Psychology and Road Design - Breaking The Law - Why Motorists Speed

The previous sections focused on unintentional mistakes

This section focuses on why we don't obey the rules, and the part road design plays in this - in particular why motorists speed



Parts of society still associate speed with success - for example linking the fast transport of goods and services with increased business efficiency and economic growth. Even the UK government consulted on raising motorway speed limits to 80 MPH in 2011, citing significant improvements in vehicle safety since the 70 MPH motorway speed limit was introduced in 1965

Other groups suggest that travel speed should be at the driver's discretion, because speed limits don't reflect real-time factors like the weather, congestion, or the presence or absence of vulnerable road users

However, this is unadvisable because most motorists cannot be objective when selecting their travel speed

For example, they consider their own safety above the safety of other road users (particularly pedestrians and cyclists), they underestimate the increased risk of accidents from increased speed (it's not a linear relationship), they ignore environmental factors like rain, or night driving, and they over-focus on time gains from small increases in speed - gains not usually worth the increased risk

Instead speed limits in the UK have come down in recent years

But reducing the speed limit alone is not always an effective way to reduce overall traffic speeds and improve safety, and psychology can help us to understand why

Recent Years


There are a number of reasons drivers speed. One is that they can get away with it without crashing

Most drivers, even those with poor driving ability, will never experience a significant road crash, which fortunately remains a rare occurrence considering the billions of miles travelled each year and millions of motorists

Therefore many motorists remain ignorant of the significant and often life-changing results of a crash. So they underestimate their vulnerability, drive too fast and take too many risks

When near misses do occur, motorists are prone to 'hindsight bias' where they remember the good things (for example not having a crash) but forget the bad things (for example their poor driving behaviour that led up to the near miss event)

Human beings also rely on 'optimist bias' where they over-estimate their ability and expect things to turn out OK. Optimist bias is a useful tool in everyday life - without it we'd never attempt anything new - but it can have undesirable side effects when applied to driving

Another reason drivers speed is the low odds of being caught - estimated to be as low as once every six years - not much of a deterrent

Indeed, with the spread of lower speed limits this likelihood will decrease as a limited number of police officers are available to enforce lower limits on ever more roads

'Deterrence theory' states that individuals will only stop doing something if they're likely to be caught. Safety cameras are an effective deterrent because they catch every speeding motorist that passes by

The immediacy of punishment can also deter undesirable behaviour. Speed cushions are a prime example of immediate punishment for speeding as the negative impacts of speeding are felt straight away - although they have less or no impact on motorbikes or wider vehicles like vans, SUVs and MPVs, which ironically are statistically the vehicle types most likely to speed

Unlikely To Be Caught


A third reason drivers speed is that in some situations speeding can even seem rewarding (for example getting somewhere earlier) which reinforces poor driver behaviour - this is known as 'learned riskiness'

Conversely slowing down can seem like punishment and deter repeating that behaviour - for example a driver for whom time is valuable slows down in a speed restricted area, but experiences no reward or safety advantage and therefore feels punished

A further reason motorists feel that they can speed is the anonymity provided by their vehicles. It enables them to behave in ways they never would among people they know

For example motorists drive more slowly in their own neighbourhood because it's more likely their vehicle will be recognised by neighbours and friends. But once beyond their neighbourhood, the same motorists will often increase their speed - even above the speed limit


Increasing Speeds

In some situations reducing speed limits can have undesirable consequences or side-effects

For example delays caused by slower moving traffic can encourage motorists to drive faster elsewhere to make up for lost journey time. This is particularly relevant on surrounding roads not subject to a speed limit reduction - affecting roads both before and after the speed restricted area

This effect can also be observed on roads with speed cushions where vehicle speeds between each cushion are sometimes higher than average speeds before the speed cushions were installed

Another undesirable consequence is increased frustration and stress for drivers, caused by longer journey times and increased congestion

Stressed and frustrated drivers are more likely to behave aggressively towards others, more prone to road rage, make more incorrect decisions, take more risks, and adopt worse driving behaviour

This can cause a self-propagating 'non-virtuous loop' where poor driving behaviour leads to decreased speed limits, increased journey times, more driver stress, poorer driving behaviour, and so on

Non-Virtuous Loop

Reduced Impact

A longer term undesirable side-effect is that spreading lower speed limits too widely means they lose their impact to slow traffic down in situations where lower speeds are vital

