predicting behaviour in traffic assume we are rational. But it appears
we are not. Guy Walker discusses how our attachment to queuing etiquette
causes huge delays in traffic at road works.
I don’t think I’ve ever been involved in such a controversial ergonomics
topic. What could it possibly be? Something hard-hitting like nuclear
power? Healthcare? No. Roadworks. A little while ago some research of
mine appeared in the press under the title ‘Polite drivers cause
queues’, and within nanoseconds I was receiving emails and phone calls
from motorists. Some were reacting positively to the story, some rather
less so. I think the phrase “waste of tax-payers’ money” might even have
been used. So let me explain, because behind the headlines and sound
bites is a genuinely fascinating ergonomics story.
driving along the motorway, with three lanes of emptiness ahead of you.
Then you see signs warning of roadworks and lane closures. As the
traffic thickens and the point arrives where the closed lanes have to
merge, what happens? Does everyone make maximum use of the available
road space and allow others to merge at the head of the line with a
friendly wave and a spirit of mutual cooperation? That’s what traffic
theory would predict. It would say the lanes would become fully loaded,
even at the point of merging, because each motorist is looking to
maximise their utility in terms of time savings and other cost
variables. Does this sound like real life though? I don’t know about
you, but drivers in my locale head for the nearside and open lane as
soon as humanly possible, and there seems to be a critical distance from
the merge where people decide not to let other motorists in. They’re
pushing in aren’t they? The British love a good queue and there are
strict rules to follow.
What we have here is a classic case of
emergence, where small, low-level behaviours magnify to truly daunting
levels, giving rise to counter-intuitive large-scale effects out of all
proportion to what’s causing them. Closing the gap and preventing
someone from merging, or heading for the nearside lane as soon as
possible might seem pretty inconsequential but we can lose up to a lane
of capacity – that’s in addition to the lanes we’ve already closed –
purely due to queueing behaviours like these. On the subject of wasting
tax-payers’ money, bear in mind that it costs something like £3 million
per mile to put those lanes there, and a further £43 thousand per year
on average to maintain them. Combine these figures with the fact that we
already have an acute capacity challenge, and you can see why we can no
longer ignore the problem. And it is an ergonomics problem. Here we
have an engineered environment where we expect people to behave in
certain ways, except they don’t. Sound familiar?
amount of effort goes into planning roadworks. A cutting-edge approach
employed up here in Scotland, and elsewhere, is something called traffic
microsimulation, where a faithful virtual model of the road network is
populated with lots of little virtual cars, which in turn follow some
fairly simple rules. When the simulation is set running, what emerges is
an incredibly lifelike representation of how real traffic will behave.
This simulation is quite a break from how transportation engineering
used to do things. In the past you would use ‘macroscopic’ models built
on origin/destination matrices and some fairly formidable algorithms to
calculate coarse-grain flow rates and speeds and so forth. The benefit
of using traffic microsimulation is that you can look at individual
vehicles, in very small time frames, giving you very fine-grained
analyses of traffic conditions.
Because microsimulations can
deliver such a detailed analysis, Transport Scotland commissioned the
Clyde Strategic Microsimulation Model, a model of the entire
metropolitan Glasgow area: 250km of road, 50 grade-separated junctions
and no fewer than 1.5 million simulated vehicles. The purpose of the
model, in use since 2010, is to help design and test roadworks for the
trunk and motorway maintenance programme in central Scotland. Obviously,
the model has to be accurate as it will help determine when, where and
how many roadworks will be performed as well as informing the travel
advice given to the millions of motorists who use the network. As luck
would have it, the opportunity arose to test the model with some
real-life roadworks on the M8 motorway to the west of Glasgow, to see
how real and virtual drivers compared.
Working with SIAS Ltd,
an Edinburgh-based company who developed the Clyde Strategic
Microsimulation Model, we were able to make use of automatic traffic
count data from a range of recording stations around the roadworks site
over a three-year period. With several million data points we can be
fairly confident in what we were observing, which was a marked
discrepancy in what the microsimulation model was predicting in terms of
traffic flow and what was actually happening. For some of the most
critical rush hour flows we were losing a substantial amount of capacity
because of the way people queued and, moreover, the microsimulation
model wasn’t capturing it.
The capacity reductions due to driver
queuing behaviours were substantial. The average capacity reduction was
22% or something in the order of 1500 vehicles per hour per lane, which
is not too dissimilar to the amount of traffic a normal motorway lane
would carry. Remember, this is a loss of a lane of capacity in addition
to the ones that have been closed.
It got worse, because the
capacity reductions were hitting peaks as high as 55%, and it had
nothing to do with hard engineering as such, it was due entirely to
seemingly irrational driver behaviour. In some follow-up work in other
locations it was clear some very powerful social norms were in play,
which affected how people wanted to queue. Norms that were far more
powerful than all the logical, rational transport engineering advice
provided to motorists in the form of signs saying “merge in turn”, “stay
in lane” and “maintain a safe distance”.
To some extent we have
been here before. We’ve known for a while that we can get bigger than
expected reductions in capacity when you reduce lanes, and of course,
there are the famous Minnesota studies all
about late and early merging. The key thing, though, is that while we
know something about the ‘what’ we really don’t know much about the
‘why’. With traffic microsimulation, which focuses on individual
vehicles and so-called ‘agent behaviour’, we need to find out what,
exactly, is happening.
Driving is clearly a social activity
performed in close proximity to others. In motorway driving, our other
research keeps showing how important the presence of other drivers is to
your own behaviour. We call this phenomenon conformity and there is a
lot of it about. Research shows that drivers approach junctions faster
and brake later when being followed compared to when they are on their
own. Other research describes the pressure we all feel to keep up with
others, sometimes even when it is not safe to do so. People the driver
knows, such as passengers, tend to inhibit speed. In other situations,
with anonymous other drivers, it has the reverse effect, as we can see
in the early merging in response to upcoming roadworks. None of us wants
to experience the aversive stimuli of being hooted at or blocked from
merging, nor being regarded as a ‘typical white van/BMW/Audi/Volvo
driver’. These factors all sound rather trivial, but they are clearly a
more powerful determinant of behaviour than the rational optimisation
we, and engineers, would like to assume. And it gets worse. Through
social learning these behaviours feed back into the wider driving
culture to themselves become local and national norms of behaviour,
continually reinforcing what people will keep conforming to.
good news is that we were able to help our civil engineering colleagues
get a handle on all this complexity and make the world a better place.
In the short term, we are able to take these insights and provide
explicit guidance on how we could use the existing features of traffic
microsimulation to make the virtual vehicles behave in more lifelike
ways. In the longer term we’ve tapped into a rich seam of research that
is providing us with small, clever, user-centred ways of increasing
capacity that are orders of magnitude cheaper and more effective than
stronger exortations to ‘merge in turn’ or building more lanes. Watch
out for future articles.
In the meantime, let’s go back to the
beginning and ask the same question: do polite drivers cause queues?
Sort of. We are certainly ready to conform to socially accepted norms
around queuing, and actively try to avoid trouble and offence by what we
see as ‘pushing in’. Then again, I’m certainly not advocating that
people become rude drivers.
What about this instead? What about
using ergonomic insights to help reduce feelings of anxiety in queues?
What about making the cognitive and perceptual environment one in which
the behaviours we want to elicit are the least effortful and traumatic
to perform? Instead of ‘merge in turn’ what about ‘drivers ahead don’t
mind you pushing in’? v
By Guy Walker, Associate Professor in Human Factors at Heriot-Watt University in Edinburgh
This article first appeared in issue 532 of The Ergonomist, October 2014.