Ronnie Cohen looks at consequences of the UK’s measurement muddle for The Highway Code.
One major issue that arises from the UK measurement mess is the seemingly random and illogical mixing of units, of which The Highway Code provides a good (or should that be bad?) example.
Recently, the Department for Transport (DfT) held a public consultation about proposed changes to The Highway Code. I responded to the consultation and suggested some changes. Let’s hope that there will be some advances when the next edition is published. There is room for improvement but issues with measurements will remain until the UK completes its metric transition.
Speed limits in The Highway Code are remarkably consistent. They are quoted in miles per hour (mph) followed by kilometres per hour (km/h) in brackets. I praise the DfT for using the correct km/h symbol throughout the Highway Code. However, even for speed limits, there are a couple of notable exceptions. In Section 273, speed limits are expressed in mph only. In this section, it says “On leaving the motorway or using a link road between motorways, your speed may be higher than you realise – 50 mph may feel like 30 mph.” In an Annex, only km/h is used for motorcycle licence information but mph followed by km/h in brackets is used for moped licence information.
The only measurement that The Highway Code consistently uses metric exclusively is weight. Wherever weights appear, they are expressed in kilograms or tonnes. No conversions are given for any weights.
Usage of units for short distances is really inconsistent. In some places metres are used exclusively, and in others metres are given with conversions.
You can find exclusive use of metres for:
- Reading a number place from a distance (Section 92).
- Height of tramway overhead wires (Section 307).
- Distance between you and the car in front when stopping in a tunnel (Section 126).
- Vehicle lengths (Vehicle markings section, Large goods vehicle markings subsection)
- Load or equipment overhang (Vehicle markings section, Project markers subsection)
You can find use of metres followed by conversions for:
- Goods vehicle travelling less than a certain distance (Section 99).
- Child’s height for seat belts/child restraints (Section 100).
- Typical Stopping Distances chart (Section 126).
- Average car length (Section 126).
- Reduced visibility (Section 226).
- Parking proximity to a junction (Section 243).
- Distance from a junction for some vehicle types (Section 250).
- Minimum distance behind broken-down vehicle to put a warning triangle (Section 274).
- Usual clearance for overhead electric lines (Section 292).
One use of height information that I find most incomprehensible is in the table in Section 99. In this table, the third row uses metres only for a child’s height but the following row uses metres followed by feet in brackets. If you wanted to give only a single conversion, you would surely provide it for the first instance, not for subsequently.
One use of yards in the text appears for the interval between countdown markers at the motorway exit in the Traffic Signs section. They appear without any conversions to other units. It reflects the regulations in The Traffic Signs Manual for countdown markers to be placed at 100 yard intervals. By contrast, emergency marker posts must be placed at 100 metre intervals on motorways and road works signs must be placed at 100 metre intervals as well even though they show the same number of yards to roadworks. Why is there inconsistent usage of units for the placing of different types of signs?
There are other metric measurements without conversions:
- Breath and blood alcohol levels in micrograms per 100 millilitres (Section 95).
- Capacity of motor vehicles in cubic centimetres (Section 253).
- General motorcycle information in cubic centimetres and kilowatts (in Annexes).
- Motorcycle licence information in cubic centimetres and kilowatts (in Annexes).
- Moped licence information in cubic centimetres (in Annexes).
- Tread depths of tyres in millimetres only (Vehicle safety, maintenance and security section).
- Compression information for first aid in centimetres only (First Aid on the road section).
It is clear that the DfT expects readers of The Highway Code to be familiar with millimetres, centimetres and metres. As metric measures have been taught in primary schools for almost 50 years, we would expect this. So why does the DfT insist that road distances must be shown only in yards and/or miles and that restriction signs cannot be shown only in metres? Lengths and heights on other signs must be shown in feet and inches as well as metres. In The Highway Code, metres come first followed by imperial units. Why not just metres?