Common sense and experience would tell us that “traffic calming” measures, such as installing speed bumps, raised crossings, in-lane bus stops, or partial street closures, would have some negative impacts.
Rational people will also recognise that most engineering changes in a city are going to have a range of associated negative impacts, trade-offs, disadvantages, and costs. Politicians, consultants, and bureaucrats can argue that the benefits will outweigh the costs, or that the positive impacts are far greater than any negative impacts. However, it is a very different approach to pretend that disadvantages do not exist, or to hide and downplay the negative impacts. That second approach undermines trust in institutions, operates in bad faith, is regarded as deceitful, and is disrespectful to the general public.
In their public documents promoting “traffic calming”, city councils, New Zealand Transport Agency (Waka Kotahi), consultancy firms, and international organisations of influence often down-play, hide, or ignore negative impacts. When negative impacts are officially acknowledged, they are often buried deep in reports, under-researched, and rarely put front-and-centre by the bureaucrats as important issues for genuine consultation with the public.
This article will cover eight negative impacts and disadvantages associated with “traffic calming” measures (or TCM). This article will provide information on where to find documents and sources where these impacts have been acknowledged.
These eight major negative impacts and disadvantages are:
- Slowing or damaging emergency vehicles.
- Causing damage to buses, plus discomfort and delays to bus passengers.
- Decreasing traffic volumes.
- Increasing stress, cognitive load, driver errors, and unsafe decision-making.
- Increasing noise and air pollution.
- Being part of “mode shift” social engineering.
- Expensive to install.
- Being controversial and unpopular, especially on major roads.
Details about each of these negative impacts and disadvantages are included below:
1. Slowing or damaging emergency vehicles.
Page 34 and 35 of this University of Utah report explains that fire engines are most impacted, followed by ambulances with patients.
“TCM [Traffic calming measures] could have negative effects on emergency response, slowing down the emergency vehicles. Some of the measures, especially vertical obstacles and closures, can have significant impacts on emergency response vehicles. Surveys found that fire truck engines are the most prone to be impacted by TCM measures. They are followed by ambulances carrying patients, ladder trucks, and ambulances without patients. The 12-foot hump has the most significant impact on those vehicles. Different measures have been taken to overcome these problems. TCM measures should not be applied on streets in the vicinity of fire stations, since those are the routes fire trucks use the most. Some design changes, such as speed cushions, split humps, or sealed down deflector islands are implemented to reduce the impact on emergency vehicles.”
Page 33 of this World Resource Institute report explains the difference between speed cushions and speed humps then mentions that speed cushions are specifically designed to cause less damage, disruption, or discomfort to emergency vehicles (and occupants) than some other “traffic calming” options.
“The basic design of speed cushions is very much like the speed hump, except the additional modifications to accommodate for wider vehicle width of cars. The width of each cushion is designed intentionally so that the wider axle of emergency vehicles or buses can pass but that the smaller passenger vehicles must ride over the raised area…
… Avoids excessive discomfort or damage to emergency vehicles and buses by making separations in the hump.”
2. Causing damage to buses, plus discomfort and delays to bus passengers.
Large vehicles such as buses and fire engines are regarded as being more susceptible to damage and occupant discomfort when going over “traffic calming’ obstacles such as speed humps and raised platforms. As page 34 of the University of Utah report explains…
“Buses have firmer suspension systems, similar to most other large vehicles carrying heavy loads. They are less maneuverable than cars. TCM can lead to increased wear and tear to buses. If buses are driven along a traffic calmed road many times a day, they can be damaged and maintenance costs can increase.
Bus operators have a duty of care to their passengers, particularly senior citizens and disabled, who may be standing or moving around the bus. In some situations, traffic calming can cause great discomfort, especially if the bus service has numerous vertical deflections.
Bus services operate by a timetable. Reliability is important if customer confidence is to be maintained. It is important that TCM do not cause excessively increased travel times to buses by requiring diversions or slowing down significantly more than other vehicles.
Speed cushions are the preferred vertical deflection measure for bus routes, as they have less impact on buses than speed tables, but slow vehicles to a desirable speed…
… Speed tables should only be used on bus routes at key locations, such as schools or shopping centers. They should not be closely spaced. The bus operators would prefer no more than five speed tables on any bus route.
