Peter Jones MInstP

I have noticed on checking this blog today that the HGV statistics have vanished from the last section.

If this again is the work of hackers they are not being very clever.
The statistics (covering the period 1994 to 1997) are in Hansard as they were supplied to the House of Commons in answer to a question from Peter Lilley MP.

The above statistics are also in  www.caravanaccidents3.wordpress.com  after the HGV section

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Tools ‹ Caravan and HGV Trailer Snaking Accidents — WordPress.

The items below have been copied ( with minor modifications) from my blog



as 20six.co.uk seem to be all too frequently

“out of action.”

powered by




Feb 2010

Institute of Traffic Accident Investigators

In 2005 the ITAI declined to present the report I had prepared for them to their International Conference.

I know that I had adopted the correct procedures as I was visited at home twice by one of their officials to make sure that I was working along the correct lines. Several of their members had requested that I investigate Caravan snaking and I was given access to the private “ internet chat room” which was dedicated to the exchange of Scientific Information between Road Traffic Accident Investigators.

It was the enthusiasm and dedication of the ITAI members I came into contact with that gave me the motivation to investigate at length the snaking/jack knifing of caravans and HGV’s.

The ITAI consists almost entirely of UK Police investigators or retired Police investigators. However, it is an independent body that regulates (amongst other things) the professional standards of its members.

The report I wrote for them has been amalgamated with much work I have produced in the last 5 years, but I have taken care to clearly identify the exact items that were in the 2005 ITAI report.


(The itai report part 2 is now also in




As HGV accidents are running at about 230/week this subject should be given a higher priority by the Government, Health and Safety Experts and Accident Investigators; the figure of 230 is likely to be a serious underestimate as the DfT have combined LGV’s and HGV’s in the table of statistics I used to

make my estimation.


DfT Statistics


Due to the size and weight of HGVs, they tend to be involved in more severe accidents. In 2007 the rate of fatal accidents was higher for HGVs (1.6 per 100 million vkm) than for all motor vehicles (0.9 per 100 million vkm). LGVs however had the lowest fatal accident rate of all vehicle types (0.4 per million vkm).



The Police Authorities have combined all HGV’s in one statistical table. We need to know which type of HGV is the safest.

Responsible Caravanners would never tow a caravan weighing more than the tow vehicle; rigid HGV’s can easily comply with the Caravan Club Safety Recommendations **, but Articulated Lorries would have to run below capacity to meet the same standards.

(** see 2009/10 Caravan Club Members’ Handbook; Page 640;Top para on left)

My 2.5 litre Tdi Diesel Land Rover Discovery is designed to tow a trailer of up to 4 tonnes in weight, if it has been adapted to link up to the appropriate braking system for HGV’s.

As the disco has 10 forward and 2 reverse gears it would be a good (small) HGV, but as the trailer would be twice as heavy as the Land Rover I would judge that 20mph is about the maximum safe air speed ( when the wind is negligble). The maximum safe side wind component (and the air speed) would vary greatly depending on the load.

The same Land Rover can tow a 3.5 tonne trailer having over run brakes on a “B+E” driving licence, but to allow for the inferior brakes the top safe air speed would need to be rather less than 20mph.

Has a committee of Professional Engineers ever attempted to assess safe air speeds for all types of HGV’s and caravans?

The accident rates for HGV’s indicate that there are serious problems to be solved. Due to their low miles/year (on average) the accident rates for caravans and small trailers give a false impression. I would expect the number of caravan/small trailer accidents to be slighter larger than those for HGV’s if the former category with their inferior brakes ran a similar number of miles/per year. The casualty rate would be vastly lower due to the low weights of these mainly recreational trailers.

HGV’s are not covering their costs to the community, are the least safe of vehicles and the rail network should be used for more long distance freight where the lines are available in the correct place.




The roads have been made much safer by the widespread use of safety cameras; I can, in the main, now travel at what I judge to be a safe speed without being subject to “Road Rage” by the irresponsible minority of motorists and HGV drivers.

The most serious exception to this is rural country “B” roads which have few safety cameras and some speed limits which have not been revised since the 1930’s.




“Fit To Tow” video published by the Highways Agency.

See Paragraph 31


Since my blog above was “hacked” I do not seem to be able to edit same. I am therefore pasting in an item below meant to be read with Para 31 in the above.

