Speedo Graduations. The Road Safety (Vehicles) Regulations 1999 and the Australian Standard Design Rule 18 – Instrumentation referenced doesn’t stipulate speed graduations.

In many vehicles the speedometer is graduated in 10 km/h increments and the speedometer needle width can represent 2-3 km/h.

The photo here is from a Surabu Liberty 2005 model, which does have an indicator needle which is 2.2 km/h thick. This significantly hampers the drivers ability to determine whether they are 0 or 3 km/h above the limit.

Eye sight. It is a medical fact that in people 40 years and over the eye starts to lose it's flexibility due to age and consequently the ability of the eye to rapidly refocus, or to refocus on close objects diminishes. The usual consequence is that these people require reading glasses.

A motorist will be using eye correction to suit distance vision when driving and consequently the instrumentation is likely to be out of focus, particularly in a situation when they glance down at the instrumentation.

I'm 51 years old and wear glasses which give me very good distance eye sight. I can see a golf ball land at 200 metres, but when I glance down at a speedometer, my eyes take a while to adjust and re-focus. Usually for the time I glance at the speedometer it is out of focus, and in my case the speedo needle looks twice it's actual width.

Parallax error. When checking speeds at the 9 o’clock position, the height of the drivers head changes the angle of view and will influence the perceived reading.

On this speedometer the height change can represent 2-3 km/h difference. A good demonstration is trying to read the speedo from the passenger seat when the needle is near vertical.

The two photos on the right appear to show the same speed reading. The car was set on cruise control on a flat road and the photos were taken 4 seconds apart, but 300mm height difference, representing a tall and short driver.

When the close up of the photo is examined there is an apparent difference of 2 km/h in the speedometer reading, the difference for being booked at 5% or 10% over in a 40 km/h zone.

Conclusion

To be practical, do you think a person glancing down from the road to check their vehicle speed can honestly determine whether they are slightly over or under the speed limit, even if they have 20/20 vision.

Remember that this is a fully compliant vehicle speedometer in a 5 month old car, and similar in style to that featured in the Transport Accident Commission (TAC) advertisements for the “Wipe off 5” campaign.