For example, applying 30 MPH speed limits to roads where there are no pedestrians, for example through sparsely populated rural villages, will make motorists more accustomed to dismissing 30 MPH limits as irrelevant. So this increases the risk that they will then speed through 30 MPH urban areas where there are pedestrians

This is similar to the problem of maintaining compliance with 20 MPH speed limits past schools when for much of the time there are no school children present

Reducing speed limits on existing roads can also generate a long-term undesirable consequence

It can be difficult to enforce a new reduced speed limit on a road where, for many years, regular drivers have experienced travelling safely at the old higher speed limit. As already explained, they won't see the benefits to others of reducing their speed, and they will only experience disbenefits themselves

So they're likely to continue to travel at, or near, the old speed limit. This creates an added problem because human beings tend to copy what others around them are doing. So even new motorists, like learner drivers, will become accustomed to travelling above the speed limit on this road and other similar roads

Therefore, if nothing further is done to enforce lower speed limits, a self-perpetuating circle of bad habits can occur

Breeds Bad Habits

Counter Measures

So what counter measures can we use to slow drivers down more effectively?

I've already mentioned physical measures such as changing the road appearance, reducing lane widths, or putting in pedestrian crossings, and punitive measures like safety cameras and speed cushions. But there are other measures we could try

Full-sized cut-outs of police vehicles located on bridges across motorways have proven successful at controlling motorway speeds. Maybe this success could be replicated by locating full-size cut-outs of pedestrians, or street art representing pedestrians, in 30 MPH areas where there are few actual pedestrians

If traffic slows sufficiently that local residents feel safer walking along their roads, then the fake pedestrians may no longer be required

As already mentioned, vehicle activated road side signs are a good way to reinforce good driver behaviour

So instead of full-sized cut-outs of pedestrians, we could use vehicle activated flashing signs showing images of pedestrians or school children to remind speeding motorists of why the speed limit exists

We can also target drivers' anonymity to encourage them to drive with more care. Vehicle activated road signs that flash a vehicle's registration if that vehicle exceeds the limit have been effective at reducing traffic speeds

This could be combined with a simple onboard computer that remembers registrations and targets repeat offenders by showing how often they speed at a specific location

When no vehicles are approaching, the sign could cycle through a list of the worst offenders so that local residents can learn to identify and shame these particular motorists into driving with more care

Counter Measures


So I hope that this shows why reducing the speed limit alone is not always an effective way to reduce overall traffic speeds and improve road user safety

I complete the presentation by looking at a case study where the speed limit was reduced from 60 to 40 MPH with no additional measures except for repeater speed limit signs. How effective has this been?

The road in question is the B2028 Selsfield Road north of the village of Ardingly in West Sussex

This road is a busy commuter route linking Haywards Heath to the south with East Grinstead and Crawley to the north. In addition, many vehicles use this road to get to Ardingly College (an independent school a mile to the south) and Wakehurst Place (a National Trust house and gardens) just to the north of the study area. An aggregates processing plant south of Ardingly also means a high proportion of HGV traffic uses this road

The road used to have a 60 MPH speed limit, reducing to 30 MPH at the edge of Ardingly immediately to the south of the study area

Selsfield Road Case Study

Selsfield Road Case Study

This road, just over 1km long, was widened and straightened in the 1960s at the same time as the South of England Centre showground was constructed along the western side of the road. The showground can also generate traffic on this road - particularly on event days

The road is 7.3m wide along its length until the northern end where it narrows to tie-in with the old 6.5m wide road

A section of the old road remains as a lay-by (just out of this picture) and there are three large flared entrances to the showground

Along much of its length the road is bordered by verges up to 8m wide separating the footway from the carriageway. The footway runs along the western side of the road for the full length of the study area

Selsfield Road Case Study

Selsfield Road Case Study

The verge widths reduce in the middle section and a footway starts here on the eastern side of the carriageway

In addition to the showground, the road is bordered by a mix of residential properties and local businesses (including the Gardeners Arms bar and restaurant) - mostly in the northern half of the study area

A few of these properties have private parking, the rest park their vehicles either on the pavement or on the verge on the opposite side of the road. The pub also has a large car park

Selsfield Road Case Study

Selsfield Road Case Study

In May 2011 West Sussex County Council (WSCC), as the highway authority for this road, held a public meeting to debate reducing the speed limit here from 60 to 40 MPH