Recopied from the Highways Agency www site “Fit To Tow” video (text version) 3/12/09




Paul Atkin (Association of Chief Police Officers) and Carol stand opposite each other in front of the line of vehicles that Carol walked past in the opening scene





Carol turns and talks directly to the camera.

Carol: Of course there are rules about what you can and cannot do and there are risks too. Last year alone accidents involving towing vehicles caused nearly fourteen hundred injuries and forty-three fatalities. So, how do we make sure when we’re towing we do it right?


Peter W Jones

I had not previously copied in the line that confirms the presence of ACPO.

Accident Statistics.

It can be seen from my written evidence (2006) to the Select Committee on Transport of the House of Commons ( see


Put ” Minutes of Evidence Transport Select Committee Caravan and HGV Trailer Snaking Accidents” in their search engine) that I quoted an estimate from the Caravan Club of “on average, less than 1000 miles per year for each of its members caravans.”

This estimate was given about 1989. Since then the price of fuel has increased substantially and this is bound to have reduced the number of miles each member tows their van.

If one accepts that 1000 miles per year is the average for a typical caravan and that the average for a typical family car is about 10,000 miles per year it can be seen that to obtain a good indication of the risk one is taking pulling a caravan compared with driving your car without the caravan, the figures given by the Highways Agency via Carol Smillie should be multiplied by at least 10.

I am not advocating that one gives up caravanning; I shall not be doing so.

However, please look at my conclusions on safe towing that will be found in


I have not allowed for other trailers with over run brakes that fall into this category. The Caravan Club estimate was “LESS than a thousand miles per caravan.”



Peter W Jones. MInstP




He has advised me that he has referred the matter of Caravan safety to Birmingham Trading Standards and the minister responsible for this area of policy. This mainly affects over run brakes and stabilisers, but other areas covered in my blogs apply to both Caravans and HGV Trailers.

B-ham and the DfT Trading Standards have failed to deal with this in the past, as I have recorded in my blogs. Both these bodies should be able to rely on advice from a Professional Engineer, but “Chartered Engineers” and above seem to be very scarce in Westminster and Birmingham.

I know that some schools and 16 to 18 Colleges in various parts of the UK will have Physics and Maths teachers whose qualifications will enable them to endorse any relevant work their students do for GCE “A” level or GCSE. It would then be a “Professional Report.”

If the students are using the school/college computers for their work they will only have to print it out and have it signed.

The teachers however would have to put in some extra work to familiarise themselves with the material I have been writing and adjust their teaching accordingly. As this area of the syllabus is unlikely to change soon this extra effort will last them some time as succeeding groups will also be very interested.

All this is explained in






Peter W Jones MInstP









According to racfoundation.org the foundation’s director, Professor Stephen Glaister, said that historically, road accidents are analysed by individual police forces with the emphasis placed on finding out if anyone has broken the law. Identifying the underlying causes of crashes seems to be of secondary importance.

The site continued to explain that in the UK we have been locking up drivers for a century and yet motorists still die in their thousands on the roads each year. The focus on solely penalising individuals rather than also identifying systemic safety failings is a serious flaw in current transport policy. Road safety should be driven by prevention as well as punishment.


I have spent some considerable time explaining in my blogs how these systemic safety failings apply to trailers particularly those with high aspects such as some HGV’s and caravans. I have obtained a great deal of help from the publications of Bath University Department of Mechanical Engineering, but mainly derive my convictions from 33 years recreational towing experience on UK roads

Further note 2/12/09

racfoundation.org have published a paper by Dr Elliot



The above reference also gives the source of Professor Glaister’s statistics.





Peter W Jones MInstP

The most recent entries to my blogs are at




See also



Advice for Physics and Maths teachers (GCSE and GCE “A” level):-















(i) Anemometers

(i *) Caravan Club Handbook extracts 2009/10

(ii) Bath University Research

(iii) HGV Trailers

(iv) Wind Induced Road Accident.

(v) Wind Induced Rail Accidents.



I have no control over, and do not profit from advertisements on this blog.

A hand held anemometer will normally only give you a reading for the air speed. As these are quite inexpensive this would be of some assistance and would be safe if used only when your vehicle is at rest.

