Sunday, 27 September 2020

The Non Signalised Dutch Junction

So here's a thing. In an idle moment, I might have a mooch around with Google Streetview and something interesting pops up. In Hilversum, The Netherlands, I stumbled upon a protected intersection without traffic signals.

The location is here, to the south of the city in a suburban shopping street. I've not visited the area and so I can only bring you an armchair visit, but the junction and the approach streets are interesting to look around.

The junction is a 4-arm crossroads of Gijsbrecht van Amstelstraat (a two-way street) and Neuweg (which is one-way). There is a mixture of cycle lanes and cycle tracks on the approaches to the junction, but at the junction itself, everything becomes cycle tracks;

The image above is Gijsbrecht van Amstelstraat looking east through the junction. The street cross section has a footway on each side, a one-way cycle track, a buffer area which has cycle parking and the carriageway itself. Just behind the viewing position, the buffer becomes a car parking area with a minimal buffer between the parking bays and the cycle track.

To the east of the junction, Gijsbrecht van Amstelstraat, the cycle tracks quickly become cycle lanes and the street becomes residential. For Neuweg, the cross section is similar, although there is a single one-way traffic lane. I'm not entirely sure how this all works at a network level, but Gijsbrecht van Amstelstraat looks like it's part of a general city ring road, but it's more like a circular distributor route made of city streets and Neuweg is a radial distributor road coming out from the city to connect with a National road which is performing a regional through route function.

Neuweg is interesting in its own right because it is one-way, but this very much looks like it was a way of providing space for a cycle track in each direction (as well as lots of on-street parking), although it becomes cycle lanes further. For drivers, they will have to enter the city via a different route. If you have a look around, there's a general principle of providing two-way cycling everywhere and if there's space, 2-way motoring; if not, one-ways for motors. In other words, there's a cycling network with the motoring network arranged around it. Neuwg is shown below.

Everywhere else, there are just streets where through traffic is either filtered or sent on long one-way loops to make it pointless for rat-running - these are access streets for the people who live there, their visitors and their deliveries - of course cycling is permitted everywhere and in both directions such as Rozenstraat which is one way for motors, two-way for cycling and a home zone street.

All very interesting and important context, but back to the junction. It's a pretty standard approach to have walking and cycling traffic moving orbitally around junctions (in many countries) and this junction is no exception. Cycling orbits next to the carriageway with walking next.

Walking and cycling have priority in each direction along the line of travel on Gijsbrecht van Amstelstraat by way of parallel zebra crossings (on speed tables and people walking have priority over the cycle track here) whereas motor traffic has priority along Neuweg over both walking and cycling and over motor traffic Gijsbrecht van Amstelstraat (where drivers are required to stop rather than give way).

Neuweg has a 50km/h speed limit with Gijsbrecht van Amstelstraat having a 30km/h speed limit - the arrangement of speed limits and priority for motor traffic here suggests to me that Neuweg does perform a somewhat arterial function. From a walking and cycling point of view, Neuweg performs the same function.

For people cycling through the junction, notwithstanding the need to give way to motor traffic on Neuweg, the junction is fully protected which is rather amazing given the compact nature of the junction. The drawing of the junction below is slightly stylized - I've left off the speed tables and not drawn the transitions to and from the cycle tracks beyond the junction. Neuweg runs up and down with the one-way from the bottom towards the top of the drawing.

If you look closely, you'll see the stop lines for drivers leaving Gijsbrecht van Amstelstraat and the triangular give way markings for drivers approaching the parallel zebra crossings. You'll also notice smaller give way markings on the cycle tracks approaching the junction giving cycle traffic on Neuweg priority over cyclists joining to head north or south.

As best as I can work out from Google Maps is the cycle tracks are about 1.5m wide, the footways perhaps around 1.8m (some narrower and some wider). The car parking bays are about 2m wide (light gray); with the carriageway of Neuweg about 3.7m and the carriageway of Gijsbrecht van Amstelstraat maybe 6.5m. 

The cycle crossings on Gijsbrecht van Amstelstraat are set back just over 5m from Neuweg which is important to ensure that drivers can see what is happening and stop to let cycle and pedestrian traffic cross as well as not blocking the other road or crossing (depending on direction of travel).

