontal axis is marked with the difference between the speed measured by the device under test and the reference installation, while along the vertical is indicated the number of times a given speed difference has occurred during a series of measurements. Vertical red lines mark the legally mandated error limits. In order to pass the conformity test, not a single one of the measurements must fall outside the error limits.The example in Figure 4 comprises 1255 individual measurements, showing that the legally mandated error limits are obeyed with extremely high statistical significance.

Figure 4: Histogram of a comparison run. Up to a vehicle speed of 100 km/h the deviation of the value measured by the device under test is given in km/h, above that limit in percent.