It would be dangerous and counterproductive to use them whilst driving.



Scroll down to

“Air Speed Indicators.”


Peter w

Jones MInstP



I am publishing the items below so that there is a public record.

As caravans get bigger and heavier, and cars get smaller and lighter, the caravan industry will be greatly tempted to alter their policies.

The only change of policy that I think is satisfactory is for great speed reductions when towing anything heavier than the towing vehicle.

The HGV Industry have never abided by the caravan club towing rules and this in my view is one of the reasons for their shocking accident statistics.


Insert 15-01-11

Politicians of all parties have turned a blind eye for decades to these statistics, and the police record on investigating causes of RTA’s, so I am not surprised that even after the visits of all the political “heavy weights” to Oldham East and Saddleworth we still had a turn out of under 50% for the recent by election. On the same day a Council by election for Camborn North in Cornwall produced the first Labour win for many years, but it was secured with a vote of 230 according to BBC Ceefax.

Our democracy is in crisis, but very few can see this, although I must praise the Conservative Education Secretary for speaking up so strongly against the obscene pay levels in parts of the banking industry when he took part in a recent “BBC Question Time TV programme.”

The MP’s expenses scandal is a minor matter compared with the number of fatalities, injuries and damage to the UK economy resulting from avoidable road accidents. I only refer of course to the accidents that will be avoided when sufficient people have read these blogs and applied the policies that I and others I have acknowledged think should be put into place.


Insert 15-01-11









Transport Research Laboratory

Extract from Evidence sent to the Select Committee on Transport.



27. Specialist accident investigation branches exist for air, rail and maritime accidents but there is no

equivalent for road accidents. Where Police Collision Investigators do investigate serious and fatal road accidents their reports are not collated and little use is made of them beyond the case being investigated,

There could be much benefit in establishing a formal link between serious/fatal crash investigation and safety research by standardising certain aspects of investigations and encouraging recommendations from these specialists. Establishing this link would increase the value of police investigations by providing high quality accident data for safety research and thereby helping to reduce casualties.













2009/10 page 639





The towing vehicle in this guide is assumed to be a car.

Mass of vehicle in running order (MRO)

(Kerbweight, kerbside weight)


The weight of the car as defined by the car manufacturer.

Normally, this includes: 90% full tank of fuel; an adequate supply of other liquids incidental to the vehicle’s propulsion; the driver but without any passengers; without any load, except tools and equipment with which the car is normally provided; but without the towing bracket. You should add about 25kg for the towbar and ball.


Some definitions used by car manufacturers do not include the driver. If you do not know the MRO for your car, add about 100kg to the driverless kerbweight of the car to allow for the driver, towing bracket, ball and additional wiring.

Maximum Authorised Mass (MAMj


Weight of the car when fully laden with driver, passengers and luggage and imposed noseweight.

Note: In other documents this may also be referred to as the Maximum Permissible Weight (MPW) or Gross Vehicle Weight (GVW).

In other documents this may also be referred to as the Maximum Permissible Weight or Gross Vehicle Weight

Maximum Permissible Towing Mass (MPTMJ


The weight defined by the car manufacturer as the maximum that the car is to tow.

Gross Train Weight (GTW)


The maximum permitted combined weight of the car and trailer as specified by the car manufacturer.

Towing load limit

The maximum trailer mass that the car can tow. Published by the car manufacturer.


The mass of the personal effects required for two people to go caravanning is approximately 100kg. This may include, for example: bedding, clothing, cooking utensils, crockery, cutlery, food and external water carrier. A further 25kg for each additional person should be allowed for items such as books, drinks, and other non essentials.

You can

a) weigh each item before it is placed in the caravan
and add the total to the MRO


b) take the fully loaded caravan to a public

The address of your nearest public weighbridge can be obtained from your local Council’s Trading Standards Department (Weights and Measures).

Note: Although regularly checked, weighbridges may give varying results as they are calibrated for much heavier vehicles. A public weighbridge operator will give you a written or printed record for each weighing.

Remember: The weight of the battery, gas cylinders and any manufacturer or dealer options must always be taken into account.

The weight of the battery, gas cylinders and any manufacturer or dealer options must always be taken into account.


