Speedometer Accuracy And Low Speed Driving 154f2k

IN SOUTH AFRICA...

Or: can I drive at an indicated 25 km/h in a 15 km/h zone?

1/5 : when we assume a vehicle’s speedometer is reading “too fast” - we may be tempted to drive at an indicated speed that is faster than the speed allowed

e.g. when we assume a speedometer is 10 km/h “too fast”, then we may be tempted to drive at an indicated speed of 70 km/h in a 60 km/h zone (or 25 in a 15)

…may have some basis in the fact that SANS 1441 sets the following limits to speedometer accuracy:

speedometers may not under read (i.e. be “too slow”) speedometers may not be “too fast” by more than 10% plus 4 km/h (this applies to speeds from 40 to 120 km/h)…

…which means speedometers are allowed to be “too fast” by:  as much as 10 km/h at a true speed of 60 km/h  as much as 16 km/h at a true speed of 120 km/h…

…BUT WHICH ALSO MEANS THAT:

at an indicated speed of e.g. 120 km/h, one could be driving at a true speed of anything between 105 km/h and up to 120 km/h if one’s speedometer conforms to SANS 1441 standards!

my speedometer indicates

then my true speed

should be at least

but

could be up to *

15 km/h

not specified, likely 10 km/h?

–

15 km/h

25 km/h

not specified, likely 19 km/h?

–

25 km/h

40 km/h 100 km/h

33 km/h 88 km/h

– –

40 km/h 100

120 km/h

105

–

120

If speedometer conforms to standards

km/h

km/h

km/h

True speed also depends on vehicle manufacturer and conditions of use; assuming no abnormal conditions, malfunctions or modifications - e.g. tyres with a larger diameter than specified could cause true speed to be above indicated speed!

… WHICH IMPLIES THAT AT LOW SPEED e.g. driving at an indicated 25 km/h, one could

Indicated speed on speedometer (km/h)

assume a true speed likely upwards of 22 km/h

25

60

80

100

120

SANS 1441 requires speedometer to be “too fast” by

No standard

<9 km/h

<11 km/h

<12 km/h

<15 km/h

Suzuki Jimny too fast by only

Likely <3

3

4

5

6

VW Golf GTI 2.0 TSI too fast by only

Likely <3

3

4

6

7

M-Benz SLS AMG too fast by only

Likely <3

3

2

2

3

Ford Figo 1.4 too fast by only

Likely <6

6

7

5

8

Fiat 500C too fast by only

Likely <2

2

3

5

6

Toyota Auris Sport X too fast by only

Likely <4

4

5

5

6

Mitsubishi Triton with bigger diameter tyres too fast by only

0

0

0

1

1



Start from the safer assumption that

indicated speed = true speed

unless you are 100% sure and can prove otherwise 

Especially in low speed zones,

drive at the speed required

e.g. drive at an indicated 15 km/h in a 15 km/h zone 

A difference of 5 km/h is not trivial at all Hitting a person at a true impact speed

Is the same as falling from a height of

15 km/h

0.85 m

20 km/h

1.55 m

25 km/h

2.5 m

Every added 5 km/h almost doubles the risk of a crash *

* Source: Kloeden, C. N., McLean, A. J., Moore, V. M. & Ponte, G. 1997. Travelling Speed and the Risk of Crash Involvement. Federal Office of Road Safety, Canberra, page 391