Introduction to speed reduction
We're all painfully aware of the dangers of speeding, but we've all been guilty of it at some point. Speed has long been known to be one of the primary factors in most road traffic collisions. This factor increases both the risk of the accident occurring to begin with and the severity once said accident happens.
I'll always remember the harrowing road safety advert with the child stating "if you hit me at 40 miles per hour there's around an 80% chance I'll die, hit me at 30 and there's around an 80% chance I'll live"
So, let's talk about some of the speed reduction measures out there. First of all there's the 'SLOW' signs, often seen painted on the road or 'reduce speed now' on the signs next to the road. The primary issue here, is that for regular users of the road, for example local residents and commuters, these signs quickly lose their efficiency as we become desensitised to them and simply stop noticing them.
Another method of traffic calming are roundabouts which we are very used to here in the UK, but our friends in the US may not be as familiar with them. Roundabouts are effective at traffic calming and segregation, however they require intensive planning and are generally much more costly. One way streets follow a similar ilk.
Following these, governments, councils and businesses have a wide range of options to choose from, but as well as doing the job at hand well, the measures must also be cost effective.
Humps and bumps and cushions
Righto, now speed bumps and humps - a lot of the time these are used interchangeably, but they are different. Let's start with the similarity, they're both examples of vertical deflections, the benefits of which are consistent. Unlike signage, bumps and humps don't lose their effectiveness over time as regardless of whether or not you are used to them being there, you must still slow down.
Speed bumps and humps are made of rubber, asphalt or plastic with metal and concrete options available (metal and concrete versions will not 'give' as rubber will so vehicle damage is more likely).
Now the differences:
Speed bumps - a shorter travel distance than humps, these essentially create TWO bumps over which the car must travel (the front axle will completely cross the bump prior to the rear axle), unlike speed humps. This results in a more turbulent crossing if moving at a higher speed, making speed bumps a more aggressive form of speed reduction. Speed bumps usually require the driver to slow to around 5-10mph.
Speed humps - these can span the entire width of the road if need be and have a much wider surface area making them much more suited to roads where speed needs to be reduced but it is not convenient to slow vehicles down to 5mph. Speed humps generally allow vehicles to keep up a speed of 15-20mph.Speed cushions - these are of a similar construction to speed humps with the exception of wheel cut outs at certain widths. These are in place to obstruct regular cars and passenger vehicles while allowing vehicles with a larger wheel width to pass freely, emergency services for example. Other vehicles are forced to slow down.
Placement of speed reduction measures
Where? Speed bumps, humps and cushions should always be placed on level surfaces as gradients can alter the total height, creating a more aggressive impact than wanted. Driveways, curbs and street lights include other considerations.
Can drivers see it? Streetlights also play a part in visibility, which is our next consideration - drivers are unable to reduce their speed, regardless of whether or not they want to if they cannot see the speed reduction measures which are in place. Signage, reflective strips and high-visibility material can and should be used where required to ensure they're highly visible.
What's the weather like? Take into consideration the weather in your country. For example, here in the UK we get a lot of rain but this year in early 2018 we had an uncharacteristically large amount of snow for a protracted period of time, followed by (what felt immediately!) an uncharacteristically long heat wave. In the snow, would the speed humps and bumps been visible? This is where signage is a necessity. It's better to have it and not need it than need it and not have it.
What should it be made of? We've looked at this briefly before - brick and concrete will not flex, resulting in a greater risk of damage to the vehicle. Rubber and plastic generally cause less damage as there is a certain degree of flexibility, as well as being able to use recycled materials.
On a final note
Speed bumps:
- More aggressive in controlling traffic
- Force drivers to reduce speed to around 5-10mph
- Travel length starts at 6inches
- Height from 3-6 inches
- Typically used around stores, schools and pedestrian zones.
Speed humps:
- The lesser aggressive of the two
- Reduce speed to between 15-20mph
- Typical height is 2-4 inches
- Speed humps help to keep traffic moving
- Suited to high traffic areas
It's been thrown around before that speed bumps are a contributor to air pollution. There is very little evidence to suggest that the implementation of speed bumps has had a negative effect on the climate. Air pollution is of course worse in high traffic areas, however this is not a result of the traffic calming. Removing traffic calming measures in the majority of cases would simply put people at risk.
Speak soon,
Danny