Round-top speed humps are not acceptable on bus routes in London as passengers experience a double discomfort when a bus is traversing the hump, one for each set of wheels.”
Page 31 of the World Resource Institute report also recommends that…
“Bus passenger comfort should be accounted for where speed humps are applied on certain bus routes. Speed cushions may alternatively allow buses to pass with limited disturbance to passengers.”
In both of these reports, the damage to buses by certain ‘traffic calming” measures is acknowledged. That these ‘traffic calming” measures cause delays and/or discomfort to bus passengers is also acknowledged as a negative impact. The authors of these reports even express the view that those negative impacts should be minimised. However, these authors appear unconcerned that these same “traffic calming” measures might also cause delays, discomfort, or vehicle damage for other road users. It is reasonable to assume that many trucks and other larger vehicles would face similar issues as known for buses or fire engines. Private car owners and users are likely also impacted with delays, discomfort, and possible vehicle damage as a result of “traffic calming” measures. On average, small cars might face less severe impacts than large vehicles. Because smaller vehicles are more numerous and transport more people in many cities, the negative impacts of “traffic calming” measures on lighter automobiles should not be so readily dismissed. However, these potential impacts appear to be largely ignored (or hidden) by officials and academics.
3. Decreasing traffic volumes.
Documents which promote “traffic calming” measures usually focus on safety improvements (fewer crashes and/or fewer serious crashes causing death and injuries). That “traffic calming” measures reduce average speeds is also emphasised, using the reasoning that lower average speeds lead to less death and injury on average from crashes. Many “traffic calming” measures also intentionally or unintentionally reduce traffic volume, as page 57 of the University of Utah report explains…
“The literature provides several results on traffic calming implementations. The most significant impacts of traffic calming are observed on traffic volumes, vehicle speeds and traffic safety. Although traffic calming measures are divided into volume and speed control, both categories have higher or lower impacts on both traffic parameters. Some studies also explored negative impacts of traffic calming, but in general they are outweighed by positive implications.”
There are going to be different perspectives on whether reducing traffic volume is a positive or negative impact of “traffic calming”. Reduced traffic volumes means few cars are moving through a given roadway over a given time. Automobile traffic might avoid the roads with “traffic calming” leading to more traffic on side streets and increased congestion elsewhere in the city. A decrease in traffic volume might represent automobile travellers getting too frustrated with delays and discomfort cause by “traffic calming”, with those issues causing some people to not travel by car or avoid travelling all-together. This behaviour change likely has a variety of social, economic, and environmental impacts. The bureaucrats may welcome those impacts; your average automobile traveller may not.
Page 32 of the University of Utah report explains that…
“Volume impacts depend on the entire network, not just the characteristics of the street itself. The availability of alternate routes and the application of other measures in area-wide treatments may have large impact on traffic volumes.
In particular, volume impacts depend fundamentally on the split between local and through traffic. TCM will not affect the amount of locally bound traffic unless they are so severe or restrictive as to “degenerate” motor vehicle trips. The concept of suppressing motor vehicle travel with increased costs is still new and it is unlikely to succeed in the United States. TCM may reroute non-local traffic instead of dealing with local.”
Many would view a road with artificially-reduced traffic volumes as a more dysfunctional road. That is if you view a road’s primary function as facilitating the efficient movement of vehicles which carry people and goods.
In the New Zealand context are we looking at situations where the installed “traffic calming” is now “… so severe or restrictive as to “degenerate” motor vehicle trips”?
Is this part of an agenda of “suppressing motor vehicle travel with increased costs” in New Zealand’s cities?
An approach that those University of Utah academics cautioned was “…unlikely to succeed in the United States”.
4. Increasing stress, cognitive load, driver errors, and unsafe decision-making.
Some “traffic calming” measures work psychologically as well as physically. Road layouts in some areas with “traffic calming” are not clear and intuitive. Confusing signage, visual obstructions, and usual road markings, as well as additional obstacles, can create uncertainty, put extra stress on drivers, and put further demands on their attention (increase cognitive load and/or perceptual load). Often this will make people drive more slowly, when they perceive the roading to be poorly-designed, confusing, or hazardous. However, the predecessor of New Zealand Transport Agency ( Waka Kotahi) was cautioned back in 2006 that such psychological manipulation of drivers can have negative impacts.