As I said earlier, I don't know the area, but the methodical approach to how the junction operates leads me to think that the safety record is probably pretty good. It also reminds us that with Neweg being subject to a 50km/h speed limit, cycle tracks are required under the Dutch system of sustainable safety and thus this extends to the junction treatment. Could this work in the UK?

In short, yes. The drawing above is the same layout, mirrored to drive / cycle on the left. It's fussier than the Dutch layout because of UK road markings and the need for zig-zags at the parallel zebra crossings - this requires the "elephant feet" square cycle markings to be 400mm from the zebra stripes. It's a daft dimension which really makes it tricky to make things work. Really, this should have been specified as between 400mm and maybe 1,200mm to give the designer flexibility. Anyway, the arrangement works and would be UK road-legal.

Of course, the key issue with this layout will be that of traffic volumes. Notwithstanding the speed limit issue I talk about above, I suspect that the junction is reasonably busy, but of course not enough to need signals. It's an old layout because the cycle tracks are made from concrete slabs rather than asphalt and giving Neuweg priority over foot and cycle traffic in an urban area like this feels a little old fashioned. This post may be little more than an academic exercise, but the junction was interesting enough to take a detailed look.

Saturday, 19 September 2020

Cycling In Town Centres Is a Network Issue

Every so often there's a flurry of complaints about people cycling through pedestrianised areas. Sometimes there might be a media item in which a talking head holds the cycling community at blame for the behaviour of a few and in some cases, the local authority will ban cycling, yet wonder why people are still there.

Discourse around this subject always comes at it from the wrong end, which is either the behaviour of a few individuals or concern about people cycling from those who maybe cannot quite articulate the issue.

A typical UK local authority response to cycling
in pedestrianised areas. (Hatfield, Hetfordshire).

I'll take the latter first. If you are walking in a pedestrianised area (in a general UK context), you're not expecting people to ride near you on a cycle. In fact, given how woeful the UK's mode share is for cycling (in general), someone cycling is a novelty to most people in most places. To some, the fact that there is someone cycling in what they thought was "their" space is enough to make people concerned - in other words, the person on foot has had their experienced safety compromised because of an unexpected interaction.

Johannes Paul II Straße, Aachen, Germany. 
A pedestrianised area where cycling is permitted. 
The street layout encouraged people to cycle 
in the centre of the street.

With the former, I am talking about the behaviour of the type of person who decides it's appropriate to cycle through a pedestrianised area at their normal riding speed. Even this isn't clear cut. There will be people (as there are in any walk of life) who are plain antisocial. If they weren't riding a cycle, they would be pushing in the queue to get on the bus or if they were driving, they would be speeding as they drove past a school at kicking out time. 

There will also be other people who are still in fight or flight mode after mixing with traffic on the ring road who haven't yet relaxed from the experience and dropped their pace - it's complex, but from the point of view of someone startled by suddenly seeing someone cycle by, there is no difference - they cannot possibly know what sort of person has gone by.

A couple of people in ordinary clothes cycling through
a pedestrianised area. Heads up, taking their time.
Leicester city centre where cycling is allowed.

I thought it might be interesting to see what the research says. Three Transport Research Laboratory (TRL) reports make for some helpful reading. 

First is a study from 1993 for the Department of Transport, "Cycling in Pedestrian Areas", which used video analysis to look at the issue. The study analysed 1 hour video recordings of 12 English sites and 9 mainland European sites and then 12 hour video recordings and questionnaires at 4 English sites. 

The English sites were a mix of places where cycling was banned, allowed during some hours and allowed all the time. The sites on the mainland were similarly arranged in terms of access time, but where cycling wasn't permitted, there were periphery routes available.

Vestergade, Odense, Denmark. One of the original
streets in the study which has different access times for
cycle traffic on different sections.

The findings from the report were;
  • Pedestrians alter their behaviour in the presence of motor vehicles where they are permitted in a shared area whereas the presence of cyclists has no appreciable effect;
  • People cycling adapt their speed to suit pedestrian density and dismount if required. Conflicts are generally dealt with by people cycling taking avoiding action;
  • Pedestrian areas have good safety records. The sites in the study had no pedestrian/ cyclist collisions in 15 years apart from one child pedestrian. No collisions were observed in the analysis of the video footage.
  • Where cycle traffic flows are higher the surface treatment and placing of street furniture and  shop displays can have a significant influence and a clearly identified section for cycle traffic aids orientation and operation with people tending to walk at the sides of the street and people cycling in the centre of the street.
The conclusion of the report stated;

The extensive observations made during this report has disclosed no real factors that justify the exclusion of cyclists from pedestrian areas and indicate that cycling can be more widely permitted without detriment to pedestrians.

