GPS Speed Accuracy
Smartrak's Automatic Vehicle Locator (AVL) velocity data is accurate to 0.36km/h.
When using data for a disciplinary process, Smartrak recommends that the overspeed events are thoroughly investigated and aspects of how overspeed events are generated in the system are considered. There are situations that can cause false-positive overspeed events and these situations should be recognized and checked prior to any action.
The following details how overspeed events are calculated. Overspeed events can be identified from reports and the map site.
How overspeed events are generated
Every overspeed event undergoes the following speed validation (any events which do not pass all these checks are automatically invalidated):
- Satellite Quality - GPS position from a minimum of 4 satellites to ensure accuracy.
- Changes in speed - Checks GPS speed hasn't increased or decreased by more than 20km/h in the last second.
- Point to Point Speed - GPS speed is within +/- 9km/h of the calculated point-to-point speed, given the previous and current position.
NOTE: Despite all this validation, there is still the possibility for false-positives due to GPS bounce. Please ensure you check for false-positives as detailed later in this document.
Overspeed Threshold Sampling
An overspeed event will occur when a vehicle exceeds the speeding threshold for more than 10 valid GPS speed samples (1 second per sample), at which point the speed in the 10th sample will be reported as speed in the overspeed event.
An example of how this sampling works could be as follows:
You could have 15 seconds of overspeed data with only 7 valid GPS speed events in a row, in this case, an overspeed will not be generated.
When this event occurs the overspeed event shows up on the map as orange arrows. Subsequent overspeed events will be reported every 5 seconds until the vehicle's speed returns under the overspeed threshold, at which point blue arrows will show on the map (indicating operating below speed threshold).
The period between the first overspeed event and the blue below speed event will give you the overspeed duration for that speeding period.
Checking for False-Positives
Due to the nature of GPS it is important to understand that speeds reported can be inaccurate on occasion. If the GPS satellite coverage is low or the vehicle is in conditions that can affect the signal, the speed can be miscalculated. Taking this into account we advise that people watch for trends in speeding patterns rather than singling out a specific event.
When singling out specific events, please consider the following to ensure their validity and that they aren't false-positives:
- Is the overspeed event on the road?
If the event is half-way up a hill or on top of a house, then the event was subject to GPS bounce and is not correct.
- Is the overspeed event in a highly built-up area?
Even if the event is on the road, sometimes a highly built up area will create GPS bounce off tall buildings, trees, canyons and gullies.
- Check the events immediately before and after the overspeed event
If there is a large jump in speed in a small amount of time, this event may not be correct.
- Does the vehicle have a pattern of overspeeding or high g-force events compared to other vehicles
Further investigation of vehicle history may be appropriate to ensure the unit is providing accurate data.
Again, Smartrak recommends that overspeed events are viewed as a trend for a driver, rather than individual events. Exceptions to this may include situations where a driver has dangerously exceeded acceptable limits.
If there is a pattern of overspending or high g-force events for individual drivers, further investigation may be appropriate.
The Overspeed Report
The Vehicle behaviour report within Smartrak's Reporting suite will print out any overspeed events that have been recorded.
The overspeed period report within Smartrak's reporting suite will assist you in identifying speeding patterns within your fleet.