Pages 81 and 82 of Land Transport New Zealand Research Report 300, “Speed change management for New Zealand roads” state…
“Reducing speeds by increasing cognitive load or perceived risk can have negative consequences. Increased cognitive load may also increase accident risk. Frequent junctions, parked cars and interventions that encourage increased levels of pedestrian and cyclist activity might reduce vehicle speeds, but this type of intervention should not be used purely as a traffic-calming measure. For example, if driving speeds are not sufficiently reduced, these interventions might actually decrease safety because of the additional hazards that accompany them and because of the extra demands they place on the driver…
…Increasing driver stress may lead to reduced speeds, but as stress is complex and multidimensional, it may only be effective for certain types of people and have negative consequences such as reduced ability to make ‘safe’ driving decisions and an increase in the frequency of driving errors.”
High cognitive load (and overload) is a major cause of the crashes attributed to human error as described in this study…
“One of the main sources of human error while driving is cognitive overload (e.g., Engström et al., 2017), next to distraction (e.g., Hancock et al., 2003) and fatigue (e.g., Gunzelmann et al., 2011). However, the effect of cognitive workload on driving is not straightforward and depends on many factors such as the specific kind and amount of workload as well as driving difficulty.”
A 2017 review paper titled “Effects of cognitive load on driving performance: The cognitive control hypothesis.” states…
“Another subtle type of error potentially induced by cognitive load relates to the inability of cognitively loaded drivers to flexibly adapt to novel or unusual driving situations.”
Confusing road layouts and the installation of new “traffic calming” could contribute to both increased cognitive load generally and present drivers with novel situations which take them away from their routine behaviours. Evidence from behavioural studies in that review indicated that driver performance is badly affected by increased cognitive load when the driver is performing non-routine tasks. This is called the Cognitive Control Hypothesis and is described as “Cognitive load selectively impairs driving subtasks that rely on cognitive control but leaves automatic performance unaffected.” Cognitive control is being used for the non-automatic, non-routine, and non-monotonous driving subtasks.
Another study on people in driving simulators reported that increased perceptual load causes drivers to miss important details about hazards and have slower reactions to those hazards…
“High perceptual load significantly increased inattentional blindness and deafness, for stimuli that were both relevant and irrelevant to driving. High perceptual load also increased RTs [response times] to hazards.”
2016-2020 data reported by Hamilton City Council demonstrates that the top two “contributing factors to fatal and serious injury crashes” were “Poor observation” (at 36%) and “Failed to giveway/stop” (at 25%).
Will additional “traffic calming” measures lead to drivers who are more stressed, frustrated, distracted, and confused?
5.Increasing noise and air pollution.
Some documents acknowledge that some “traffic calming” measures” can increase noise and air pollution.
Pages 89 and 90 of Land Transport New Zealand Research Report 300, “Speed change management for New Zealand roads” features one study which mentions community feedback…
“A study in Connecticut, US, assessed the general impact of traffic-calming devices. The study investigated the acceptance, effect, and impact on route choice behaviour of traffic-calming devices. 183 people were interviewed in two towns in Connecticut. Overall, traffic calming was seen as an effective way to slow down traffic and make a friendlier community. Speed humps were the most commonly used traffic-calming device, but they were also the most controversial, with many residents even more unsatisfied with humps than traffic passing through. They may object to them for reasons such as noise, aesthetics, or pollution caused by extra acceleration and deceleration.”
The New Zealand Transport Agency website does acknowledge the potential of raised crossing to increase noise with the following statement…
“May increase noise as vehicles brake, slow, pass over them and then accelerate (particularly heavy vehicles).”