It is important not to exclude cyclists from pedestrian areas and force them to use dangerous alternative routes. There are a wide variety of appropriate and satisfactory solutions (in terms of design and regulation) the choice of which will vary from place to place, and depend on local circumstances.

The Narrow Way (Mare Street) in Hackney is
pedestrianised with cycling permitted. 
There is a hint of where to cycle with the 
street layout. The alternative routes aren't 
great, so it's a useful link.

In terms of the pedestrian and cyclist interviews, the main point of concern was around the place where motor traffic had access (Oxford) and both groups felt safer in Chichester where motor traffic was banned.

In 1998, TRL published a report titled "Alternative Routes for Cyclists Around Pedestrian Areas". This study looked at 9 English towns where there were comprehensive bans on cycling in their cores and the quality of the routes available to people cycling to bypass them. Three towns were examined in more detail where there were signed diversion routes around the pedestrianised areas. 

Gallowtree Gate, Leicester, had a cycling ban at the time of
the study with a signed diversion for cycling,
but it's now open to cycling at all times.

There is some interesting commentary (p13) in the report which states;

Shopping trips (52%) and leisure trips (13%) are the cycle trips most likely to be foregone or transferred to another mode. Commuter cyclists are less likely to be deterred from travelling by cycle due to the pedestrian area restriction: whereas the main journey purpose of 26% of cyclists interviewed is commuting/business, only 11% of trips foregone or transferred are commuting trips. 

For those journeys that cannot be made by bicycle, because of the pedestrian area cycling restriction, 31% are transferred to walk, 24% to car (driver) and 15% to bus. 26% are not made at all as the respondents travel only by bicycle.

Notwithstanding the age of the study, this suggests that town centres lose out by banning cycling twice because some people don't make the trips at all and some will drive rather than cycle which adds to local traffic congestion.

Friarsgate, Winchester. Off-peak cycling is allowed
in some parts of the city core and banned in others. 
The alternatives include busy one-way streets which
are hostile to cycling.

The study noted that there was a lack of reported collisions between people cycling and walking as a side note; with the most interesting conclusions for me being;
  • The alternative routes created additional risk from traffic and inconvenience to people cycling compared with the pedestrianised route;
  • The alternative routes were longer, with almost half involving dismounting, and a different route was almost always necessary for the return trip;
  • Where signed alternative routes for cyclists were provided, most people found them to be safe and convenient;
  • Cyclists using the pedestrian areas and the alternative routes tend to choose routes on the basis of directness and minimising delay.
  • Because of the diverse journey patterns of cyclists in the town centres, safety needs to be improved throughout the road network. This would assist cyclists in town centres and reduce the incentive to cycle through the pedestrian area
The third report is from 2003 and titled "Cycling in Vehicle Restricted Areas", again from TRL. the study was three cities (Cambridge, Hull and Salisbury) and was more concerned about behaviours. The study used video, manual speed surveys and interviews of cyclists and pedestrians. There were also discussions with the local authorities and others around nine other places.

In the summary, the following statements are made;

It was shown that pedestrian flow, regulations, the types of cyclist and the characteristics of the site influenced dismounting and cycling speeds. The majority of cyclists tended to slow down or dismount and push their bicycles when pedestrian flows were high. However, a minority (mostly young males) continued to cycle quite fast.

The interviews with 300 pedestrians and 150 cyclists, showed that the majority of pedestrians said that they were "not bothered" by cyclists using the VRA [vehicle restricted area]. However, a number of people had witnessed collisions between cyclists and pedestrians in the VRA and a majority of pedestrians at two of the three sites said they would like to see cyclists excluded for at least part of the day.

When you dig into the report, the wish for exclusion seems to come in where cycling flows are higher and only then after prompting. There is some useful discussion around how people cycling should be managed in terms of the position they take in the street. Visual impairment groups suggested that there should be a fence or barrier providing separation, but the report notes that this and indeed, a strong visual delineation would not meet urban design objectives; however, the use of street furniture could usefully channel people cycling to the centre.

