The total estimated cost for implementing the speed limit change from 30 mph to 20 mph, including the cost for changing signs and markings on roads, has been published in the Regulatory Impact Assessment (RIA) undertaken by the Welsh Government on 20 mph speed limits. The published document gives a breakdown of road sign costs. These costs are several times lower than the Department for Transport (DfT) cost estimates of conversion of road traffic signs to metric units in cash terms despite 50% inflation during the 16-year period between the two reports.
In February 2006, the DfT produced a report to estimate the cost of conversion of road traffic signs to metric units. They estimated that it would cost £680 – 760 million to convert half a million signs, which works out at approximately £1400 per sign.
On 21 June 2022, the Welsh Government published the “Explanatory Memorandum to the Restricted Roads (20 mph Speed Limit) (Wales) Order 2022” report about the expected impact of the legislation. Unlike the DfT metric conversion report, the Welsh Government report examined the benefits of the policy.
Section 6.2 of the report covers the direct costs of the policy. Values are discounted present values over a 30-year appraisal period from 2022 presented at 2022 prices. No other direct costs are expected by the Welsh Government or other parties beyond the fiscal year 2027. While there have been arguments for and against the policy, the validity of the cost figures provided by the Welsh Government has not been challenged by the DfT.
The following items will be needed to implement the 20 mph policy:
- 30 000 replacement signs at intersections
- 5000 posts at intersections
- 2300 new signs including repeater signs for locations where 30 mph will remain
- 5000 replacement signs around school areas
As the DfT report included the costs of overheads and an optimism bias adjustment, the figures quoted here include the same to enable direct comparisons between the DfT and Welsh Government figures. These figures are quoted in cash terms with no adjustments for inflation.
The total cost of the 20 mph policy is estimated at £32.5 million. This figure is divided between capital costs (one-off expenditure on assets) of £29.1 million, including a 22% optimism bias adjustment, and transitional costs (one-off costs which relate to the implementation of the policy) of £3.5 million. There is a more detailed breakdown of costs in Appendix A4 in the report.
Transport for Wales (TfW) provides the following breakdown of the £32.5 million of direct costs of the 20 mph policy in Appendix A4:
- Communications: £1.6 million including 10% for optimism bias
- Gateway signs: £5.5 million including 22% for optimism bias
- Road markings: £22.6 million including 22% for optimism bias
- Existing 20 mph signs and school signs: £1.1 million including 22% for optimism bias
- Exceptions: £1.1 million including 22% for optimism bias
- Monitoring: £0.4 million including 10% for optimism bias
- Government staff costs: £0.4 million with nothing added for optimism bias
The A4 Appendix table provides detailed notes about the items that need to be implemented and their associated costs. Based on the information provided, the following table shows a breakdown of costs:
| Item | Cost per Item | % for Overheads | Cost incl. Overheads | % for Optimism Bias | Total Cost per Item |
|---|---|---|---|---|---|
| New gateway sign | £62 | 50 | £93 | 22 | £113 |
| New gateway post | £235 | 50 | £353 | 22 | £430 |
| Removed/replaced sign at 20 mph zones and school signs | £120 | 40 | £168 | 22 | £205 |
| Repeater sign for exceptions | £25 | 50 | £38 | 22 | £46 |
Remember that these are figures are based on 2022 prices. Despite general price increases of more than 50% since the February 2006 DfT report, the cost per item (including overheads and optimism bias) are substantially less than the £1400 per sign implied by the DfT report in 2006, which has not been adjusted for inflation.
The Welsh Government cost estimates at 2022 prices for signs are £46, £113 and £205 per sign (including overheads and optimism bias) depending on the type of sign. If we take the lower end of the DfT cost estimate per sign of £1360 and adjust it for inflation, the cost would be over £2000 per sign at 2022 prices according to the Bank of England Inflation Calculator. The Welsh cost estimates at 2022 prices are approximately one tenth of the DfT cost estimates for signs (and for some signs, a lot less) and one fifth for posts when the DfT figures are adjusted for inflation.
At August 2023 prices, the DfT cost estimates for converting half a million signs would be between £2200 to £2500 per sign. These are grossly inflated estimates, which bear no relation to actual costs incurred in the real world. The work undertaken by the Welsh Government casts more doubt on the credibility of the DfT cost estimates for the metrication of road signs.
NOTE: Figures do not always add up due to rounding.
Source for table in Appendix A4: Transport for Wales & Arup calculations
You can find the DfT report, “Estimating the cost of conversion of road traffic signs to metric units” at http://tinyurl.com/7bqczxa.
You can find the Welsh Government report, “Explanatory Memorandum to the Restricted Roads (20 mph Speed Limit) (Wales) Order 2022” at https://senedd.wales/media/fo3ibze5/sub-ld15187-em-e.pdf.
You can find the Bank of England’s Inflation Calculator at https://www.bankofengland.co.uk/monetary-policy/inflation/inflation-calculator.
Metric Views 
What a shame that the DfT agencies are unwilling to combine converting to metric signs along with the expense of changing the existing signs to display a new speed limit.
How bizarre and frustrating that their unwillingness to alleviate the metric muddle is so short-sighted.