Page 58 of the University of Utah report featured this table which summarises some of the disadvantages associated with various “traffic calming” measures”, including increased noise pollution and air pollution…
Page 44 of the University of Utah report, in the context of Transit Oriented Development, argues that once people start using their automobiles less that this will reduce air and noise pollution…
“Reductions in automotive trips and VMT come primarily from either mode shifts or reductions in trip length. These reductions lead to further energy savings, air and noise pollution reductions, and an overall improvement in the quality of life”
[Note: VMT is “Vehicle Miles Travelled”, in New Zealand we are seeing recent use of the phrase “Vehicle Kilometres Travelled” or VKT in official documents]
In early 2024, a “raised safety platform” was removed in Auckland City due to it causing “excessive noise” issues for local residents. The NZ Herald reported that Auckland Transport claimed that removing the raised crossing would cost $133,000, though full costs will be known once the work is complete. The NZ Herald also reported a local resident’s claims that vibrations from trucks going over the raised crossing were causing property damage and the platform was contributing to flooding.
6. Being part of “mode shift” social engineering.
The phrase “mode shift” is often used to describe schemes to reduce private automobile travel and ‘encourage’ people to walk, cycle, scooter, or take public transport as an alternative mode of transportation. This is related to the previous section on reducing traffic volumes and some of the concepts mentioned in Page 32 of that University of Utah report, such as making “traffic calming” installations “… so severe or restrictive as to “degenerate” motor vehicle trips” and “suppressing motor vehicle travel with increased costs”.
It is interesting to note that in the first decade of the 21st century, the motivation for ‘encouraging’ mode shift was to reduce carbon emissions. More recently, the “traffic safety” messaging has become more prominently used. This is evidenced in the following quote from Page 17 of the World Resource Institute report, called Cities Safer by Design…
“Changing the current paradigm involves a process by which cities can limit vehicle travel while maximizing safety for those who are traveling. A framework for this approach is the Avoid-Shift-Improve paradigm (Dalkmann and Brannigan 2007). The framework was created as a way to reduce carbon emissions from transport, but it can also be adapted to traffic safety. Cities can find synergies in policies to address both climate change and traffic safety. In terms of traffic safety, this means avoiding unnecessary vehicle trips, shifting trips to safer, less threatening modes, and improving the existing environment and operations to be safer for all road users.”
In the New Zealand context, the agenda to “encourage mode shift” is a major focus of some reports by the New Zealand Transport Agency (NZTA or Waka Kotahi). One report, The Hamilton-Waikato Metro Area Mode Shift Plan 2020, acknowledges that discouraging and disincentivising the use of private vehicles is part of the plan as these quotes from page 30 and 41 demonstrate…
“Influencing travel demand and transport choices – Changing behaviour may also require a mix of incentives and disincentives (or ‘push’ and ‘pull’ factors) to either discourage use of private vehicles (by making them less attractive relative to other options) or making people more aware of their options and incentivising them to try something new. This may include parking policies, road pricing, travel planning and education.” (page 30)
“Ensure financial incentives and disincentives support mode shift” (page 41)
The table on Page 58 of the University of Utah report acknowledges that many “traffic calming” features reduced carparking spaces. Although many car users value carparking spaces in the city and would consider reduced carparking space to be a negative impact, bureaucrats may see discouraging car travel as a positive impact as this contributes to the “encourage mode shift”agenda.
Brett Frischmann describes one type of social engineering, called “nudging” like this…
“Nudging is the ascendant social engineering agenda pioneered by Nobel Prize winning economist Richard Thaler and Harvard law professor Cass Sunstein. Nudging permeates private and public institutions worldwide…
… Nudging has become a popular approach for (re)designing environments, procedures, and tools to get humans to behave better.”
“Nudging” is often considered one of the more subtle and mild types of social engineering approaches, relying on physiological manipulation and designing “choice architecture” to cause people to making better choices.
Who decides what is “better” and are they correct?
Although, “traffic calming” involves elements of “nudge” psychology in its implementation, it is often harsher than most “nudging” techniques. Many road users do not have other viable choices for easy transport alternatives and yet are still disincentivised (punished or penalised) physically, psychologically, and financially for using their automobiles.
What other negative impacts could result from widespread psychological manipulation of a city’s population?
How much increased stress and frustration is this citywide, psychological manipulation causing?
7. Expensive to install
It can be difficult to get exact figures on how much each raised platform or raised crossing costs to install as they are usually part of larger projects.
When asked about general installation costs, Waka Kotahi (New Zealand Transport Agency) had this as their response in June 2022…
The actual cost of specific “traffic calming” measures is often lost in large projects with large budgets, as seen in this example of an Auckland Transport response to questions regarding the New Lynn to Avondale Shared Path project…
A Rough Order Cost Estimate provided by AECOM to Auckland Transport in 2018 indicates that installing a single raised crossing at the Tamaki Drive/Turua Street intersection would cost about $120,000. Anecdotally, the New Zealand price per raised crossing was closer to $300,000 in 2023.
According to a Christchurch City Council response in November 2023 to questions concerning “…additional costs involved in installation compared to an intersection without a raised platform”, installing a large raised safety platform at an intersection can cost between $195,000 and $950,000 depending on the difficulties encountered during construction…
“For the Wigram/Hayton intersection, the scheme design stage cost estimate indicates that the installation of the proposed raised platforms and associated stormwater drainage at the intersection is approximately $195,000. This cost may be reduced following further investigation at the detailed design stage.
-For the Lincoln/Whiteleigh/Barrington intersection, the “post scheme design” estimate (the same stage of the project timing wise as Langdons) was $750k. This was prior to detail design, stormwater complications etc. The total project cost was approximately $900k; however, it is not possible to definitively say of this cost what was intersection and what was platform. There was a lot of difficulty with levels and making the platforms wheelchair usable (moving power poles, widening footpath, modifying design, etc).”
News articles in early 2024, have revealed that a single raised safety platform can cost approximately $500,000 to be installed in Auckland City. The large items in the cost breakdown reported by the NZ Herald include $172,000 for “Traffic management” during construction and $87,000 for “Stormwater upgrades”. As raised platforms often interrupt water flow along gutters and can create flooding risk, extra stormwater upgrades are often needed to mitigate those risks.
Increased maintenance costs due to the addition of “traffic calming” measures are rarely mentioned specifically in official documents. However, we should expect a responsible and transparent city council to produce such estimates and maintenance cost breakdowns before installing so many of these “safety upgrades” in our cities.
8. Being controversial and unpopular, especially on major roads.
A council can have high levels of local support when they install “traffic calming measures” on quiet, residential side streets. However, the recent trend of authorities installing raised crossings and other speed bumps on busy main roads is often unpopular with the general public. An extreme case was this proposal in Auckland to install a raised crossing over six-lanes on Pakuranga Road. According to 1News, this proposal received 90% opposition from the public during a four-week consultation.
Simeon Brown, (MP for Pakuranga, National Party) was quoted as saying…
“The reality is we are just going to add significant congestion to a road that already carries tens of thousands of vehicles a day. There’s already a crossing 300 metres from where they want to put this speed bump crossing, it’s just unbelievably crazy”
In the same article, 1News quoted Auckland Transport’s Executive General Manager of Service Delivery as saying…
“While no decision has yet been made, our challenge from here is to weigh up that community feedback and consider it against our unshakeable commitment to keeping Aucklanders safe and the robust evidence about safety risks provided by our expert engineers and road safety specialists.”
Auckland Transport also justified the proposed policy as being tied to “Vision Zero”.
Conclusion
This article has covered some of the major negative impacts and disadvantages of “Traffic calming” measure with an emphasis on speed humps and raised platforms. Although rarely directly acknowledged, there are some mentions of the negative impacts and disadvantages in some official documentation. Given the numbers of raised platforms being installed in New Zealand and internationally, and the obvious negative impacts experienced by people, the small numbers of academic studies and limited official acknowledgement of these issues is deeply concerning.
Major Source Documents
The University of Utah report. X. Zhou, P. T. Martin, M. Zlatkovic, and I. Tasic, Traffic Modeling of Transit Oriented Development: Evaluation of Transit Friendly Strategies and Innovative Intersection Designs in West Valley City, UT, 2013, https://rosap.ntl.bts.gov/view/dot/27998/dot_27998_DS1.pdf
S.G. Charlton, Research Report 300 Speed change management for New Zealand roads, 2006, https://www.nzta.govt.nz/resources/research/reports/300/
The World Resource Institute report. B. Welle, Q. Liu, W. Li, C. Adriazola-Steil, R. King, C. Sarmiento, and M. Obelheiro. World Resource Institute, Cities Safer by Design, 2015, https://www.wri.org/research/cities-safer-design