In all three reports, there's obviously a fair bit to read through. Despite when they were undertaken, I think we have got some very useful information and discussion arising from the research.

From my own experiences visiting some of the places in the research and from my trips to northern European cities, I wasn't particularly surprised by the contents of the reports. The third report was one I was familiar with (but hadn't reviewed for a while) and the first two were new to me in researching this post.

So, we have evidence to suggest that most people on foot aren't generally bothered about people cycling unless flows get high and maybe concerns only get expressed when pushed. We can see that there are people who are basically just antisocial when they cycle. We've seen that visually impaired people have concerns (fully legitimate in my view). 

There's evidence to show that cycling bans are bad for the town centre in terms of losing visitors or some switching to cars. We have also seen that some local authorities ban cycling but do nothing about the diversion routes which are often hostile, indirect and with poor accessibility.

Rue de l'Eau, Luxembourg City. Pedestrianised with some 
motor vehicle access, but two-way cycling allowed.

I mentioned "interaction" at the start of this post. The evidence suggests that most people cycling take responsibility for their own behaviour and that of the person on foot - by that I mean, people are dropping their speed and dismounting when it get busy as well as anticipating the unpredictability of other people. We've also seen that by and large, there are very few collisions where people can cycle through pedestrianised areas.

Design Implications
This is a two-fold issue. The thing that most people tend to look at is the pedestrianised street itself, but in fact, it's a network level design issue. People with no business in the town centre won't be immediately thinking about slowing down too much, their minds are somewhere else and so we should be catering for them with proper routes around core pedestrianised places. These routes should feel safe, direct and legible and if done properly, we've cleared out the people who don't need to be in the the town centre which reduces the overall flows that some pedestrians find concerning.

Vaartscherijnbrug, Utrecht, The Netherlands. If you don't
need to cycle into the city centre, you can cycle
around the edge to get somewhere else.

Then with the pedestrianised areas themselves, we can use street furniture placement or perhaps textural cues to guide people cycling to the centre of the street away from stop fronts where people walking may have their attention elsewhere. Having a clutter-free central area is also pretty useful for fire access, maintenance and (where permitted) servicing; so it's a win-win.

Storgatan, Malmö, Sweden. This is the entrance to
a pedestrianised area where cycling is permitted.
The street layout still hints at the historic road layout
with different paving in the centre and with street furniture
well-positioned, people cycling stick to the centre.

The important thing is that we should be welcoming people to our town centres because it is good for them both. People obviously work in town centres and there are people who find cycling easier than walking - bans on cycling directly discriminates Disabled people who rely on cycles at their mobility aids; could you imagine the public reaction to banning mobility scooters?

The challenge we have is that local authorities that ban cycling in the UK don't provide decent alternatives (notwithstanding the needs and wants of people wanting to cycle to shops and businesses). Even this week, Worcester County Council has extended it's ban on cycling in Worcester City Centre to 10am - 6pm, from 10:30am - 4:30pm. This is apparently to make things safer for pedestrians and it has the backing of businesses after consultation (whether anyone else had a say isn't clear). 

Worcester follows the pattern of the hostile places in the 1998 TRL study because their ban doesn't come with safe and direct routes around the pedestrianised area. The way around is a heady mix of one-way streets, including the A38 where people cycling get to play with buses, HGVs and general traffic. Your outgoing route would also be different to your incoming route. Clearly the City and County are happy to turn away customers, workers and disabled people. Presumably they can produce evidence supporting the ban and the equality impact assessment they did before extending it.

Hovedgaden, Nordby, Fanø, Denmark. This village
centre is largely pedestrianised and cycling is
welcomed. There is actually a cycling bypass on the
road which skirts the village to the ferry terminal which
takes some of the through-cycling out.

You may feel that until and unless alternatives are provided, cycling shouldn't be permitted in pedestrianised areas. I have to disagree because the evidence shows that it is a low risk thing to allow (as well as being good for town centres and people). 

If there is a problem with behaviour, then this is tackled through design of the space and designing out through cycle traffic. Then you can switch the enforcement you were going to deploy on the ban onto the idiots who don't behave and you target this activity when pedestrian flows are highest because that's when the potential for interaction will also be highest.

