ULEZ is all metric unlike UK road signs

The Mayor of London Sadiq Khan has expressed his commitment to expand the Ultra Low Emission Zone (ULEZ) this year to make London’s air cleaner and improve public health. Transport for London (TfL) is planning to expand ULEZ across all London boroughs from 29 August 2023. As ULEZ expansion has been in the news recently, we take a look at the measurements that ULEZ is based on. Whatever happens to ULEZ, measurements play a central part in making it work. Unlike UK road signs, the measurements used in ULEZ are all metric.

There is a daily charge of £12.50 to drive vehicles within the ULEZ that do not meet emissions standards. The ULEZ emissions categories depend on the vehicle type (and weight for larger vehicle types) and are:

  • Euro 3 for motorcycles, mopeds, motorised tricycles and quadricycles (L category)
  • Euro 4 (NOx) for petrol cars, vans, minibuses and other specialist vehicles
  • Euro 6 (NOx and PM) for diesel cars, vans and minibuses and other specialist vehicles

As their names suggest, these are all European emission standards. These standards are based on g/km for most vehicles and on engine energy output in g/kWh for lorries and buses. Vehicle weights are all expressed in kilograms and tonnes. Other measurements used in European emission standards are:

  • NH3 measured in parts per million per kWh
  • NH3 measured in mg per kWh
  • kW used in the Euro 1 tier
  • Nanometres (nm) used to express the size of particulates
  • Litres for engine sizes
  • Milligrams (mg) for pollution emissions tests

Data in the TfL’s London Atmospheric Emissions Inventory (LAEI) 2019 is used to inform policy making for ULEZ. This is part of the Mayor of London’s Data Store and is all metric. The measurements used in the data store include:

  • vehicle-kilometres for 2019 for each vehicle type
  • emissions of key pollutants in tonnes/year for 2013, 2016 and 2019 for each source category at a 1 km grid square resolution
  • modelled 2019 ground level concentrations of annual mean NOx, NO2, PM10 and PM2.5 in µg/m3 (micrograms per cubic metre) at 20 m grid resolution
  • estimations of the number of Londoners and number of schools, hospitals and care homes in London exposed to an annual average NO2 concentration above the Air Quality Strategy objective of 40 µg/m3 and PM2.5 concentration above the interim World Health Organisation (WHO) Guideline of 10 µg/m3, based on the modelled 2019 ground level concentrations

WHO limits for various pollutants are expressed in µg/m3.

TfL has analysed NO2 concentrations along the edge of roads at approximately 4-metre distance to test compliance of the legal annual average limit of 40 µg/m3. The NO2 concentration has been calculated as an average within a 150 metre buffer of each educational establishment.

ULEZ is a subsystem of the road transport system that is purely metric but all this metric usage is hidden from public view. The kilometre-based emissions limits for cars and other small vehicles are incompatible with mile-based odometers and imperial distance signs. It is ironic that you will see the number of miles or yards to the ULEZ area such as this sign:

This imperial sign points to the ULEZ, a system that is based entirely on metric units. The studies carried out to inform policy, WHO pollution guidelines, the London Atmospheric Emissions Inventory (LAEI) and the ULEZ itself are entirely metric. None of these systems use the imperial units seen on UK road signs.

We use the metric system, including metres and kilometres for distance, in ULEZ and the imperial system for speed limits and distances on UK road signs. This is another example of using one measurement system for one purpose and another measurement system for another purpose. This is a unique and typical British mess that we are stuck with for the foreseeable future because our political leaders lack the courage to make the case for completing the UK’s transition to the metric system.


