When travelling at a steady speed on a level highway, your GPS may indicate a speed about 10 kph less than the speedometer. Which speed is the correct one?

Question 1:  
When travelling at a steady speed on a level highway and the GPS indicates a speed slower/faster than what is indicated on the speedometer, what is the correct speed to use?

Many motorists are unclear about what speed to use – the speed on your GPS or the speed on your car’s speedometer. The calibration of a car’s speedometer and the calibration of a GPS may differ, therefore your vehicle speedometer may indicate a speed different to your GPS. As a rule of thumb, you should always use the reading on your car’s speedometer as the correct reading.  
In the instance of speed traps, the measuring equipment used to determine if your vehicle is speeding is also calibrated. This means that the calibration of the speed measuring equipment may also differ from the calibration of your vehicle’s speedometer, with both showing different speeds. Therefore, the Prosecution Guideline for Speed Measuring Equipment gives an allowance of 10 kph.

Question 2:  
When can you use your hooter?

Regulation 310 of the National Road Traffic act deals with excessive noise. It states that no person is to operate a vehicle on a public road in a manner that causes any excessive noise that can be avoided if the person exercised reasonable care.

Regulation 310A goes further in discussing the use of hooters, and states that no person on a public road shall use the sound device or hooter of a vehicle except where it is necessary or on the grounds of safety. 

The public is therefore prohibited from causing unnecessary, excessive noises while driving unless they are complying with a provision of the National Road Traffic Act or are doing so on the grounds of safety. If a person uses their hooters unnecessarily they can be charged for public disturbance.

The Act does not unfortunately provide a list of scenarios where a person may legally use a hooter. The legislation uses the phrases ‘necessity’ and ‘grounds of safety’ to allow for interpretation of the regulation by the relevant authorities on a case-by-case basis.

Here are some examples from the AA legal use of a hooter (please note that this is not an exhaustive list and the relevance of each scenario will depend heavily on the circumstances of each and every incident):
  • Following the instruction of a traffic officer
  • Witnessing or trying to avert a hi-jacking, armed robbery or any suspected criminal activity
  • Trying to avoid an accident with a pedestrian or another vehicle on the street
  • If a motorist is in an accident and has no other means of alerting people to their location
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