Vismarkt, Utrecht, The Netherlands. There is a peak
hours cycle ban here, but people ignore it if they
feel they're not causing a problem.

In the final analysis, trying to ban a fairly simple form of human-powered transport is doomed to fail. If you think about it, the people who manage to get to a hostile town centre are already pretty committed and will have dealt with equally hostile roads getting there. A local authority putting up a sign won't stop them and let's face it, the chances of getting caught are going to be slim unless they really are being stupid at peak teams. 

In my view, managing and nudging people in a situation like this is far more productive than trying to ban them and we have already done the research to back all of this up.

Wednesday, 9 September 2020

Speeding Culture

I've often said that infrastructure creates the culture and for much of the UK, this is a motoring culture. However, as many people who have been cycling throughout the Covid crisis will tell you, we also have a speeding culture.

It will therefore come as absolutely no surprise that the latest "vehicle speed compliance statistics" from the Department for Transport shows a disregard for speed limits. Of course, it isn't a matter of "vehicle compliance" it's a matter of "driver compliance" and why the DfT seeks to remove this agency in the headline is beyond me.

Of course, a non-compliance will be 1mph over any given speed limit and to some extent this should be borne in mind, but 1mph over is still speeding, but we have developed a culture that a little bit of speeding isn't a problem because it's not widely enforced. However, as can be seen on the interactive graph in this ProPublica article, the risk of being killed really does go up with every mile per hour of speed increase.

So what are the headlines? Well they aren't great as shown in the first figure;

Again, we've the lack of agency, but certainly within the "professional" vehicle classes, there are plenty of drivers who should know better and the only reason that the drivers are articulated lorries appear saintly on motorways is that they have speed limiters (see the bottom of p11 here for more information and remember there are different limits for different classes on motorways and within National speed limits).

The data for car and van drivers is interesting with around half of each speeding on motorways and 30mph roads. There is no data for van drivers on National speed limit single carriageways, but I would expect the level of non-compliance to be similar. The reason I am so confident is in the nature of what these roads look like (as a result of their design or evolution). Even single carriageway roads with a National speed limit which are relatively wide and straight have geometric limitations on the maximum speed it feels to safe to drive at, whereas many motorways and 30mph roads are not uncomfortable to speed on;

The design of this 30mph street doesn't send a message to
drivers that they should drive below 30mph.

The data suggests that in terms of drivers exceeding the speed limit by 10mph on motorways is 12% on motorways, 6% on 30mph roads and 1% on 60mph roads. Of course, in terms of the percentage above the speed limit this equates to 14%, 33% and 17% above the speed limit respectively which is a better way of presenting the same thing. 

In terms of compliance generally, it is best single carriageways with the National speed limit and worst on 30mph roads. The level of compliance has been the same since 2011, although this year the DfT has used fewer sites and the document warns about comparing and the data is based on "free flow" - the speeds drivers can travel at when they are free to do so.

For 20mph limits, the picture is far worse, although there is a caution that free flow may be less clear on these roads because of the presence of traffic calming and that free flow sites are more likely to be through routes rather than residential streets (which will be under represented in the data). 86% of car drivers exceeded the speed limit with 20% being 10mph over the speed limit (or 50% over the speed limit). Compliance was better with lorry and bus drivers.

Again, the type of roads chosen will look no different from the 30mph roads I have mentioned above. As a generalisation (of road layout and driver behaviour), this is suggesting that drivers are taking cues from the street layout when selected their speed rather than what the speed limit is posted as. 

This is Hornchurch High Street. It's a busy A-road and the
comfort for cycling is questionable, but the layout of the street
tells drivers that this is a place to drive slowly and the 20mph
speed limit has a decent level of compliance.

In terms of enforcement there were 2.11 million speeding offences in 2018 in England & Wales. The RAC Foundation suggests there are 15.3 million drivers in England and Wales to put that into context. There was an 88% conviction ratio and 28% of motoring offences were for speeding. In terms of collisions (DfT still calls them accidents), 5.1% included exceeding the speed limit as a contributory factor and for fatal/ serious collisions this rises to 6.6%. Of course inappropriate speed can still be a contributory factor within the speed limit. 