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According to this article, Ireland spent about €10 million on replacing 58,000 speed limit signs, which would work out at about €172 on average per sign. It was estimated that there was 440,000 speed limit signs in the UK in 2013. So extrapolating that and accounting for inflation/conversion would give you a cost estimate of about £103 million, even assuming that there are 500,000 speed limit signs, it would still only be £118 million. To put that into perspective, the whole government expenditure for 2020/19 was £928 billion, with £44 billion going onto transport. With them already spending about that much on HS2 and cost estimates for that going over £100 billion.
Of course this doesn’t account for distance signs but those are still only a few hundred pounds to replace and unlike speed limits signs aren’t safety critical so can be phased in at a longer period of time. With plenty just requiring the plate underneath to be replaced and only part of the sign can be changed with adhesive stickers. With height restrictions already require both imperial and metric so they can simply move to make imperial optional and only require that metric to be shown in the vehicles in question.
The idea that road signs are still in imperial because it would be too expensive, is quite frankly a laughable and bogus excuse, that nobody would care about once it is done. The real reason is because there is a lack of political will as unfortunately it is just something that a lot of people just don’t care about. The British public are use to miles and don’t see the point of change and politicians are too concerned with short term ramifications. When in reality people would quickly get use to it and move on.
https://www.irishexaminer.com/news/arid-30176612.html
Click to access traffic-signs-england-research.pdf
Click to access Budget_2020_Web_Accessible_Complete.pdf
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Just found out that in Puerto Rico petrol is sold in litres and distance signs use kilometres but speed limit signs show miles per hour. So, here is a political entity where all residents are US citizens and where there is a fair penetration of metric but still a mind-boggling muddle. What madness! <:-0
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Alex,
Are you aware that Canada converted road signs using an overlay sticker and only replacing the sign after it wore out? Thus saving millions.
Has anyone attempted to calculate the cost of using a sticker? How much would it cost with this method? How much would be saved? Why always go the most expensive route?
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@Daniel Are you aware that Canada converted road signs using an overlay sticker and only replacing the sign after it wore out? Thus saving millions.
Road signs are not that expensive to replace, both the Irish example and this Welsh cost estimate work out at a little over £200 per sign so all things considered any cost savings from using adhesive stickers over replacing the plate would be quite marginal. Don’t forget you also need to factor in things like labour and prep work. The problem is also that in the UK, like Ireland the mph signs look like km/h signs used elsewhere. Therefore there needs to be a way to distinguish them in order to avoid confusion. My personal preference would be to have km/h on the sign, like in Ireland and then eventually phase them out when the issue of confusion is no longer an issue.
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Although the Irish metrication program was spread over many years, she changed from mph to km/h over one weekend.
As Alex M has said, the 50 km/h sign and the 30 mph sign look almost identical (apart from the “km/h” text). I agree with Alex M that having “km/h” on the sign would be essential, but I would go one step further – why not give the km/h signs a yellow background instead of a white one? The yellow could be replaced with white over time. Given that there has been a proliferation of speed limit signs over the past few years, especially of repeater signs, it might well work out cheaper to use adhesive overlays because they are quicker to apply and then to replace the signs with more permanent material with white backgrounds without having to pay overtime rates.
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If the “km/h” symbol is to be included on the roundel, rather than shown using a temporary supplementary plate, then it will not be possible to use overlay stickers because the roundels will need to be replaced with larger ones in order to maintain the font size of the numbers.
The new 20 mph signs in Wales were installed over a period of months (no overtime rates needed), but with temporary 30 mph stickers, which were able to be removed quickly and easily on the eve of the speed limit change.
At night, signs with yellow backgrounds would be practically indistinguishable from signs with white backgrounds.
The Welsh Government figures show that most of the cost involved in replacing speed limit signs lies in the removal and replacement of the associated road markings, which they put at £2800 per location.
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Alex said: “…any cost savings from using adhesive stickers over replacing the plate would be quite marginal.”
Do you have any proof of this claim? Can you provide a cost estimate for just using and adhesive sticker? I’m sure there is a significant difference, seeing Canada went the sticker route. If the cost difference is quite marginal, then Canada would have just replaced the signs.
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Martin,
Instead of going to the expense of changing the background colour, I would just cover over the “3” up with a “5”. This would be the quickest and least costly method. Mentioning that the signs were changing from mph to km/h on social media and the media should eliminate the need for an additional km/h sign or sticker to be added. Canada did add a km/h shield below the main speed limit sign and they are still used today in some Canadian cities close to the US border. You can see this on google Earth Street View in Windsor, ON at the Intersection of Huron Church Road and Millen Street. It’s located on the Huron Church, just north of the intersection with Millen attached to the Street Lamp post. Canada does not use Vienna Convention signs for speed limits.
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@Daniel
In terms of speed limit signs, I think replacing the whole sign is the best way to approach it. Both the Welsh and Irish cost estimates put it at just over £200 per sign, which is already fairly cheap in the grand scheme of things. I personally prefer having the km/h on the sign, like in Ireland, and as the UKMA has suggested before. which, as another commentor has mentioned, would require replacing the sign in order to fit it on.
However, adhesive stickers are still useful in replacing parts of distance signs, as only the units on them would need to be changed.
@Martin Vlietstra “On the sign would be essential, but I would go one step further—why not give the km/h signs a yellow background instead of a white one?”
That is not what I meant; I was merely pointing out that old Irish mph signs looked like km/h signs used elsewhere, hence why km/h was added to the sign. Putting km/h on the sign is perfectly sufficient in order to avoid issues of confusion with the old mph signs. Changing the colour in addition to that is unnecessary.
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