Twenty’s Plenty (of Headaches for Sign Practitioners)

I’m not debating the presence of using 20 mph as a speed limit in urban areas, that is not the point of this article. I am writing to differentiate between 20 mph Zones and 20 mph Limits, as plenty of signing practitioners still get this wrong. Here is a brief overview of the history and differences between the two systems and what I feel would be better.

A brief history

Originally, 20 mph was only permitted as a speed limit if part of a self-enforcing Zone. These were first trialled in 1991 in Kingston-upon-Thames, Norwich, and Sheffield having been given consent to do so by the Secretary of State. At the time, any speed limit lower than 30 mph had to be specifically authorised. These Zones contained numerous trials of differing traffic calming measures to ensure that the limit was adhered to as the intention was to reduce child casualties in residential areas.

In 1999, an amendment to the law permitted the use of 20 mph Limits without consent from the Secretary of State. This now meant that there was a two-tier system of 20 mph Zones, intended for residential areas and heavily self-enforcing with vertical or horizontal traffic calming features; and 20 mph Limits, intended for locations like town centres which did not warrant traffic calming but reduced speeds would be beneficial to provide a better public realm experience. An area covered by a 20 mph Limit could have vertical or horizontal traffic calming features but these were not mandatory, and had to be signed separately as they would in a regular 30 mph area. What it did have to have, unlike a Zone, was repeater signs like 40, 50, and (on dual carriageways) 60 mph limits.

However, all guidance tended to steer authorities away from Limits as research demonstrated reductions in traffic speeds were often negligible and could bring the speed limit into disrepute. They were seen as only effective where traffic speeds were already 24 mph or lower. This meant that authorities were generally still installing expensive Zones with their self-enforcing features until the 2011 amendment TSRGD was issued.

The amendment changed the approach to 20 mph Zones. Previously, self-enforcing traffic calming in Zones had to be at prescribed maximum intervals, otherwise the Zone was effectively invalidated. This was scrapped, and only a single feature would be needed for the entire Zone to be compliant with the law. The amendment also redefined repeater signs and road markings as traffic calming devices, meaning a 20 Zone could have repeater signs but no horizontal or vertical devices.  This also meant there is the now bizarre situation where a 20 mph Limit adjoins a 20 mph Zone and a pair of regular terminal signs saying 20 mph are placed back to back with the 20 Zone entry sign because otherwise the Zone is invalidated.

So, to summarise:

20 mph Limit:

  • Start of Limit signed with regular Dia 670 sign; it may be part of a gateway sign provided it is designed in accordance with TSRGD
  • Repeaters must be used in accordance with TSRGD
  • Physical traffic calming features are not required but may be used; if they are warning signs in accordance with Traffic Signs Manual Chapter 4 are required
  • They work best when speeds are already below 24 mph

20 mph Zone

  • Start of Zone signed with Dia 674 sign and terminated with Dia 675; this may include a slogan or image on the entry sign provided it is designed in accordance with TSRGD
  • Must be self-enforcing through the use of traffic calming features
  • Repeaters are not required but may be used as a traffic calming feature
  • They work on roads with previously higher speeds due to the very nature of physical measures

20 mph sign (CC BY-ND 2.0 licensed by Tony Hall_Flickr)

This is not a Limit, despite someone cover-plating the word ZONE! This limit is technically unenforceable as this is no longer a prescribed traffic sign.


This is a Limit, using permitted Gateway type signing.

The problem is, as you can see, there is now virtually no practical difference between a Zone and a Limit. Yet if you sign one incorrectly you open up all kinds of legal loopholes.

My recommendation

Quite simply, we do away with 20 mph Limits and sign them all as Zones. The inconsistency has no value to road users, it confuses people in charge of putting signs up, and brings the system into disrepute. In order to further reduce the amount of needless clutter on the roads I’d also argue that any road that warrants speed humps should be 20 mph so we can get rid of the hundreds of needless warning signs for them.

20-CalmingWhere physical measures are used a supplementary panel to the Zone entry sign would be required as shown above. 

In the meantime, the current unsatisfactory mess looks set to continue, which is unfortunate.