Although the data is broad, I think it is reasonable for us to conclude a few things from it;

  • We have a culture of speeding, especially with car and van drivers;
  • There are issues with how we design our streets for 30mph and 20mph in terms of drivers taking cues from the environment on how they should behave, notwithstanding there are speed limits to obey;
  • Speeding is a significant issue in terms of motoring crime and the police are right to take enforcement action to deal with it;
  • Drivers who are caught speeding are highly likely to be convicted.

Personally, I would be very happy if we could free up police resources from dealing with speeding drivers, but this requires changes to our streets, especially the urban 20mph and 30mph I have discussed above - those which are probably going to be through routes. Of course, vehicle technology with speed limiters will help, but this is going to take years and of course, even if a road has a 20mph speed limit which is universally obeyed, it may well be very hard to cross if it's busy.

You may think it tenuous, but I think rolling out low traffic neighbourhoods is a pretty cost-effective treatment to deal with side streets in comparison with traffic calming which leaves more resources to tackle main roads where we can narrow traffic lanes to slow drivers down, make space for other modes and to change the nature of how a street feels. In turn, the police can spend more time dealing with those who exhibit the very worst driving behaviours.

Sunday, 6 September 2020

Traffic Signal Pie: Begging The Question

Push buttons at signalised crossings is a subject which pops up all the time and when it comes to accommodating cycle traffic, we find yet again lumping it with people walking disadvantages both modes.

One of the recent US-imports to the UK has been the increasing use of the term "beg-button"; in other words, people wishing to cross the road have to beg people driving to let them cross by pushing the button. I sympathise with the premise, but what is worse, is not being able to reach the button in the first place. I posted a photograph of my cargotrike next to the push button of a toucan crossing on Twitter yesterday with the complaint that I simply couldn't reach it.

Sadly, someone suggested that I could easily dismount and while that is true, it demonstrates some of the attitudes which prevail in the UK because people simply do not realise that there are people who cannot dismount - this is a clear example that the environment disables people. I went through the same crossing today on a completely different cycle;

As you can see, the saddle is much closer to the push button, but it is still a stretch - I am not welcomed by this layout at all. It's a specific layout issue because in using this crossing (a two-stage non-staggered arrangement) I am moving towards the push button rather than arriving next to it as I would be on the other side of the road;

As you can see above, I am right next to the push button which is easy to press, although on the tricycle, I risk being tipped by the dropped kerb, so it isn't ideal. One issue which probably doesn't appear immediately obvious is that my position here does two things. Being a nearside crossing, my body would block the view of the crossing display to anyone else wishing to cross. This can be dealt with by the use of a second high level display, although if I am on my cycle, I may well block this too;

More importantly, I am blocking access to the tactile cone sitting under the push button box which rotates when the green man is showing; a feature which many visually impaired people rely on;

Of course, this problem has been created by lumping walking and cycling together. If the modes are properly catered for with separate crossing provision, then the use of push buttons means we can more sensibly place them in relation to the seating position of someone cycling;

The photograph above shows a layout which has a parallel cycle and pedestrian crossing (with a decades-old layout). The push button for the cycle crossing used to be on the right hand traffic signal pole (next to the guardrail) which meant that anyone on a cycle wanting to cross had the whole issue of not being able to reach the button and blocking the footway. By setting the push button further back with a stop line, these two problems are dealt with. The crossing really needs a complete redesign for a number of reasons, but this relatively modest change makes things work a little better.

Of course, the ultimate cycle-friendly layout is one where there is no push button. The photograph below shows a parallel signalised crossing where people cycling are detected by a sensor on top of the traffic signal pole. The metal frame next to the cycle track is a foot/ hand rest to make it a little more comfortable for people waiting to cross (product here).

The choice of push button versus detection will be down to site layout. If a cycle track runs towards the crossing point, the detection should be quite simple. If the layout is tight with other modes then a push button avoids other traffic being detected by mistake (depending on the type of detection) - this is an area better known to signal engineers! Of course, there is the whole question of whether we should be using push buttons routinely, but assuming they are with us for now, let's at least improve the layouts.

Saturday, 29 August 2020

Life On The Edge

As a kerb nerd, I'm going to talk about something which is a bit counter-intuitive this week and that is we should not use kerbs!

