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The abominable Dr Vibes

I thought to offer my comment as a vibration consultant, before you reply to many more of these types of queries. This will avoid perpetuating a potential error. Whilst agreeing wholeheartedly with potentially damaging effect of road humps on vehicle tyres and suspensions and also on their possible effects on people (spinal column injuries for example, particularly for taxi and ambulance drivers), I think I ought to correct one misapprehension regarding their potential effect on building structures, which has been alleged - cracked walls and burst water mains.

I have worked on vibration effects on structures, vehicles, people and many other aspects of road-induced vibration over many years and was the principal author of the relevant British Standard, under the guidance of the appropriate BS technical committee.

This BS is "BSI BS 7385-2 Evaluation and measurement for vibration in buildings — Part 2: Guide to damage levels from ground borne vibration". Credible damage (not alleged damage) to building structures or water mains has very rarely, if ever, been demonstrated when I last looked at this problem, although it is often alleged. This is because people are generally much more susceptible to vibration than structures in the frequency range of vehicle-induced vibration.

Home owners are naturally concerned about their property, and when they perceive incoming vibration from vehicle passing by, they naturally assume that damage could be occurring. If any of your correspondents have any measured data and properly verified damage (before and after inspections), rather than simply alleged damage, the appropriate BS committee would be interested to hear from them. All of the hundreds of cases which we researched whilst preparing BS 7385 revealed very few actual cases of substantiated vibration-induced building damage, from any of the typical sources of vibration, including road traffic, so some care is needed here.

The vibration threshold (12 mm/sec peak particle velocity -ppv) for cosmetic damage (plaster cracking for example) is invariably well above the vibration level in nearby buildings caused by heavy or light vehicles crossing humps. In fact, cracked walls, classified as minor damage, would require significantly higher vibration inputs (25 -50 mm/sec ppv according to the type of structure). Road rollers passing within approximately 1m could be of concern, but rarely solid or pneumatic rubber tyred vehicles.

This is not to say that sensitive equipment may not be adversely affected, but that is not at issue here. Most water mains can also sustain much higher vibration levels (at least 20 mm/sec ppv, and possibly up to 50-100 mm/sec ppv) than that caused by vehicle passage over a hump (typically 0.2- 5 mm/sec ppv, even at 1 m standoff distance). Most vibration levels that I have measured personally due to road discontinuities, imperfections or undulations, are well below 10 mm/sec ppv.

This is because we as people can perceive vibration above approximately 0.15 mm/sec and are relatively very sensitive compared with structures, at most frequencies of vehicle-induced vibration. As you will see the research carried out by TRL, given in this link, http://www.dft.gov.uk/adobepdf/165240/244921/244924/1000, table 2, shows that the building would need to be less than 1 m distant to have any possibility of minor damage being caused.

Most humps (but not all) are considerably further away from private homes, due to intervening gardens and pavements than 1m, I think you would agree. The design of the hump profile of course should be optimised to minimise the vibration impact on people travelling in the vehicle. The vibration transmitted through the tyres, suspension, body structure and seat will of course vary according to the stiffness and damping in each of these elements along the transmission of road discontinuity to the human seat. This why higher speeds over the humps are possible of course with certain vehicle types, without sustaining excessive vibration input into the occupant.

Having said all this, I am completely in agreement with you and most of the readers of your column that humps should be removed, due to the health effect on people and damage to vehicle components and general annoyance to motorists. Thanks for a most interesting column. I am right behind most of your answers and views and I really enjoy it.

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The attached collapse occurred recently adjacent to s 'speed table' on Berkhmamsted High Street that is regularly traversed by heavy trucks. So this is definitely something that should be investigated by the committee. The collapse was put down to a structural fault, but could have been finally triggered by vibration.

As regards damage to the sub structure of the road, I have photographic proof of numerous potholes occurring in and around speed humps and speed cushions. Vehicles traverse the humps. The pounding creates shockwaves that crack the sub structure of the road.

Water seeps in. In winter it freezes and expands. The substructure of the road then becomes unstable. I would not trust any research into humps by the TRL after its pathetic whitewash report concluded that speed cushions do not damage vehicles in any one. (One of the vehicles in its useless one-day test was, in fact, damaged by the humps.).
Tags: road humps
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