This ratio is the actual laden weight of the caravan expressed as a percentage of the MRO of the car, i.e.:

Actual laden weight of the caravan x 100% = MRO of the car

Towbar load limit

The maximum vertical static load that the towbar can support through the towball. Published by the towbar manufacturer and shown on a plate on the towbar.Towball height

The height of the centre of the towball should lie in the range of 350mm to 420mm above ground level when the car is laden. Some large cars may require a towbar drop plate to achieve a height within this range. See your towbar fitter for advice.Why is this ratio important?

It is used to ensure your fully loaded caravan is appropriate for your towing experience as it has a major influence on towing stability.The caravan industry recommends:

For a novice caravanner, ideally, this ratio should not exceed 85%.

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cc Handbook page 640


For an experienced caravanner the maximum
recommended ratio is 100%, provided the figure
is permissible in respect of the tow car’s published
capability. –


Remember: The weight of your personal effects and the optional equipment you have fitted will affect the result:

Keep the caravan as light as possible – the lower the weight, the better the match and, provided items are stowed properly, the safer it is to tow. The greater the actual laden weight of the caravan and the greater the train weight set for the car, the more careful and experienced in towing the driver needs to be.

Note: Care must always be taken not to exceed the car’s loading and towing limits including the Gross Train Weight which takes preference over the weight ratios above.


Check your driving licence is suitable for your car
and caravan combination-see page 647.

Stay safe and legal:

To ensure that the combination of car and caravan is legal for use on the public highway you must ensure that:

Your car’s MAM is not exceeded,

Your caravan’s MTPLM is not exceeded,

Your caravan’s MTPLM does not exceed your car’s


The combined laden weight of your car and caravan

does not exceed your car’s GTW.

The lowest load limit of these four elements is not exceeded:

The car’s towbar and ball

The caravan’s drawbar limit

The caravan braking system overrun device The caravan hitch coupling.

Then make sure that the caravan noseweight lies within the 5% to 7% range (see below). Information on car and caravan limits may be found in the handbooks issued by the car and caravan manufacturers, from the Clubs and from caravan dea ers.


“Can my car tow this caravan safely?”

There are a number of computer programs available on the internet that will match a car to a caravan or vice versa. Some will demonstrate the car’s performance when attached to specific caravans. Some of these can also allow a car owner to analyse generic caravan types and to study caravan profiles.

The car/caravan match is produced by a calculation similar to the caravan/car towing ratio explained above.However, since it is not possible to know the actual laden weight of the caravan, particularly if the question is being asked of a caravan that is to be bought, the ratio of the caravan MTPLM to the MRO of the car is used to generate this match. The industry recommendations on ratios for novices or experienced drivers also apply in this calculation.


Your best source of information is your caravan dealer who should be able to advise you. The Clubs are also invaluable sources of additional information.

^Tollowthese guidelines:

• Never exceed the MTPLM. Remember that the
weight of any optional extras that you may have
added will need to be considered in your weight
calculations, e.g. a caravan mover

the Remember that theweight of any optional extras that you may haveadded will need to be considered in your weightcalculations, e.g. a caravan mover

P* — ,i . _^ _ — _–,


In general, the higher the noseweight, the better
the stability of the combination. Experience has
shown that a noseweight in the region of 5% to
7% of the actual laden weight of the caravan is
safest.However, the maximum noseweight will
always be limited by the lowest of the car, towbar
or caravan coupling vertical load limits.

The way the caravan is loaded is critical.

Heavy items placed at the front and rear or stored at height could destabilise the caravan undertow.

Visit our website and book online at

In general, the higher the noseweight, the better
the stability of the combination. Experience has
shown that a noseweight in the region of 5% to
7% of the actual laden weight of the caravan is
safest.However, the maximum noseweight will
always be limited by the lowest of the car, towbar
or caravan coupling vertical load limits.

The way the caravan is loaded is critical.

Heavy items placed at the front and rear or stored at height could destabilise the caravan undertow.

Visit our website and book online at


CC handbook page 646


Experience of towing is not essential for taking up caravanning. Any driver should soon find towing a caravan both rewarding and pleasurable. If you are at all unsure, the Clubs offer towing and manoeuvring courses.

The caravan should always be towed either level or slightly nose down.

