How are speedometers designed to accommodate the change from mph to km/h? In this article I will examine a variety of techniques and at the end of the article I will propose a 21st Century solution.

Australia and South Africa

I lived in South Africa when that country adopted the metric system. I believe that there were many similarities between the Australian and the South African approaches, so I will lump them together as a single approach. From what I can recall, cars sold up to and including 1971 had mph speedometers and those sold from 1972 onwards had km/h speedometers. There were no dual unit speedometers. Drivers were expected to “know” the conversion factors – the only one of real consequence was that the 35 mph urban speed limit became a 60 km/h speed limit. On the open road, the old 70 mph speed limit became 110 km/h, but was rarely enforced.

Various gadgets were available to help people understand the new system, in particular I remember an adaptor that one could insert between the speedometer cable and the speedometer itself which had the effect of speeding up the internal rotating cable by a factor of 1.6, making the mph speedometer into a km/h speedometer (a pity that it still read “mph” on the speedometer face).

Although the South African approach appeared a little rough at the edges, South African drivers were implicitly given the message “The country is going metric – just get used to the idea and stop clinging onto the imperial system”.

The Yom Kippur War of 1973 brought a big change when oil world-wide production dropped sharply. As a fuel conservation measure, the South African authorities introduced a regime of high fines for anybody breaking the reduced speed limits (50 km/h in urban areas and 80 km/h on the open road). Drivers with mph speedometers very quickly learnt how to convert into km/h!

United Kingdom and United States

The United Kingdom and the United States (and certain other countries) took the approach that drivers should not have to do mental conversions and so introduced dual unit speedometers with mph as the principal unit and km/h as a secondary unit. This change was introduced many years before the planned conversion dates, putting the onus on the car manufacturers to prepare for a conversion date at some unspecified time in the future.

On the surface this appears to be a solid compromise. Once the conversion took place, drivers would not have to remember the conversion factors and as the old cars were replaced, new cars would have km/h only speedometers. In theory this looked fine, but in practice it gave governments “wriggle room” – they could (and did) postpone metrication of road signs because the circumstances were not “quite right” (i.e. a “sterling crisis”, or an upcoming election). The result is that the conversion has not taken place and certain elements of society who have no real interest in units of measure hijacked the anti-metrication movement for their own agenda.

One of the design flaws (in my opinion) of some speedometers such as the one in the above diagram is that the lettering “MPH” and “km/h” are not adjacent to each other. I heard of an Australian driver who, seeing the “km/h”, but not noticing the “MPH” thought that the outer display was in km/h.

European Train Control System (ETCS)

In the 1990s, the EU expressed concern each country had its own railway signalling system (or in some cases multiple signalling systems) forcing high-speed international trains to carry a multitude of signalling devices for in-cab signals. Thus, the first Eurostars had to carry British, French and Belgian-compatible signalling systems on their high-speed (above 200 km/h) trains. As a result, the EU instructed the rail industry to develop a pan-European signalling system for high-speed trains where the drivers relied on in-cab displays rather than line-side displays. The result was ETCS which is now in use in many parts of Europe as well as certain routes in Australia, India, Philippines and many other countries.

Being a pan-European project, it is hardly surprising that it is all-metric.

ETCS relies on picking up information from standard units (known as “Eurobalises”) that are mounted on the railway sleepers. ETCS is designed to be introduced over a period of time and has different “levels” depending on the degree of sophistication that is incorporated into the local system. All levels use the same driver-machine interface (DMI) unit which consists of a large display unit and associated electronics (see above diagram).

• ETCS Level 0 is used where the train has a DMI installed to replace a legacy speedometer, but there is no associated lineside equipment and, as a result, the speedometer is the only ETCS component that is operational.
• ETCS Level 1 is used where lineside signals and in-cab information are used in tandem. As far as I can see, Level 1 will not be used in the United Kingdom.
• ETCS Level 2 is used when lineside signals are dispensed with and all the information that the driver needs is in the cab.

At the time of writing, ETCS Level 2 is operational on the Moorgate spur of the East Coast Main Line (ECML) and is being installed as far as Peterborough. Once that has been completed, trains will run under Level 2 from Kings Cross to Peterborough and under Level 0 onwards to Edinburgh.

In the United Kingdom, ETCS is programmed to display mph when operating under Level 0 and km/h when operating under Level 2 with the switchover occurring automatically. As can be seen from the above diagram, there is little confusion as whether the unit is displaying mph or km/h – there is no supplementary information at Level 0! One of the design features of the ETCS DMI is that the angular position of the needle is independent of the units being displayed – thus when the train goes from a Level 0 section of track to a Level 2 section of track, the needle itself will not change position.