Some recent reports about ULEZ Expansion:

9 thoughts on “ULEZ is all metric unlike UK road signs”

  1. UKMA has widely covered and reported on the cost of upgrading road signs across the UK to show metric units, costs which, as we know, have been grossly inflated in any estimates put out by the DfT. But what other costs would there be? The Highway Code seems to include metric distances for stopping distances and the like, so that is not a problem, but what other costs would there be, if any, in completing this changeover? I presume some legislation referring to speed limits on the roads would have to be amended to show metric units. How was this done in other countries? What costs would be involved? Clearly any costs involved did not stop a country such as Australia from metricating their road signs, speeds and distances when they switched to metric in general. What really is the reason why the DfT has not made any progress on this issue? It is quite infuriating to see even more new imperial road signs on the roads these days when everything else involving weights and measures is, officially at least, done using using modern metric measurements.

    Liked by 1 person

  2. @metricnow confusion is often cited as a reason in some cases, one interesting point is the continuing use of ‘m’ to mean miles. I seem to recall reading a DfT document from many many years ago (don’t ask me to remember precisely where) when this practice was actually questioned and may have even been interpreted as a criticism, but no solution was given and there has clearly been no attempt to correct this.

    Until somebody does so there will always be somebody who says ‘Well if one sign says the distance to the next services is 10m and the next sign says roadworks in 800m won’t this confuse the driver’. Most drivers will likely laugh this off as an anomaly though!

    Liked by 1 person

  3. The last major revision of traffic signs took place in 1958, for the opening of Britain’s first motorway, the M6, Preston by-pass. There were subsequent minor revisions, but there has been little change in the design standards since the 1970s. The few changes that have taken place have been sticking plaster upon sticking plaster. For example, the sign that used to mean “no speed limit” was redesignated to mean 60 mph on a single carriageway or 70 mph on a dual carriageway. This would have been fine as a temporary expedient, to allow time to change the signs to indicate the actual speed limit with a number. Yet we still have it. What do motorists think of it?
    In town where I live there was concern over the revised road layout at a large roundabout. Again the problem is in the signage. The lane layout, though slightly unusual, makes sense in view of traffic patterns, but the signage does not give prober indication of the lane layout sufficiently far ahead. Destinations appear on signs in a haphazard order, giving no clue as to what to expect at the next junction.
    Basically, the signage has failed to keep up with the more-complex requirements of modern junctions. The failure to have any plan to metricate distances or speed limits is just one part of this problem.

    The cost? Well Ireland metricated its signs over a period of about 20 years on a piecemeal basis, costing relatively little money. Signs have a limited life; they either wear out or need to be replaced sooner if there are significant changes to junction layout. It changed its speed limit signs to metric over a period of one weekend, at a cost of about 12 million euros. At an educated guess I would say that the UK could change its speed limit signs for about £80 million. The total annual budget for UK highways is £24 000 million, according the the Government website. So our speed limit signs could be metricated for a one-off cost of about one three-hundredth of the total annual budget. The Government could find that money. It is a case of one department, the DFT, dragging its feet.

    Liked by 2 people

  4. The insistence of using m for miles doesn’t make sense given that most signs don’t even mention units and it can be abbreviated as mi, like it is in the USA. I also trust that people are smart enough to easily tell from context so it is not really that confusing. While using m for miles is particularly confusing to visitors who are use to it being used for metres. A good thing about metres is that it uses a one letter symbol which makes things more clearer . As opposed to yards or yds so that sign could just say 200m, never mind using convoluted fractions such as writing it as 1/2 mile as opposed to 800m.

    It seems odd to me that when this subject comes up, people are more than happy to dismiss it on the assumption that it would cost too much (without putting any critical thought) when it is such a terrible argument against it. Even when the DFT were inflating the estimate in order to find excuse to not do it, they came up with about £700 million which is hardly anything in terms of government expenditure, especially for a one off cost that has such a significant noticeable impact. To put that into context government expenditure for 2019/20 was £842 billion, with £37 billion on transport so the idea that they are still in imperial units because it would cost too much is quite frankly laughable.

