Speed bump

Local authorities have cited disadvantages to speed bumps:

• The city of Modesto in California, produced a fact sheet which contains the following disadvantages:[7]
  • Slow response time of emergency vehicles;
  • May divert traffic to parallel residential streets; and
  • Possible increase in noise and pollution for residents living immediately adjacent to the speed bumps.
• The English town of Eastleigh states the following as disadvantages:[8]
  • Can cause damage to some vehicles;
  • Can increase traffic noise, especially when large goods vehicles pass by;
  • Required signs, street lighting and white lines may be visually intrusive;
  • Can cause discomfort for drivers and passengers;
  • Can cause problems for emergency services and buses.

Other sources argue that speed bumps:

• Distract drivers from other hazards such as children
• Increase pollution as traffic travels in a lower gear using significantly more fuel per mile;
• Are a compromise for more active enforcement;
• Increase noise by creating tire-to-bump thumping and increasing the amount of engine-revving;
• Cause spinal damage[9] and aggravate chronic backache.

In 2003, the chairman of the London Ambulance Service, Sigurd Reinton claimed that delays caused by speed bumps were responsible for up to 500 avoidable deaths from cardiac arrest each year. He later denied the statement.[10]

In Sweden, an evaluation of spinal stress in bus drivers against ISO 2631-5 required on health grounds that:[11]

• bus drivers avoid certain streets until the humps were modified; and
• the maximum acceptable speed be reduced to 10 kilometres per hour (6.2 mph; 2.8 m/s) on one street for drivers encountering 150 humps in a day.

Speed bumps can also have adverse environmental impact. A study found that in one north London street with a speed limit of 20 miles per hour (32 km/h; 8.9 m/s) and fitted with road humps, a petrol driven car produced 64 per cent more nitrogen dioxide (NO₂) than in a similar 20 miles per hour (32 km/h; 8.9 m/s) street fitted with road cushions. It also produced 47 per cent more particulate matter (PM) and nearly 60 per cent more carbon monoxide (CO) emissions.[12] Another study estimated that, for a private automobile, the increase in fuel consumption due a pass over a speed bump is responsible for fuel waste of 10ml.[13] This multiplied with the number of vehicles going over a particular speed bump every day suggests significant annual fuel wastage for a single speed bump.

Dynamic speed bumps differ from conventional speed bumps in that they only activate if a vehicle is traveling above a certain speed. Vehicles traveling below this speed will not experience the discomfort of a conventional speed bump. Dynamic speed bumps may allow the passage of emergency vehicles at higher speeds.

The Actibump system, successfully used in Sweden, is based on powered equipment integrated into the road surface, which operates a platform that is lowered a few centimeters when a speeding vehicle approaches. Any vehicle approaching at or under the speed limit will pass on a level road. The system measures the speed of an oncoming vehicle by using radar.[14]

In another design, a rubber housing is fitted with a pressure relief valve that determines the speed of a vehicle. If the vehicle is traveling below the set speed, the valve opens allowing the bump to deflate as the vehicle drives over it, but it remains closed if the vehicle is traveling too fast. The valve can also be set to allow heavy vehicles, such as fire trucks, ambulances, and buses to cross at higher speeds.[15][16]

Speed humps typically limit vehicle speeds to about 25–30 km/h (15–20 mph) at the hump and 40–50 km/h (25–30 mph) at the midpoint between humps, depending on spacing. Studies show an average 18% reduction in traffic volume and an average 13% reduction in collisions.[17]

While similar to speed bumps, humps are less aggressive than speed bumps at low speeds. Humps are often used on streets, while bumps are used more in parking lots.[20] While speed bumps generally slow cars to 10–15 km/h (5–10 mph), humps slow cars to 25–30 km/h (15–20 mph). The narrow traverse distance of speed bumps often allows vehicles to pass over them at high speed with only mild disturbance to the wheels and suspension, and hardly affecting the vehicle cab and its occupants. The relatively long slopes of speed humps are less disruptive at low–moderate speeds, but they create a greater, more sustained vertical deflection; at higher speeds, a more sustained deflection is less-absorbed by vehicle suspensions and has a greater effect on the vehicle as a whole.[21]

One problematic aspect of speed humps is their effect on emergency vehicles. Response time is slowed by 3–5 seconds per hump for fire trucks and fire engines and up to 10 seconds for ambulances with patients on board.[17] Speed humps are thus usually not placed on primary response routes. Speed cushions may be placed on these routes instead.

Occasionally, there is an increase in traffic noise from braking and acceleration of vehicles on streets with speed humps, particularly from buses and trucks. Other effects include increased fuel consumption and emissions as well as increased wear and tear on brakes, engine and suspension components.

Damage caused by snow plows during the winter months is an additional concern.

Heavy sedans, trucks, and SUVs are less affected by speed humps, and may not have to slow down as dramatically.

Thin cuts are sometimes placed in the middle of a hump in order to allow bicycle traffic to pass through. However, forcing cyclists to take a particular line on the road compromises their ability to position themselves safely according to the other traffic on the road at the time.

Speed cushions have several distinct advantages over similar traffic calming devices. Many municipalities are challenged by opposition to speed humps and speed tables since they slow down emergency vehicles and buses. Speed cushions address this problem by allowing larger vehicles to straddle the cushion without slowing down. This is also an advantage for buses, as lower floor vehicles can sometimes ground out on traditional humps.[24] Speed cushions are often less costly than speed humps or tables, and most cities report them to be just as effective. In some jurisdictions, narrower speed cushions are placed at more frequent intervals to allow ambulances to pass while driving over the centre line. Large trucks are also not slowed down.

Development of speed cushions has focused primarily on the European context. European vehicles typically have a narrower track width than American vehicles, meaning their left and right wheels are closer together. Emergency vehicles still feature a wide track width, and the difference between them makes speed cushions more applicable.

In North America, however, consumer vehicles have a track width of 1,300–1,500 millimetres (50–59 in). Many emergency vehicles are also equipped with dual tires on their rear axles. The additional tires limit track width to as narrow as 1,200 millimetres (48 in), meaning speed cushions may not be suitable for their intended use.

Typical speeds resulting from 7-metre (22 ft) speed tables are 32–48 kilometres per hour (20–30 mph). One sample of 8 sites found a 45% decrease in accidents per year with the use of speed tables.[26] Wombat crossings may reduce casualties by 63%.[27]

Speed tables are effective in calming traffic on streets where the speed limit needs to be maintained rather than slowing cars more significantly. Traffic speed, volumes, and accidents have been shown to decrease with the use of tables. Although not as responsive to emergency vehicles as speed cushions, speed tables cause less of a delay than humps and are typically preferred by fire departments over speed humps.[25]

Speed bumps in some areas have been removed after protests by local residents. Such protests cite the lack of any consultation as one factor.[29] For example, complaints from Derby residents prompted the removal of 146 speed bumps from streets at a cost of £460,000. Similar incidents have been reported elsewhere in the UK.[30] UK news sources reported a cyclist being killed in a crash while attempting to avoid a speed bump.[31]