Yesterday, consultation closed on the (sadly all-too-modest) proposed changes to our national Traffic Signs Regulations and General Directions (TSRGD). Today, consultation begins on the (much-anticipated) London Cycling Design Standards (LCDS). It seemed like a good day to write about just how hard it can be to get new ways of thinking into how streets are actually laid out.
| Earlier this week, I was asked to advise some master-planners about some of the transport aspects of a proposed new development in a village on the edge of a large town in Devon. Amongst the matters we covered was a new access street they were promoting, which would create a new junction with the main street that runs through the village. The current (long-standing) speed limit is 30mph, and the location is both a comfortable distance and a helpful, traffic calming, bend away from where the 40mph limit ends as the road enters the built-up area. On consultation with the local authority highway engineers, the master-planners were told that the visibility splay for the new junction should be based on a default Stopping Sight Distance (SSD) of 90m. I should have been shocked, but I wasn’t; even though the appropriate default SSD is 43m. |   |
As to the 90m figure, the Design Manual for Roads and Bridges (DMRB) – specifically TD 42/95 – states that the SSD for highways with a 30mph design speed should be 70m. Interestingly, while stating that this distance “shall always be provided”, TD 42/95 goes on to add that “there is little advantage in increasing it, as this can induce high approach speeds”. So, if DMRB (which is intended for trunk roads and motorways) says the SSD should be 70m, where does this 90m figure come from? Well, it comes from both Design Bulletin 32 (Residential Roads and Footpaths) and Planning Policy Guidance 13: Transport. These state that 90m is the SSD to apply for roads with a 30mph limit where the actual speed of traffic on the priority road is unknown.
In other words, the highway engineer in my example is choosing to promote the application of a design standard that TD 42/95 considers likely to induce higher approach speeds; and this is at least partly because he hasn’t measured the actual speed there or isn’t prepared to take action to bring actual speeds down to the legal limit.
Where standards like this apply, the instinct of many engineers (in my experience) is that less is never more, but that more usually is more. In the specific example of SSDs, there’s a remarkable, yet entirely unfounded, belief that there is a positive relationship between applying higher SSDs and improving safety. This despite the facts (a) that TD 42/95 suggests the contrary and (b) research undertaken in 2006 showed conclusively that there is no such positive relationship. But that’s just evidence, of course.
But this isn’t the chief issue that I want to highlight. My main point, for this piece, is the simple fact that DB32 was formally superseded by the Manual for Street (MfS) in 2007. This is stated formally and unequivocally on page 5 of MfS – which is a Department for Transport document replacing a Department for Transport document. (And for anyone still trying to cling to PPG13, this is also now withdrawn.)
It was to inform MfS that the SSD research I mentioned above was undertaken. The product of that research was that the appropriate default SSD figure for 30mph is 43m. You can look it up in Table 7.1 on page 91 of MfS. Now, although MfS focused on lightly-trafficked residential streets, it was made plain that “many of its key principles may be applicable to other types of street”.
Arising from this, Manual for Streets 2 (MfS2, “Wider Application of the Principles”) was published in 2010. Although MfS2 wasn’t formally a DfT document, the Chartered Institution of Highways and Transportation, which did publish it, are hardly crazy highway design anarchists. Also the DfT sat on the MfS2 Steering Group, helped fund its production, and had the Under Secretary of State for Transport write a Ministerial Foreword to it.
Table 1.1 on page 8 of MfS2 states very clearly that MfS standards for SSDs are the appropriate ones to apply on highways with a 30mph limit. In short, since September 2010, guidance to highway engineers has been clear that the default SSD for junctions on 30 mph streets should be 43m.
I’ll pause here briefly to consider this matter of ‘default’ values for things like SSDs. When a table in a highway design document contains a number, this is almost always taken by practitioners as THE number to apply. Ideas like discretion, judgement and allowing for context are deeply unsettling for many, taking them way outside their comfort zones. So, although page 90 of MfS explains clearly and patiently how anyone can actually work out what the most appropriate SSD is for the 30mph street they’re thinking of, page 91 shows a table with the number 43 in the 30mph column. As a result, the ‘default’ number will, in practice, become the only number.