    Personally I would prefer to keep the timeframe quite brief for both speed and distance signs. While Ireland changed the speed limits over a weekend at about €12 million, distance signs were phased in more gradually and the Metric Signs ahead booklet suggested a figure of about £80 million with distance signs converted over a 5 year timespan.

    Liked by 1 person

  5. Alex M:

    You touch on something that grates on me too. I find it weird and irrational to use fractions of a (rather) long distance to indicate relatively short distances on roads. I find the ‘mile’ very big as a concept of distance in the first place. So to show ‘1/2 of a long distance’ to mean 800 m, or, even more absurd, ‘1/4 of a long distance’ to mean 400 m seems to be, almost literally, approaching the distance ‘from the wrong end’. It assumes, for a start, that you know what the long distance, the mile, is. You have to conceptualise that and then think, OK, it’s a half or a quarter of that. Your odometer in your car won’t help, as it is clocking up 176yds at a time in the form of decimal tenths of the long distance. Perhaps we need to look a bit further back into history to when long distances in miles were actually relevant as a ‘thing’ in their own right. I’m guessing here, but if that dates back to the days of stage coaches, I can understand a sign showing a long distances of several miles to the next staging post could be meaningful to the driver of the horse and coach. It would mean so many hours or minutes more of journey time. There’d be no other long distance traffic on the highway at that time anyway, so the fractions of the long distance shown today wouldn’t have been there, I don’t suppose. But surely today, for today’s purposes and today’s traffic, it would make more sense simply to indicate the distance ahead to whatever is being signposted, so many hundreds of metres, rather than fraction of a much longer distance which you may not be travelling anyway. Final example, if I haven’t made myself clear: it makes more sense to me to know that the next motorway exit is 500m ahead, rather than 1/3 of a longer distance that I will not be travelling and do not need to know.

    Liked by 1 person

  6. @metricnow, @AlexM:

    The Wikipedia article on Roman numerals (https://en.wikipedia.org/wiki/Roman_numerals) gives a clue to this mess. Most of us are quite familiar with what I, II, IV , V, IX, X, XL, LX, C, D, MMXXIII etc mean, even if it is cumbersome. At least it worked to a regular base (10), even if was no symbol for zero.

    Even though the Romans used a decimal notation for whole numbers, they used a duodecimal notation for fractions. The symbol S was used for a half, while 1/12 was represented by a single dot, 2/12 by two dots and so on up to 5/12, often arranged like dots on a die. Thus 3/4 (or 9/12) was represented by “S···” (ie 1/2 + 1/12 + 1/12 + 1/12). There were further symbols for values down to 1/288, but as you can imagine, it made things rather cumbersome.

    The value for 1/12 was known as a “unica”, hence the word “ounce”, and the value of 1/288 was known as a “scruple”, (beloved of apothecaries).

    Simon Stevin (1548–1620) in 1570 extended the Arabic concept of decimal number to include decimal rather than duodecimal fractions and John Napier (1550 – 1617) in 1614 introduced the dot notation with which we are familiar today.

    Liked by 1 person

  7. I completely agree metricnow. The usage of fractions is an unhelpful hangover from another time and is indicative of the failure to move on from it. Knowing that there are 1000 metres in a km makes life simpler and you know exactly how far you travel in relation to it. While how are you supposed to relate to yards, miles and fractions of a mile? Especially given that imperial hasn’t been taught as a separate system for 50 years and the vast majority of British drivers don’t know how many yards in a mile. Never mind how it relates to fractions of it and when you are driving your focus needs to be on the road not getting your head around what antiquated units are supposed to mean.

    Liked by 1 person

  8. According to this section of a Wikipedia article the introduction of metric seems to be in full swing in Myanmar (Burma):


    However, a recent issue of the Global New Light of Myanmar newspaper has an article listing the water supply from one of their dams as benefiting “27,000 acres” of crops. So, it appears Myanmar still has its own metric muddle to contend with.

    I doubt Liberia is doing much better but at least they are being pushed in the direction of metrication by their neighboring countries.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: