Category Archives: Traffic calming

Architecture, urbanism, design and behaviour: a brief review

by Dan Lockton

Hollywood & Highland mall

Continuing the meta-auto-behaviour-change effort started here, I’m publishing a few extracts from my PhD thesis as I write it up (mostly from the literature review, and before any rigorous editing) as blog posts over the next few months. The idea of how architecture can be used to influence behaviour was central to this blog when it started, and so it’s pleasing to revisit it, even if makes me realise how little I still know.

“There is no doubt whatever about the influence of architecture and structure upon human character and action. We make our buildings and afterwards they make us. They regulate the course of our lives.”
Winston Churchill, addressing the English Architectural Association, 1924

In designing and constructing environments in which people live and work, architects and planners are necessarily involved in influencing human behaviour. While Sommer (1969, p.3) asserted that the architect “in his training and practice, learns to look at buildings without people in them,” it is clear that from, for example, Howard’s Garden Cities of To-morrow (1902), through Le Corbusier’s Ville Contemporaine and La Ville radieuse, to the Smithsons’ ‘Streets in the sky’, there has been a long-standing thread of recognition that the way people live their lives is directly linked to the designed environments in which they live. Whether the explicit intention to influence behaviour drives the design process—architectural determinism (Broady, 1966: see future blog post ‘POSIWID and determinism’)—or whether the behaviour consequences of design decisions are only revealed and considered as part of a post-occupancy evaluation (e.g. Zeisel, 2006) or by social scientists or psychologists studying the impact of a development, there are links between the design of the built environment and our behaviour, both individually and socially.
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Staggering insight

Staggered crossing in Bath

I’ve mentioned a few times, perhaps more often in presentations than on the blog, the fact that guidelines for the design of pedestrian crossings in the UK [PDF] recommend that where a crossing is staggered, pedestrians should be routed so that they have to face traffic, thus increasing the likelihood of noticing oncoming cars, and indeed of oncoming drivers noticing the pedestrians:

5.2.5 Staggered crossings on two-way roads should have a left handed stagger so that pedestrians on the central refuge are guided to face the approaching traffic stream.

When I gave this example of Design with Intent at Lancaster, the discussion – led, I think, by Lucy Suchman and Patricia Clough – turned to how this arrangement inevitably formalised and reinforced the embedded hegemony of the motor car in society, and so on: that the motorist is privileged over the pedestrian and the pedestrian must submit by watching out for cars, rather than the other way around.

Now, all that is arguably true – I had seen this example as merely a clever, sensible way to use design to influence user behaviour for safety, for everyone’s benefit (both pedestrians and drivers) without it costing any more than, say, a crossing staggered the opposite way round – but this is, maybe, the nature of this whole field of Design with Intent: lots of disciplines potentially have perspectives on it and what it means. What a traffic engineer or an ergonomist or a mistake-proofer sees as a safety measure, a sociologist may see as a designed-in power relation. What Microsoft saw as a tool for helping users was seen as patronising and annoying (at least by the most vociferous users). It’s all interesting, because it all broadens the number of interpretations and considerations applied to everything, and – if I’m honest – force me to think on more levels about every example.

Multiple lenses are helpful to designers otherwise stuck at whatever focal length the client’s prescribed.

Back to the crossings, though: the above crossing in Bath is a bit unusual in how it’s arranged with so many control panels for pedestrians. But in general, with simple Pelican and Puffin crossings in the UK, there is a design feature even more obvious, which only struck me* the same day I photographed the above crossing in Bath: the pedestrian signal control panel is usually also to the right of where pedestrians stand waiting to cross, i.e. (with UK driving on the left), in order to press the button, pedestrians have to turn to face the oncoming traffic.

The guidelines actually mention this as helping people with poor vision, but it would seem that it really assists all users, even if only slightly. It means you can watch the traffic as you decide whether or not you actually need to press the button, and will be more likely to be standing in a position where you can see the oncoming traffic at the point when you walk out into the road.

5.1.7 To assist blind and partially sighted pedestrians, as they approach the crossing, the primary push button/indicator panel should normally be located on the right hand side. The alignment should encourage them to face oncoming vehicles. The centre of the push button should be between 1.0 and 1.1 metres above the footway level.

This is the sort of ‘hidden’ intentional, strategic design detailing which fascinates me. It is obvious, it is quotidian, but it’s also thoughtful.

Staggered crossing in Bath

*Looking back through my notebooks, I see that someone actually mentioned this to me at a seminar at Sheffield Hallam in September 2007 but I forgot about it: many thanks to whoever it was, and I should be better at reading through my notes next time!

Hard to handle

Open door using outside handle

Open door using outside handleBritish Rail’s drop-the-window- then-stick-your-hand-outside- to-use-the-handle doors puzzled over by Don Norman in The Design of Everyday Things are still very much around, though often refurbished and repainted as with this delightful/vile pink First Great Western-liveried example.

I’m assuming that this design was intended to introduce an extra step into the door-opening procedure, a speed-hump, if you like, to make it less likely that a door was opened accidentally while the train was in motion (before central door locking was introduced – which makes it less necessary). From a usability point of view, we might immediately dismiss any system which has to have such detailed instructions to inform the user about performing such a simple task, but it’s certainly interesting to consider this kind of poka-yoke. Being forced to lowering the window to get to the handle is almost like a modal ‘Are you sure you want to delete this file?’ dialogue box.

Open door using outside handle

However, other concerns come into play and now need to be considered in addition: this sticker suggests keeping the window closed to cut drag and save fuel, but as I walked along the train, almost all these windows were dropped down, left in that position by the last person to close the door. The urgency of scrabbling to lower the window, stick your hand out and use the handle, with a crowd of commuters behind you probably overwrites any intentions to close the window again engendered by the ‘Make a small change’ sticker.

Open door using outside handle

Cyclepathology

A lot of architectures of control / design with intent examples are trying to enforce what I’ve termed ‘access, use or occupation based on user characteristics’. Not all designs are especially successful at achieving that target behaviour: users will not always be persuaded, or will find ways to avoid being coerced.

Mud, footpath, cycles and kissing gate

Bicycles can churn up the surface of footpaths…

Mud, footpath, cycles and kissing gate

…You can put up signs to tell cyclists not to do it…

Mud, footpath, cycles and kissing gate

…or you can put in gates (kissing gates as they’re known in the UK) to try to stop them (along with livestock)…

Mud, footpath, cycles and kissing gate

…but it doesn’t mean anyone will take any notice!

Some more architectures of control for traffic management

Many of the ‘built environment’ examples discussed here over the last year-and-a-bit have been intended to control (or “manage”) traffic in some way, e.g to slow drivers down, force them to take an alternative route, or force them to stop. I thought it would be worth mentioning a couple of other methods, the rationales behind them, and some of the problems:

Monmouth Thame
Amersham Thaxted
Top row: Monmouth, Monmouthshire and Thame, Oxfordshire; Bottom row: Amersham, Buckinghamshire and Thaxted, Essex. Images from the sites linked.

Historical example: market places

Mediæval market towns commonly had a wide market street, or square, with narrow entrances at the ends, to make it more difficult for animals to escape, and also easier to control when herding them in and out. It may not be immediately obvious from the above photos, but in each of these towns (as with many others where the old layout has been preserved), the market area was, and still is, laid out in this way. It may also have made it more difficult for a thief to escape, since with only a few exit ‘pinch points’, it would make him easier to spot.

This is, of course, almost the opposite rationale to Baron Haussmann’s Paris, with its wide, straight boulevards which prevented effective barricading by revolutionaries and allowed clear lines-of-sight to fire on them.

References: Thaxted at ‘Rural Roads’; History of Thame; Monmouth on Wikipedia.

Pinch point with car overtaking cyclist Pinch point with car overtaking cyclist
Stills from video clips of cars overtaking cyclists at pinch points, from the Cambridge Cycling Campaign website.

Pinch points and other road narrowings

In modern use, pinch points are often installed (along with centre hatching) to force drivers to slow down, usually in built-up areas or at the entrance to them, where there may also be a speed limit change. Sometimes they also force one stream of traffic to stop to allow the other priority, for example when crossing a narrow bridge. Sometimes there are built-out kerbs on both sides of the road; sometimes just a central island; sometimes all three. In general, they prevent drivers overtaking other cars by putting a physical obstruction in the way, even though otherwise it might be legal to overtake. (This is a built environment example of Lessig’s “Code is law” – regardless of what the law might permit or prohibit, it’s the way the system is coded which actually defines what behaviour is possible.)

The problem is that – something which as a driver and a cyclist (and bike designer) I experience a lot – the sudden narrowing of the carriageway causes (forces) drivers to move towards the nearside. And if there’s a cyclist on the nearside, even cycling close to the kerb, he or she will suddenly have a driver passing very close, braking very hard, possibly clipping the bike or actually hitting it. It’s even worse if the kerb is built out as well, since the cyclist has to swerve out into the path of the traffic which may also be swerving in to avoid a central island. In cities such as Cambridge with a lot of cyclists and a lot of traffic, the pinch points are a major problem.

A lot of injuries and deaths have been caused by this ‘safety’ measure. Someone very close to me was knocked off her bike and hurt after swerving onto the kerb to avoid a large truck bearing down on her as the driver tried to fit through a pinch point (similarly to the situation in the photo at the top of Howard Peel’s detailed assessment of pinch points at the Bike Zone). As with so many architectures of control, the designers of these layouts seem to view most users (both drivers and cyclists) as ‘enemies’ who need to be cajoled and coerced into behaving a certain way, without actually looking at what their needs are.

The North Somerset Cycle Campaign’s article on “Good and bad practice” with pinch points shows a far superior layout, for both drives and cyclists (photo reproduced below), from the Netherlands – cycles and cars are kept apart, neither cyclist nor driver is forced to deviate from his/her path, but drivers must give negotiate priority with their oncoming counterparts.

Pinch point in the Netherlands Astonishingly dangerous hatching in Devon
Left: A better pinch point implementation from the Netherlands – image from the North Somerset Cycle Campaign; Right: A very dangerous (and ridiculous) real-world example of hatching-with-obstacles from Devon – image from Richie Graham, discussed in this thread on SABRE

Looking further at centre hatching, this too often causes drivers to pass much too close when overtaking cyclists, since (in the UK), most drivers are reluctant to enter it to overtake even though (with broken lines along the side) they are legally entitled to do so. The reluctance may come from ignorance of the law, but in many cases it is often because there may suddenly be a central concrete island in the middle with no warning. (This is certainly why I’m very careful when using the hatched area to overtake.) Again, this is a de facto imposition of regulation without a legal mechanism enforcing it. As Peter Edwardson puts it:

Two reasons are normally advanced to justify hatched areas, neither of which is entirely convincing. The first is that they separate streams of traffic, but how many head-on collisions occur on single carriageway roads anyway, and surely in the vast majority of cases they involve a driver who has recklessly crossed the white line. The second is that they slow traffic down, which may be true to a limited extent, but again is of no value unless it reduces accidents at the same time…

However, I have recently seen a document from the Highways Agency… that stated clearly that one of the aims of hatched areas was to “deter overtaking”. They daren’t go so far as to actually ban it on straight stretches of road by painting double white lines (although no doubt that will come) but instead they put in confusing paint schemes that have the practical effect of doing just that.

There is of course one entirely sound and legitimate reason for painting hatched areas on the road, to provide a refuge for vehicles turning right, something that in the past has been a major factor in accidents. However such areas should only extend at most for a hundred yards or so on either side of the right turn, and should not be used as an excuse to paint a wide hatched area for a long distance.

In the case of the astonishing (to a UK driver’s eyes) implementation of hatching on the A39 (soon to be A361) Barnstaple southern bypass in Devon – the right-hand photo above – actual bollards have been embedded in the road surface to ‘enforce’ a de facto ‘no overtaking’ intention, though the hatching area actually makes it perfectly legal to overtake. (It makes it worse that the reflectors on the bollards are the wrong colour as well.) Motorcyclists could overtake by weaving between the bollards into the hatched area, but this wouldn’t be especially easy or safe. It would certainly be more dangerous than the alternative situation of wider lanes with no hatching and no bollards. So what’s the point of the scheme?

Shared space at Seven Dials, London Shared space at Seven Dials, London
A Shared Space implementation at Seven Dials in central London, by Hamilton-Baillie Associates

Psychological techniques

We’ve looked before at ‘Shared Space’, ‘naked roads’ and other ‘psychological techniques’ to encourage drivers to be more alert, but Mike Morris sends me a link to this Spiegel story going into more detail and discussing Europe-wide pilot projects:

The utopia has already become a reality in Makkinga, in the Dutch province of Western Frisia. A sign by the entrance to the small town (population 1,000) reads “Verkeersbordvrij” — “free of traffic signs.” Cars bumble unhurriedly over precision-trimmed granite cobblestones. Stop signs and direction signs are nowhere to be seen. There are neither parking meters nor stopping restrictions. There aren’t even any lines painted on the streets.

“The many rules strip us of the most important thing: the ability to be considerate. We’re losing our capacity for socially responsible behavior,” says Dutch traffic guru Hans Monderman, one of the project’s co-founders. “The greater the number of prescriptions, the more people’s sense of personal responsibility dwindles.”

About 70 percent of traffic signs are ignored by drivers. What’s more, the glut of prohibitions is tantamount to treating the driver like a child and it also foments resentment. He may stop in front of the crosswalk, but that only makes him feel justified in preventing pedestrians from crossing the street on every other occasion. Every traffic light baits him with the promise of making it over the crossing while the light is still yellow.

The new traffic model’s advocates believe the only way out of this vicious circle is to give drivers more liberty and encourage them to take responsibility for themselves. They demand streets like those during the Middle Ages, when horse-drawn chariots, handcarts and people scurried about in a completely unregulated fashion.

I think that’s the key to a lot of ‘control-versus-the-user’ debate. Allowing users to take responsibility for their own actions is encouraging them to think. Encouraging people to think is very rarely a bad thing.

One of the simplest consequences of the shared space situations I’ve come across (whether deliberately planned implementations such as at Seven Dials, shown above, or just narrow old streets or village layouts where traffic and pedestrians have always mixed) is that drivers and pedestrians, and drivers and other drivers start to make eye contact with each other to determine who should have priority, or to determine each other’s intentions. Eye contact leads to empathy; empathy leads to respect for other types of road users; respect leads to better understanding of the situation and better handling of similar situations in future. Shared space forces all of us (pedestrians, cyclists and drivers) to try to understand what’s going on from others’ points of view. We learn to grok the situation. And that can’t be bad.

Mike Dickin, the legendary British radio talk-show host who was very sadly killed earlier this week after a heart attack at the wheel, often made the point in his frequent discussions on motoring issues that there should be no need for speed limits in many villages, towns and cities, because in many cases the ‘natural’ limit imposed by pedestrians, other traffic, road layouts and so on, should be enough to slow drivers down to well below the imposed ‘safe’ limits of 20 or 30 mph which lull drivers into a false sense of safety. Of course, he was right, and of course, in most small villages this is still the way things are done, as they were centuries ago, and as Hans Monderman suggests in the above quote.

The age of hyper-regulated behaviour, and treating the user (driver, cyclist, pedestrian) as an idiot incapable of thinking for him or herself, is largely coincident with the age of bureaucratic, centrally planned urban dystopia which sees individuals as components which must all perform identically for the system to operate. I would like to think we can move beyond that view of humanity.