12 thoughts on “Twenty’s Plenty (of Headaches for Sign Practitioners)

  1. “any road that warrants speed humps should be 20 mph”

    My issue was that is that too many roads have speed humps but don’t warrant them! Limiting these roads to 20mph by default would be an even more unnecessary restriction.

    Liked by 1 person

  2. Bryn, the relaxation for 20 zones in 2011 came not in the Amendment Regulations of that year, but in special directions sent to every traffic authority in England. Wales and Scotland had to wait until the 2016 TSRGD to be able to use a repeater sign or marking in a 20 zone in place of a physical calming feature.

    But you’re right that the distinction between the two types of 20 is now so blurred that they might as well be recombined into a single category.

    Liked by 1 person

  3. Oh sorry, another point: a 20 zone still has to have at least one physical calming feature within it (but it could be a sharp bend or mini roundabout that was already there). All the remaining ‘calming’ measures except that one can be replaced by a repeater sign or marking, but these repeaters need to be spaced a maximum of 100 m apart.


  4. Hi Simon,

    You are of course right; 2011 was a long time ago and a very busy year in terms of ad hoc paperwork being sent to (English) local authorities!

    That said, when I was responsible for a sign only 20 limit in Blackburn we found speeds on two of the “major” roads (defined as being yellow on an OS Landranger) through the area increased by 1 mph. I had originally wanted to exclude those roads but political pressure prevailed. The council has since baulked at the idea of any more sign only 20 mph areas but can’t afford tried and tested Zones on the classic model.

    It rather proves the old belief that without proper engineering measures to enforce compliance, simply reducing a speed limit is a somewhat blunt and useless tool yet we keep doing it…


  5. I went to have a look at the sign only 20 I referred to above and found it has been revoked.

    The hoop type sign post illustrating a limit was from that scheme.


  6. I, too, wondered why we still have “Zones” and “Limits” when the distinction between them is now almost nil.

    Maybe Bryn or Simon can confirm this point – the Road Humps Regs 1999 make it unlawful to construct a road hump near a culvert (among other things) unless the road is in a 20mph “Zone” i.e. signed with Diag 674. Soooo, unless these Regs have been amended, does this mean a road hump in a 20mph “limit” must still be constructed away from a culvert to satisfy Regulation 4(6) of the 1999 Regs?


  7. I’m afraid you’re right. The Road Humps Regs haven’t been revised. As they were written before the days of 20 mph limits, the only relaxations they provide on hump positioning (and warning sign use) relate to 20 mph zones. The solution is to continue to use 674 “20 zone” signs if you’ve got physical calming measures.

    As well as that inconsistency, most people think the requirement for warning signs for humps outside a 20 mph zone is ‘over the top’ and contrary to the spirit of Traffic Signs Manual Chapter 4, which tries to minimise their use. All in all, the Road Humps Regs belong to a different era when councils were seen to need restraining from doing silly things with traffic calming, and are way overdue for complete revision.


  8. Hi Bryn
    Regarding your comment : “any road that warrants speed humps should be 20 mph so we can get rid of the hundreds of needless warning signs for them.”

    Looks like that happened here using a sign combination similar to your suggestion :


    1. Hi Stu,

      Interesting – I’m fairly sure, but I expect Simon Morgan will tell us for sure, that if you have vertical features as this site does within a 20 mph limit you should (but not must) place the warning signs to Dia. 557.1 as TSM Chapter 4 hasn’t been re-written to say otherwise. Anything else under the Schedule 10 General Directions’ definition of traffic calming is presumably covered by Dia. 883 and kosher.

      More of a mess than ever if there’s another layer of sign options available.


  9. Hi Bryn,

    Just a question: does this mean if there’s a 20 mph limit but no (or very few and far apart) repeater signs, it’s legally unenforceable? Also, in what other circumstances is 20 mph, either as a limit or a zone, unenforceable?


    1. Chapter 3 of the Traffic Signs Manual covers this :
      “8.3.1. Whilst there is no specific requirement to provide repeater signs, it is for the traffic authority to determine how many are required and where they are placed.”

      However, it does go on to recommend the level of repeater signing for each speed limit.

      Other circumstances? Anything that doesn’t adhere to TSRGD. And the further you veer from the guidance in the TSM without good reason, the harder time the police will have enforcing a speed limit.

      Liked by 1 person

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