There had been a fatal crash on this road in January 2006, though not attributed to speeding, and between 2007 and 2011 there had been three slight injury collisions

I attended this meeting as one of three formal objectors to the proposals (a fourth objector spoke at the meeting but left early due to losing his temper). My objections centred on how the proposals could reduce overall road user safety (such as making drivers in queues overtake north of the study area where the road is narrower, instead of on this section of wide road where visibility is excellent)

Sussex Police had also expressed concern, in writing, that road signs alone would be insufficient to reduce traffic speed

However, local residents and parish council representatives at the meeting supported the speed limit reduction, and it was subsequently passed

Selsfield Road Case Study

Selsfield Road Case Study

In May 2012 the speed limit was reduced to 40 MPH with the erection of speed limit reminder signs at regular intervals. No other speed reduction measures were made

A speed survey, commissioned by the council prior to the changes, measured average speeds of between 43 and 45 MPH - this was when the speed limit was still 60 MPH

At the start of May 2013 I carried out my own survey, measuring the speed of 517 free-flowing vehicles at various times of the day over a three day period. Here is a graph of the results

The average speed I measured (both mean and median) was 43 MPH. So, as Sussex Police had originally feared, average traffic speeds have not reduced

In addition this graph shows that over half of the vehicles I measured did not comply with the new speed limit (the black vertical dashed line in the middle of the graph)

Click here for a CSV text file of the speed survey results

Selsfield Road Case Study

Selsfield Road Case Study

In April 2013, I also hand-delivered questionnaires to the 27 local properties and businesses, asking for their opinions about the effects of the speed limit reduction

Just over half of the questionnaires were completed and, during a subsequent site visit a further 22% of residents declined to comment. The remaining 26% were either out or didn't answer their front door

Click here for a PDF of the residents' survey results

The questionnaire started by asking about conditions before the speed limit was reduced. Almost all respondents agreed or agreed strongly that traffic used to travel too quickly, that it felt unsafe parking, unsafe walking or cycling along the road, and most unsafe crossing the road

Selsfield Road Case Study

Selsfield Road Case Study

Then the questionnaire asked for their opinions of the road after the speed limit was reduced

The red figures here are the majority responses, and the green ticks or arrows are the preferred options

A majority thought traffic speeds had gone down and felt that walking or cycling along the road felt safer. But few people thought traffic levels had decreased, or felt safer when parking. As for feeling safer crossing the road, the results are evenly split

Respondents were also invited to submit their own comments. Almost all the comments were negative, or wanting further action either to make the changes more effective or for someone to enforce the reduced speed limit

So, I suggest that these results confirm that reducing the speed limit alone is not an effective way to reduce overall traffic speeds and improve road user safety

So what could have been done, assuming available funding?

I would suggest three simple changes:

(i) First, put in a controlled pedestrian crossing at the mid-section where there are footways to both sides of the road - a puffin type crossing rather than a zebra crossing because of the higher vehicle speeds

(ii) Second, use gateway features, different surface colour, and remove the central line marking to emphasise the change in road environment where most of the dwellings are located in the northern half of the study area

(iii) Third, mark out on-road parking bays adjacent to properties so that residents feel safer parking on the road, and they no longer need to cross the road to reach their cars parked on the opposite verge. Build-outs at one end of each parking bay could further improve protection to parked cars. In addition, the reduction of the overall available road width from 7.3m to 5.5 or 5.3m at these locations could significantly reduce passing vehicle speeds

The speed limit on the southern part of the study area (between the edge of Ardingly and the Prince's Gate entrance) could also be increased to 50 MPH to enable cars to overtake here in safety - for example when stuck behind HGVs from the nearby aggregates plant - instead of overtaking on the road north of the study area where the carriageway is narrower, more winding, and overtaking is more dangerous. The 85%ile speed from my vehicle speed survey is 48 MPH (although the 85%ile is no longer the measure that WSCC use to calculate road speeds). So traffic almost travels at this speed here already

But this southern length of road is quite short (only about 600m) and there are a few isolated properties on this section. Also, this suggestion would most likely generate vehement, vocal opposition from Selsfield Road residents and the local parish council, and therefore not garner WSCC support, as WSCC councillors are of course elected by the local residents and must therefore listen to their concerns

Selsfield Road Case Study

The next section is the conclusion and includes suggested further reading if you want to find out more about this topic

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