Back to the issue of psychological techniques for traffic management, Jim Lipsey left a comment a couple of months ago mentioning the use of progressively closer painted stripes across the road in Chicago to cause drivers to slow down on a dangerous curve:

In a few weeks, dozens of new pavement stripes will be laid down. At first they’ll be 16-feet apart, but as drivers get closer to the curve, the stripes will only be eight feet apart. “They provide an optical illusion that vehicles are actually speeding up and that causes motorists to slow down, which is of course, the intended effect that we’re trying to have at that location.”

The Chicago example appears to be using only the visual effect to provide the illusion, but a similar technique is often used with raised painted ‘rumble strips’ on the approach to junctions or roundabouts in other countries – e.g. in my (poor) photos below, on the A303 in Somerset, and clearly in this Google Maps image of Ottawa (via this thread).

I remember reading a story once in which someone cycling along an avenue with regularly spaced trees, late one afternoon, had an epileptic fit (I think) as a result of the frequency of the shadow flicker on the road (this is clearly something considered by wind turbine planners [PDF]). Have there been any cases of epilepsy triggered by stripes painted on the road?

Progressively closer rumble strips on the A303 in Somerset Progressively closer rumble strips on the A303 in Somerset
Progressively closer rumble strips on the A303 in Somerset.

Bollardian nightmare?

Rising bollards in Silver Street, Cambridge

Rising bollards near Darwin College, Cambridge. A man was killed here in May 2006 when his car hit the right-hand bollard; see third photo below.

Many thanks to Steve Portigal and Josh for suggesting this subject!

Bollards which automatically retract into the road surface to allow certain vehicles to pass, and then rise again, are becoming increasingly common on public roads in the UK; whereas previously, they might have been used at the entrance to a private car park as a more visually appealing alternative to an automatic barrier, many authorities are now using them to enforce traffic control in urban areas, with the category of permitted vehicles including buses, emergency services, postal vans, and so on. (I’m not sure about taxis; I think this varies with city*). The recent compilation of CCTV clips by the Manchester Evening News (link via BoingBoing) showing ‘non-permitted’ cars and vans hitting rising bollards in Manchester, as the drivers try to follow close behind permitted vehicles has got a lot of attention, with reactions ranging from “stupid drivers deserve what they get” to “how is causing thousands of pounds’ worth of damage to punish a minor crime ever justifiable?” (There’s also this video showing a higher-speed crash – not sure if this is in Manchester too).

Rising bollards in Manchester Rising bollards in Manchester
Stills from the CCTV compilation: the rear nearside wheel of the black 4×4 is off the ground. The van’s windscreen has been damaged by the driver’s head hitting it.

As an architecture of control, what can we say about the rising bollard? Is it merely a ‘restriction of access’ device, like a padlock? Or is it actually intended (to some extent at least) to damage the vehicles of non-permitted drivers, and injure them?

The official line would be the former, of course, but going by the dominance of the “stupid drivers deserve what they get” viewpoint in public comments on the Manchester video, I would suggest that a vindictive streak is pretty significant, and there’s no reason to think it might not also be among traffic planners.

The extensive (25 page) discussion at the road enthusiasts’ site SABRE also contains both points of view, and others. One logical argument which is well made, I think, is that if the drivers expected to damage their cars, they wouldn’t have tried to follow the buses. Therefore, the warning signs/road markings (knowledge in the world) or their prior experience of these systems (knowledge in the head) cannot have been sufficiently clear to discourage them from trying to sneak through. Yes, they knew that they “weren’t supposed to” drive through, and knew that buses were, so they tried to sneak in behind, but the drivers can’t have been fully aware of how quickly the bollards rose, or they wouldn’t have attempted it, would they? Most people don’t deliberately wreck their cars.