Actually, it's more specific than that because kerbs remain very useful components of our toolbox. The issue I am interested in this week is where a footway or cycle track meets a verge - a situation which on the whole doesn't actually need kerbs to be used.

It's perhaps worth reminding ourselves about what kerbs are for;
  • Retain the edge of the top layers of a pavement,
  • A demarcation between different areas or uses of a highway,
  • To provide a check or channel for surface water management,
  • To provide restraint to prevent vehicles leaving the carriageway.
For a footway or a cycle track meeting a verge, we might need to manage surface water, but we don't need demarcation (the edge of the surface does that), we don't need to restrain out of control people walking or cycling and it's pretty rare we actually need to retain the edge of the top layers of the pavement (the term used in its technical sense).

In my view, it is the first use which is where things go wrong and it is usually down to the specification being used. Footways and cycle tracks are usually fairly lightly constructed which reflects their users, although there are problems which motor vehicles are driven onto them.

When we build carriageways (roads), we are concerned with transferring loads from motor vehicles from the point where their tyres meet the surfacing, through the pavement layers to the weaker underlying soils.

The sketch above shows how the load from the wheel is transferred down through the pavement layers into the underlying soil (or sub grade) on what is called a "flexible" construction with asphalt surfacing. The underlying soils will normally be too weak to support traffic loading directly and so we're building up layers of materials to compensate.

The immediate surfacing material is of the highest quality and deals with sealing the pavement from water ingress and provids skidding resistance. In many cases, there will be two further asphalt layers which do structural work, but with cheaper materials which need to have a certain thickness to transfer the load as well as resist wheel rutting.

At sub base level in a flexible pavement, we are using a graded stone (or reclaimed crushed concrete and other materials) from 75mm in size down to dust (to a specification for good compaction) and then below that a capping layer of lower quality stone or other materials. The red triangle essentially shows that the load at the surface requires an ever larger area of the lower (and cheaper) layers to support the wheel which has a small contact area with the surfacing.

I hope you're still with me. The layers are varied and sometimes omitted such as the capping where the underlying soil is better at supporting loads. There are other designs of pavement available such as reinforced concrete and all manner of clever sheet materials to add strength, but let's keep it simple.

For footways and cycle tracks, we're generally not needing to transfer any loading from people - they simply don't cause damage as they walk or cycle along. We don't just leave grass or mud at the surface of course, a surfaced path is more comfortable to use and we can shed water off the surface (or we can use permeable materials and clever underlying materials and systems to deal with water).

In fact, the limiting issue is the ability to construct footways and cycle tracks in the first place because at the absolute minimum, we need to be able to use a roller (usually vibrating) to compact the sub grade and the layers above. In fact, I think all asphalt surfaces should be laid by machine unless it is localised repair or reinstatement. If we simply compacted surfacing on the soil below, it would just get bashed into the ground when we roll it, so we need a bit of sub base to give a minimum pavement depth (There are different types and classes of roller depending on what the situation is).

The other dimension is that top soil is no good for building paths on. It has organic materials which rot down leading to settlement and plant seeds or roots can grow through to the pavement - we need to be founding our paths on the subsoil which means a certain thickness of construction. Beyond that, we need to assess maintenance - how heavy will the street sweeper be that cleans the footway or cycle track? How likely are we to get maintenance vehicles using it? What is the likelihood of people parking on it (cars, vans or lorries)? A perfectly competently built path for only walking and cycling use can be destroyed in one sitting by an HGV being driven over it.

In the Design Manual for Roads & Bridges, there is a whole document around footway and cycle track pavement design which goes into detail of the options available based on the strength of the sub grade soil and the type of loading. I'll not go through the detail of this because I have given a flavour above but I'll pick on one of the combinations because it gets me back to kerbs. With decent ground conditions and no chance of over-run, the recommended specification is 20mm of asphalt surfacing, a 50mm asphalt layer below that (called the binder course) and a 100mm sub base - a total thickness of 170mm.

The standard edging kerb is 50mm wide and 150mm deep. In general terms, it will be laid on a 100mm concrete bed with a 100mm concrete surround - in other words, to a depth of 250mm. This is where it all goes wrong. 

The photograph below shows some recent repair work to a shared-use path near where I live. It's not immediately obvious what's going on, but essentially the old surfacing was removed (which looked like two asphalt layers to me) and some of the edging kerbs were replaced. 