At the MfS launch in 2007, I asked one of its authors why they put the table on page 91 since they’d already put a nice, simple equation on page 90. His answer was that the DfT officers felt practitioners would simply want a table, because they’d always had one (in TD 42/95, DB32, PPG13 or wherever). The concern was that, if MfS didn’t have an SSD table, engineers would go back to their old tables, rather than mess with a bit of entry-level algebra.
I’ve already written too much, but I do just want to give another quick example of how engineering custom-and-practice can be much more influential on street design than evidence, common sense, or balanced judgement. This example concerns what are termed, in the current TSRGD, signs to Diagram 610. 610s are those ‘keep left’ (or indeed ‘keep right’) roundels (white arrow on blue circle) that are so ubiquitous on almost every highway in the UK.
This ubiquity needs to be understood against the fact that there is never, ever (EVER) any legal or regulatory requirement for their implementation. Let’s be clear: there are no circumstances (AT ALL) in which anyone absolutely has to put a 610 sign on anything. The DfT’s Traffic Advisory Leaflet TAL 1/13 emphasis this point. And yet – they’re on bollards and poles everywhere.
When used, their formal purpose is to show that drivers and cyclists must pass to the left of the object the sign is on. However, bearing in mind that we drive on the left in this country, and bearing in mind also some pretty basic UK highway laws and highway code rules, the instruction to keep left is almost always simply re-stating the obvious. The practical purpose of most such signs is not actually to say ‘keep left’, but rather ‘please try to miss the object that this bollard is standing on’ – typically a kerbed island. When we put such objects in the streets, we put keep left bollards on them. Don’t tell us we don’t need to; we just do, because it’s our thing.
One way to address this is obviously to have fewer objects in the street for drivers to have to miss. Picture, then, my pleasant surprise, when I encountered the scene shown in the first photo in a small Buckinghamshire town in the spring of 2011. The objects (splitter islands at this mini-roundabout) have a very low profile, and so some imaginative engineer had apparently concluded, quite rightly, that no 610/bollard was needed. Picture also, therefore, my dismay when I encountered the scene shown in the second photo a couple of years later.
Doubtless, some ‘bright spark’ had come along, seen a nude pair of islands, suffered a culture shock episode, feared the collapse of his entire value system, and issued the order for a couple of shiny, reassuring, 610/bollards on the spot. They weren’t needed before, they’re not needed now, and wouldn’t in any case really be needed in such a well-lit 30mph urban street even if the splitter islands did have raised kerb edges. Yet the 610s were summoned to the rescue of some engineer’s instinctive sense of what is right and fitting in such circumstances.
Whether in relation to SSDs, 610s, or a whole host of other things affecting our streets, very many unnecessary and unhelpful things are done out of habit. Not because of evidence; indeed often despite it. Not because rules insist; indeed often there are none. And not because of independent thought; indeed such a thing is frowned upon. But because of convention.
Old habits die very hard in street design. It may be helpful for you to understand that next time you’re tempted to shout “WHY?!?” into the void…
In other words, the highway engineer in my example is choosing to promote the application of a design standard that TD 42/95 considers likely to induce higher approach speeds; and this is at least partly because he hasn’t measured the actual speed there or isn’t prepared to take action to bring actual speeds down to the legal limit.
Where standards like this apply, the instinct of many engineers (in my experience) is that less is never more, but that more usually is more. In the specific example of SSDs, there’s a remarkable, yet entirely unfounded, belief that there is a positive relationship between applying higher SSDs and improving safety. This despite the facts (a) that TD 42/95 suggests the contrary and (b) research undertaken in 2006 showed conclusively that there is no such positive relationship. But that’s just evidence, of course.
But this isn’t the chief issue that I want to highlight. My main point, for this piece, is the simple fact that DB32 was formally superseded by the Manual for Street (MfS) in 2007. This is stated formally and unequivocally on page 5 of MfS – which is a Department for Transport document replacing a Department for Transport document. (And for anyone still trying to cling to PPG13, this is also now withdrawn.)
It was to inform MfS that the SSD research I mentioned above was undertaken. The product of that research was that the appropriate default SSD figure for 30mph is 43m. You can look it up in Table 7.1 on page 91 of MfS. Now, although MfS focused on lightly-trafficked residential streets, it was made plain that “many of its key principles may be applicable to other types of street”.