This is an important point. The system is not designed to be forgiving of mistakes. Now, we can say “well, why should it be? Those drivers shouldn’t try to break the law,” but in the real world, people do make mistakes. That’s why pencils have erasers. A car driver following a bus may have his or her vision of the warning signs obscured, and may be driving at a perfectly sensible speed, but still hit the risen bollards. A car driver following another car which externally looks like any other, but which is (for whatever reason) a permitted vehicle (with a transponder on board) may see the warning signs, and take them in, but, seeing the bollards lowered and the car in front driving at a constant speed, may assume that the bollards are disabled or permanently down, and so continue at exactly the same speed, and not be able to brake in time to avoid hitting the risen bollards. Bollards in some cities are only operational at certain times of day, and in certain directions; unless the warning signs themselves have a very clear variable display (Cambridge’s are fairly good), can we really expect drivers to read the times from the sign as they go past following another vehicle?

There are two much more sensible systems suggested in the SABRE discussion:

‘PeterA5145′

If this system is needed, then surely there should be conventional red lights with the bollards only rising a few seconds after the lights have changed to red (as with a level crossing). If you’ve never come across such a thing before it is not remotely obvious.

‘True Yorkie’

Perhaps a better idea would be to have a ‘vestibule’ system, with 2 bollards, spaced exactly a bus length apart. As the bus passed over the first lowered bollard, it’d stop immediately for the next bollard. As it waits for the second bollard to lower, the first would raise. Any car that tried to get in with the bus would have to shunt the bus to fit into the vestibule.

Either of these would be much better than the existing system. Both ‘design out’ the likelihood of mistakes – the vestibule system especially so.

Rising bollards in Silver Street, Cambridge: the driver of the Shogun was killed; image from Véro
The fatal accident at the Silver Street bollards in Cambridge (photo from Véro’s blog); this photo of the same bollards as my photo at the top of this post.

To a large extent this issue seems to come down to a debate on the old “if you’re not doing anything wrong, you’ve got nothing to fear” argument. Again, it’s a subject for a future post** but I find the repeated use of this, by politicians especially, to justify every erosion of established freedoms, both sly and egregious: there’s a reason why I can’t legally shoot you if you walk up my garden path, or electrify my car body shell (OK, it’s fibreglass, in fact, but the same principle applies). Perhaps the “if you’re not doing anything wrong, you’ve got nothing to fear” brigade would let happily let the authorities read all their personal correspondence, and indeed would be happy to have all private property covered in mantraps and landmines to enforce “trespass prevention”? After all, “if you’re not doing anything wrong, you’ve got nothing to fear.”

Jeremy Wagstaff applies this kind of thinking to the bollard issue to demonstrate its absurdity, and its distasteful corollary:

My tuppenny’s worth: I think traffic maiming (as opposed to traffic calming) is a great idea but doesn’t go far enough. We need similar measures to punish, sorry deter, drivers who routinely flout the law and common decency. Why not, for example, deploy the retractable bollards elsewhere, like

* the centre of a restricted parking space, so it would rise at the end of the designated period, impaling the vehicle if the driver had overstayed his alloted time;
* at random points on the hard shoulder on toll roads/motorways so that cars illegally using it as a fast lane would be impaled, or flipped over into an adjacent field

Where necessary, bollards could be replaced by other features such as

* a mechanical arm, installed on the roadside and connected to a speed sensor, which would crush cars passing by too fast or too slow, depending on what irritated other drivers the most.
* or cars driving through built-up areas too fast would be taken out by snipers deployed in trees/tall buildings. If necessary the snipers could be automated.
* cars straddling two lanes or changing lanes without indicating first would be sliced in half by retractable blades intermittently rising out of the demarcating lines
* motorbikes using the sidewalk (a particular bane in my neck of the woods) would risk having their tyres slashed by strips of spikes activated by the annoying sound of approaching underpowered Chinese-made engines.

Norms, restriction and punishment

It’s worth thinking about the norms of restrictions and warnings we encounter in everyday life.

Many – in fact most – signs indicating the prohibition of particular behaviour do not suggest immediate punishment to us. The sign may say “Do not drop litter” but most people who do drop litter know that unless someone is watching, and decides to do something about it, they will get away with it. A road sign may say ‘No Parking”, but if no-one’s around, is it wrong to stop? Is it a crime if no-one finds out, and it doesn’t affect anyone?