A 21st Century Proposal

Both the South African/Australian car speedometers and the ETCS speedometer have the advantage of only displaying speeds in one system of units, thereby reducing clutter. In the case of the South African and Australian cars, drivers were required to mentally convert km/h back to mph if they were driving older cars, whereas ETCS switched between units as appropriate.

Many modern cars provide the driver with the facility to switch between mph and km/h on a digital display, but I have not seen any proposals to enable such switching to be done on an analogue (or pseudo-analogue) display. Drawing on features of the above, I am making the following proposal.

In the 1970s, speedometer backing plates were pieces of metal with numbers painted on them. The technology of the 21st century makes frequent use of liquid crystal displays (LCDs). They appear on every laptop computer, mobile phone and on a host of other devices. I suggest that the backing plates for switchable speedometers should use LCD technology, and that the driver would be able to select the units displayed at the flick of a switch. First and foremost, such a display would have to comply with UK (and therefore EU) rules, which in turn comply with the UNECE recommendations for speedometers.  The recommendations for speedometers that do not go above 200 km/h include:

• It is mandatory that the speedometer shall be able to show km/h. The display of mph is optional (required for new cars sold in the United Kingdom). It is permitted to have switchable displays.
• Graduations shall be in multiples of 1, 2, 5 or 10 km/h [or mph].
• Speed values shall be in multiples not exceeding 20 km/h [or 20 mph].
• Graduations need not be evenly spaced.
• The indicated speed may not be less than the true speed.

As can be seen in the above diagram, the proposed speedometer complies with all these recommendations. The most important feature (borrowed from the ETCS specifications) is that the position of the needle is independent of speed. Thus, the speedometer could be a physical needle whose angle is directly proportional to the rotational speed of the wheels.

In the diagram, the text “mph” and “km/h” are in different colours. This could of course be extended to all characters on the display and even to the background colour. Such changes could be subtle, but sufficient to alert the motorist as to which units are being displayed. (Any students of ergonomics out there?).

The other feature that I have added to the display are the red lines. The first of these is at 25 mph – 40 km/h and will be in exactly the same position when switching between units. Thus, the driver will get used to seeing the speedometer needle a little above the first red line when driving in Dover at 30 mph or in Calais at 50 km/h. Subsequent red lines are at multiples of 25 mph and 40 km/h. Thus, drivers who take their cars across the Channel or who drive between Northern Ireland and the Republic could mentally tune themselves to using the red lines as a first approximation as to a safe speed.

One could of course use multiples of 30 mph – 50 km/h. This would have the advantage of the first red line showing the default urban speed unit in both Dover and Calais but would result in a thicker line (the ratio between 25 mph and 40 km/h being 1.006 and the ratio between 30 mph and 50 km/h being 1.034).

This proposal has the advantage that it would cost very little and would be of immediate use to British drivers when using their cars abroad.

Today marks the 20th anniversary of the metrication of speed limits in the Republic of Ireland. 20 January 2005 was the day that all speed limits in Ireland switched from mph to km/h.

The following summary of events is reproduced from our main website:

The Republic of Ireland’s metrication programme started much later than that of the UK. However by completing the metrication of its roads in January 2005, it has left the UK well behind.

In contrast to Canada, which started its conversion programme with speed limits, Ireland made changes of speed limits the final stage in the changeover.

Ireland replaced imperial distance signage over a period of 10 years. Since the lifetime of most road signs is also 10 years this meant that there was virtually no cost to this conversion.

The final stage of the programme was to convert the speed limits on 20 January 2005. The Irish Department of Transport took the opportunity afforded by metric conversion to comprehensively review speed limits. As a result, the speed limits should be better tuned to local traffic conditions and hence helping road safety.

The key to a successful changeover was believed to be a very rapid conversion of the actual signs preceded by a blitz of information and publicity in the two weeks preceding the change. Drivers were therefore left in no doubt as to what was happening and were warned that ignorance or confusion about the new limits would not be accepted by the police as an excuse for breaking the new limits.

Like Australia and Canada, the changeover passed without incident.

RTÉ Archives – 2004-02-05

“Traffic signs to indicate speed limits in kilometres per hour instead of miles per hour.”
Road Sign Review 2004

RTÉ Archives – 2005-01-20

“Motorists in Ireland adapt to the introduction of speed limits set in kilometres per hour rather than miles.”
Mind Your Metric Speed 2005

Further reading

UKMA Newsletter – Volume 3 Issue 1, February 2005

Metric signs ahead – A report by the UK Metric Association by Robin Paice, 2006 (Ireland, page 55) – ISBN 978-0-9552351-0-8

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