Arising from this, Manual for Streets 2 (MfS2, “Wider Application of the Principles”) was published in 2010. Although MfS2 wasn’t formally a DfT document, the Chartered Institution of Highways and Transportation, which did publish it, are hardly crazy highway design anarchists. Also the DfT sat on the MfS2 Steering Group, helped fund its production, and had the Under Secretary of State for Transport write a Ministerial Foreword to it.
Table 1.1 on page 8 of MfS2 states very clearly that MfS standards for SSDs are the appropriate ones to apply on highways with a 30mph limit. In short, since September 2010, guidance to highway engineers has been clear that the default SSD for junctions on 30 mph streets should be 43m.
I’ll pause here briefly to consider this matter of ‘default’ values for things like SSDs. When a table in a highway design document contains a number, this is almost always taken by practitioners as THE number to apply. Ideas like discretion, judgement and allowing for context are deeply unsettling for many, taking them way outside their comfort zones. So, although page 90 of MfS explains clearly and patiently how anyone can actually work out what the most appropriate SSD is for the 30mph street they’re thinking of, page 91 shows a table with the number 43 in the 30mph column. As a result, the ‘default’ number will, in practice, become the only number.
At the MfS launch in 2007, I asked one of its authors why they put the table on page 91 since they’d already put a nice, simple equation on page 90. His answer was that the DfT officers felt practitioners would simply want a table, because they’d always had one (in TD 42/95, DB32, PPG13 or wherever). The concern was that, if MfS didn’t have an SSD table, engineers would go back to their old tables, rather than mess with a bit of entry-level algebra.
I’ve already written too much, but I do just want to give another quick example of how engineering custom-and-practice can be much more influential on street design than evidence, common sense, or balanced judgement. This example concerns what are termed, in the current TSRGD, signs to Diagram 610. 610s are those ‘keep left’ (or indeed ‘keep right’) roundels (white arrow on blue circle) that are so ubiquitous on almost every highway in the UK.
This ubiquity needs to be understood against the fact that there is never, ever (EVER) any legal or regulatory requirement for their implementation. Let’s be clear: there are no circumstances (AT ALL) in which anyone absolutely has to put a 610 sign on anything. The DfT’s Traffic Advisory Leaflet TAL 1/13 emphasis this point. And yet – they’re on bollards and poles everywhere.
When used, their formal purpose is to show that drivers and cyclists must pass to the left of the object the sign is on. However, bearing in mind that we drive on the left in this country, and bearing in mind also some pretty basic UK highway laws and highway code rules, the instruction to keep left is almost always simply re-stating the obvious. The practical purpose of most such signs is not actually to say ‘keep left’, but rather ‘please try to miss the object that this bollard is standing on’ – typically a kerbed island. When we put such objects in the streets, we put keep left bollards on them. Don’t tell us we don’t need to; we just do, because it’s our thing.
One way to address this is obviously to have fewer objects in the street for drivers to have to miss. Picture, then, my pleasant surprise, when I encountered the scene shown in the first photo in a small Buckinghamshire town in the spring of 2011. The objects (splitter islands at this mini-roundabout) have a very low profile, and so some imaginative engineer had apparently concluded, quite rightly, that no 610/bollard was needed. Picture also, therefore, my dismay when I encountered the scene shown in the second photo a couple of years later.
Doubtless, some ‘bright spark’ had come along, seen a nude pair of islands, suffered a culture shock episode, feared the collapse of his entire value system, and issued the order for a couple of shiny, reassuring, 610/bollards on the spot. They weren’t needed before, they’re not needed now, and wouldn’t in any case really be needed in such a well-lit 30mph urban street even if the splitter islands did have raised kerb edges. Yet the 610s were summoned to the rescue of some engineer’s instinctive sense of what is right and fitting in such circumstances.
Whether in relation to SSDs, 610s, or a whole host of other things affecting our streets, very many unnecessary and unhelpful things are done out of habit. Not because of evidence; indeed often despite it. Not because rules insist; indeed often there are none. And not because of independent thought; indeed such a thing is frowned upon. But because of convention.
Old habits die very hard in street design. It may be helpful for you to understand that next time you’re tempted to shout “WHY?!?” into the void…
RSS Feed