We can laugh about falling trees in the forest and Schrödinger’s cat, but I think that for most of us, there is a very clear mental distinction between “incorrect” behaviour which we know will be “punished” immediately by the realities of the system (e.g. pointing a gun at our face and pulling the trigger, or driving through a level crossing barrier when a train is coming), and “incorrect” behaviour which we know is technically wrong, but which only the fear of being caught (and punished) stops us doing. Most drivers speed, but they wouldn’t speed if they knew there was a police car behind them.

So, our mental model of a ‘No Entry’ sign is that it signifies an arbitrary restriction, but one which carries no immediate punishment, unless, say, it’s a single-track road and something’s coming towards us at speed. If we ignore the sign, we might find we’re going the wrong way down a one-way street, or we might get caught (by camera or by police on the ground), but that’s a risk that people may take if they perceive it to be very low. If we can see that it’s not a one-way street, and we can see other vehicles passing that way, then there is apparently a fairly low risk to ignoring it. Our expectation is that we will get away with it.

When bollards then rise out of the road immediately in front of us, our mental model is proved wrong. The norm is shattered. This is a system that immediately punishes those who infringe the No Entry sign. This is a familiar, apparently docile ‘Keep off the grass’ sign accompanied by snipers watching very carefully.

Now, the UK’s Department for Transport clearly recognises something along these lines: the norms of the bollard systems are different to those drivers may have come to expect, and an additional aspect is also identified: that of the “control of all vehicles/control of individual vehicles” distinction:

Established practices of traffic control using traffic signals do not control separate vehicles; streams of vehicles are controlled with drivers able to see the signals from a significant distance. Rising bollards are normally used to control individual vehicles in that they are raised each time a vehicle has passed over them. The requirement, therefore, is for short range signalling… Unless drivers have a clear view of the bollards, an indication should be given to drivers that the bollards have fully retracted.

In conjunction with the vestibule system suggested in the SABRE quote above, that seems the most sensible approach to take. The DfT also has some other sensible guidelines:

Three wheeled vehicles, motorcycles and vehicles with trailers, for example, may not be sensed by the vehicle detectors used with automatic bollard systems. It will almost certainly be necessary to provide alternative means of access for some classes of road users or vehicles. The possibility of a device rising under a wheelchair or pushchair should be taken into account. The risks could be mitigated to some extent by providing suitable alternative access adjacent to the bollards, and by using a coarse road surface to divert pedestrians away from the bollard installation [interesting! - see also the pebble paving to make barefoot walking uncomfortable, mentioned here]. Whilst most applications will be to enable the passage of one vehicle at a time, there will be instances where two or more vehicles attempt to pass through in close succession. The system should ensure that bollards cannot rise beneath a vehicle because of the danger this would create. It is better to risk a certain amount of violation by “tailgating” vehicles, rather that put road users at risk. Any system, however well designed, will fail to operate correctly on occasions. The system should fail to a safe state, ideally with the bollards retracted. In the event if an accident the emergency services may need to override the control system and retract the bollards.

Still, for all the effort (and costs) involved in installing and running the bollard systems, would it really not be better to look at the subject from a greater distance? The roads drivers want to use are in many cases roads which used to be open to all traffic – indeed, in Cambridge, Silver Street used to be one of the main routes into the city centre, part of the old A603 from Bedford. The current alternative route from west to east is significantly longer and almost always very congested. It passed close outside my window when I was a student; I know. It’s understandable in many cases why drivers want to use the old route.

The real issue that needs to be addressed is why people want to drive into these areas. There is always a reason; people are rarely “stupid” with no explanation.

*I’d like to get into the “are taxis public transport?” debate another time: not now though.
**There was a quite astonishing article I read about a year ago where a police chief in a small US town had (seriously?) suggested putting CCTV inside every home in the community, for constant monitoring, and used the same “if you’re not doing anything wrong, you’ve got nothing to fear” argument; if I can find this again, I’ll post the link.