Hi, I’m starting this thread to hopefully settle a ongoing argument in changeset 160145534. The basic question is should a roundabout that interrupts a national speed limit duel-carriageway also be considered a duel-carriageway or a single-carriageway and should the speed limit of the roundabout be 70mph or 60mph.
Nathan_A_RF in the changeset argues that the roundabout is in fact a circular single-carriageway that interrupts the duel-carriageway and therefore should be a 60mph speed limit.
I and others have argued that roundabout normally adopts the settings of the interrupted road.
For context, the national speed limit on a dual carriageway (a road with a prolonged physical central barrier dividing traffic flows) is 70 mph, with the national speed limit on a single carriageway being 60 mph. There are never signs indicating this change in national speed limit from one to the other, so the national speed limit on roundabouts is debatable, as is the national speed limit on slip roads to and from dual carriageways.
For the latter, I have contacted both National Highways and my local police force relatively recently about whether dual carriageway slip roads are subject to a 60 mph or 70 mph national speed limit. The police linked me to the standard government website about speed limits (Speed limits - GOV.UK) which does not give any answer. National Highways got back to me and asked me to be more specific in my query, so I asked about a single junction in Kent. They responded saying “the speed limit on the slip road will remain the same as the main carriageway, unless signposted otherwise”, which given the change between a 70 mph and 60 mph national speed limit isn’t signed, doesn’t help (maybe suggests a slip road has a 70 mph speed limit?). Assuming similar responses would be received if I asked the same questions about roundabouts, then there is no real answer about whether slip roads and roundabouts have national speed limits of 60 mph or 70 mph, and so a consensus needs to be reached here.
It could be argued that a roundabout signed explicitly with a 60 mph speed limit suggests that the national speed limit is 70 mph (as per Mapillary), however there are similar small sections of single carriageway roads attached to dual carriageways with the same signed limit (as per Mapillary), and such a speed limit will have been covered by a single legal order covering the dual carriageway either side of the roundabout (easier and probably cheaper to do) rather than two separate orders with a gap in the middle at the roundabout.
The Road Traffic Regulation Act 1984 states that a dual carriageway road is “a road part of which consists of a central reservation to separate a carriageway to be used by vehicles proceeding in one direction from a carriageway to be used by vehicles proceeding in the opposite direction”. The interpretation of “part” is entirely subjective. As such, I will argue that a roundabout is not part of the dual carriageway - it is a break within it - the central reservation terminates as a result of the roundabout. This, combined with the fact that a roundabout is a single circulatory carriageway with traffic all travelling in the same direction around its centre - i.e. not in opposite directions as per the RTA definition - dictates that the speed limit should be 60 mph if it is subject to the national speed limit.
maybe suggests a slip road has a 70 mph speed limit?
A slip road, for all practical purposes at minimum, has a 70mph speed limit. It is treated as such by drivers, police, driving instructors, everybody. It’s necessary for cars to get up to speed with traffic on the main road when merging.
That is what I (and likely most) would presume through your logic in safely merging with traffic (and the definition in the RTA). However, user Pink Duck seemed to think they were 60 mph due to some speed awareness course they went to, and as such have tagged many slip roads around Norwich as 60 mph.
National Highways answered your question - the speed limit will remain the same unless signposted otherwise. You’ve also posted an example of where this has been used on a roundabout. This seems pretty clear cut, and the fact that someone in Norwich has misunderstood something that they heard on a course doesn’t change that.
This is an example of the roundabout being part of a different road to the NSL dual carriageway, and the speed limit changing as a result:
Swansea Road to the A4102 was the main road before the A470 dual carriageway was built, and has kept the priority. There are no speed limit change signs coming from either of the 30 mph roads, as the roundabout is classed as part of that road. The A470 dual carriageway has signs changing the speed limit down from the national speed limit to 30 mph, then back again after the roundabout.
There are several other roundabouts on that stretch of the A470, and they don’t have signage to change the speed limit, as the A470 is the main road that includes those roundabouts.
Another discussion is taking place at the SABRE forum to gather more information about the subject. I refer those who are interested or have commented on the discussion to take a look at the relevant thread and my post over there using this link.
Although this is tangential to the discussion at hand - we are discussing the application of NSL roundels to roundabouts, not the speed limit of roundabouts in general - in a similar vein to what Chris5156 said, we need to default to the lowest possible speed limit where the speed limit is ambiguous. This would mean that roundabouts would be tagged as having the lowest speed limit appropiated from one of the arms.
This discussion is very academic, pedantic, geeky and so on of course, but within the context of roundabouts designed NSL this is somewhat important from a perspective of accuracy, precision, as well as making sure that OSM data consumers to be able to access safe data, even if it is impossible to reach 70 mph, let alone 60 mph on a roundabout.
Some may also find Chris5156’s post quite useful for the discussion too.
When National Highways said that, it can really be thought of as they meant “the road will remain the national speed limit unless signposted otherwise”. Otherwise they would have clearly answered my question to them about what that national speed limit was.
There is also no such thing as a road taking “priority” at a roundabout. As I explained already the limits will be defined by speed limit orders. In the case of the A470, it may have been simply that the 30 mph speed limit order wasn’t changed when the dual carriageway was built, and as such the new roundabout had the existing 30 mph speed limit applied. Same thing for other speed limits, such as the 60 mph signed limit on the roundabout I mentioned above.
I have to agree. I don’t think it correct that the speed limit is 60 instead of 70 but we need a higher authority to make the decision which practically will probably never happen. Most likely way it would happen would be a challenge to a speeding ticket with a actual speed of between 60 and 70 on a dual carriageway roundabout. So sadly I agree we should take the fail-safe option of 60.
I agree. While I think that the discussion has clarified the ambiguity of the national speed limit on a roundabout, I don’t believe that it has been particularly productive given that it’s such a niche issue. For the vast majority of roundabouts, the speed limit is clearly signposted unlike the national speed limit, which leaves it up to interpretation. Not as if that’s a significant issue here given the physical constraints of a roundabout preventing someone from getting remotely close to that speed on a typical roundabout, but it’s still something to talk about nonetheless.
Well at the very least if every roundabout is mapped as 60, there is consistency and nothing is left up to interpretation (or at least, only a handful of odd cases will be)