In the photographs above and below you can see that for the replacement edging kerbs, a trench was dug and new kerbs laid on concrete. In some places new sub base was laid between the edging kerbs and the whole thing was resurfaced (badly).

Around 8 weeks later, there are signs of failure in terms of cracks along the path returning as well as dips forming in the new surfacing (photograph below). It's not a location where vehicles tend to get and hopefully me riding my Dutch bike isn't that heavy!

It's not an isolate case. Elsewhere along this road, there have been attempts to repair cracks in the shared-use path. The photograph below shows a repair where the edging kerbs were replaced within a trench. It has failed again with a new crack as well as the joint between the old and new surfacing opening up.

The photograph below is slightly different because the failure is in an area which the edge of the path was rebuilt following water main works. But, it's the same pattern of failure. The water company would have only replaced what it took out.

It's not easy to see what is going on, but the photograph below is a close up of the crack. At the top you can just make out the concrete backing which is helping to support the edging kerb, which is a nice little clue in the mystery.

Now, my area is built on London Clay which is notorious for moving about as it dries out and shrinks in the summer and then absorbs water and expands in the winter (and when frost penetration could be another problem). This means that whatever we do, we have to accept that paved surfaces might be impacted - worse where there are tree roots sucking water out of the ground. There is a limit to how deep anyone should dig to find a decent sub grade and around my way, it will be metres before you get something better which means we work with what we have.

The sketch above shows a little of what is happening. The edging and the concrete surrounding it are not acting flexibly - that is, if the clay is shrinking or expanding then it's acting almost like a solid beam along the path and so a crack is induced roughly along the edge of the concrete surround. The bedding concrete is laid straight on the soil which is moving around because the kerb and its bed are deeper than the pavement construction.

In the photographs, the reason the crack isn't always straight (aside from the stone within the surfacing being irregular) is the lack of quality control on the edging kerb backing concrete - you can see the ragged line of this in the second photograph of this blog - they are wasting concrete and not keeping everything neat and tidy.

There are two ways to design this out. First, the edging kerb sitting on its concrete bed needs to be supported on sub base rather than directly on the soil which is created by the condition of having the minimum path thickness. In the image below, the sub base goes under the edging kerb and its concrete bed. It has to extend beyond to make sure it is fully supported.

If you think about it, this is what would happen with a road because everything is so much thicker to take motor traffic loading which means kerbs are supported. The following image is an extract from a standard construction detail showing this for both the road edge kerb and edging kerb (it is a very heavy duty detail);

It's not immediately obvious, but if you look at the photograph above, the three kerb lines are all supported on the sub base which extends beyond the two outer kerb lines.

The second way to design out the problem is to dispense with the kerb altogether which does run counter-intuitive to some people;

The image above has our 100mm of sub base, 50mm of asphalt binder and 20mm of asphalt surfacing. Assuming the sub grade soil is appropriate for the job and we're not getting vehicles on this (other than very lightweight sweepers maybe), we simply don't have the conditions for an edge failure. Each layer is a bit wider than the last to ensure support and the asphalt layer edges stand up on their own (because that is what asphalt does when there are no vehicles running on the edge).

The photograph above is (yes you've guessed it) of a Dutch cycle track which hasn't got edging kerbs. You can clearly see the red surfacing and the black binder course below which is stepped out in support. The dotted lines you can see will have been left by the asphalt crew making sure the surface was properly set out. The photograph below shows the same cycle track without edgings on both sides.

Unless a kerb is needed to manage water or there is a local issue such as trying to prevent soil from an embankment washing onto the path, the lack of edging kerbs makes things far easier to maintain because there will be failures that need to be repaired from time to time. Vegetation management is important because if it encroaches, it can break the edge of the path and sometimes stop water flowing away (but that's no different to where you've a flush edging kerb anyway. 

If the path is constructed from block paving or small slabs, then edging kerbs are used to restrain them, but can still be subject to movement which leads to failure in the paving, so it is no less important to get the detailing correct.

So, the next time you see a crack along an asphalt path in a bit from the edge, you'll know what has probably happened and how it should be fixed. Unfortunately, what will probably happen is the same mistake will be made again and people will be left scratching their heads.