You should always build up speed gradually to
get used to the different handling and braking
characteristics when towing. A caravan will alter the
performance of the car and the driver will have to
anticipate potential hazards at an earlier stage. This
experience will allow the driver to eliminate sudden
speed and/or course changes by using earlier
anticipation to create a calmer and safer driving
environment. ^

/The speed at which a caravan is towed is very important. The aerodynamic forces that act on the caravan at speed may tend to reduce the noseweight, increasingly as road speed increases. At a critical speed any loss of noseweight can cause instability and this could occur at a lower speed when driving into a head wind. Gusting cross winds, exposed bridge sections, valleys and proximity to large goods vehicles may also initiate aerodynamic instability. Slow down, but do not brake, to return to stability.

•. -“•

Speed limits:

National speed limits


Car and caravan

Single carriageway

Dual or more carriageways











Additionally, caravans must not be towed in the outside lane of a three or more lane dual carriageway or motorway.

A good reserve of power is necessary for towing up gradients at altitude

When going uphill, change gear in good time. If your car is running short of power or is behind a slower vehicle, keep well into the nearside and out of the way of other vehicles. Remember that some hills which can be ascended with relative ease often pose an unexpected challenge if you come to a standstill in traffic and then have to re-start from scratch.

When going downhill, take extra care to ensure you do not gain speed. This can be avoided by changing down a gear and reducing speed as you approach tht slope. Don’t leave this gear change too late. Using low gears throughout the descent will reduce the strain on the car’s brakes. For automatics, you may need to manually change to a lower gear in anticipation of th effect caused by the gradient change.

Note: If you plan to tow in countries where long, high altitude climbs can be anticipated, you should seek expert advice on the suitability of your car for such journeys. Further experience and training should be gained before tackling some of the more difficult elements of towing (mountain passes, difficult terrain, etc.).

Reversing competently comes with practice. Watch for slopes, cambers, potholes and other irregularities that can cause the caravan to deviate. As with all manoeuvres, the secret is to do things smoothly and at low speed. At night, additional lighting and/or a second pair of eyes may be needed.

competently comes with practice. Watch for slopes, cambers, potholes and other irregularities that can cause the caravan to deviate. As with all manoeuvres, the secret is to do things smoothly and at low speed. At night, additional lighting and/or a second pair of eyes may be needed.

Steering a caravan in reverse is counter-intuitive:

To cause the rear of the caravan to turn one
way, the steering wheel has to be moved initially
in the opposite direction from how you would
normally steer the car.

One technique is to remove the hand from the
steering wheel in the direction in which the turn
is intended and pull downwards on the steering
wheel with the other hand.

Once the initial turn is established, the steering
wheel is turned through the straight ahead
position to provide a small amount of positive

Note: Reversing in a straight line may be difficult if there is a significant camber or irregularities on a road surface.


When a caravan is attached to the towball on the back of a car, some of the weight of the caravan is borne by the rear suspension of the car. This may cause the car to achieve a ‘nose up’ attitude, in addition to any change caused by a load carried in the rear of the car. This attitude can be seen from the side of the car and may well be sufficient to mis-aim the headlights significantly and potentially reduce the effectiveness of the steering system.

Visit our .1 book online competently comes with practice. Watch for slopes, cambers, potholes and other irregularities that can cause the caravan to deviate. As with all manoeuvres, the secret is to do things smoothly and at low speed. At night, additional lighting and/or a second pair of eyes may be needed.

competently comes with practice. Watch for slopes, cambers, potholes and other irregularities that can cause the caravan to deviate. As with all manoeuvres, the secret is to do things smoothly and at low speed. At night, additional lighting and/or a second pair of eyes may be needed.

Peter W Jones MInstP

(ii) Bath University Research

To day I noticed that there had been the most significant advance since I started investigating snaking caravan accidents in 2003.

A considerable number of people must be reading the Bath University research as the Google Search Engine

has moved the item below into a much more prominent position.


University of Bath

School of Mechanical Engineering



An experimental study into caravan snaking

Final year project

submitted by Christopher J Killer

for the degree of MEng

of the University of Bath

23 May 2003

Assessor RFN

Supervisor JD

Technician PC


11 Conclusion


The performance of the Alko 2004 tow ball friction damper has been shown to be very effective at smoothing down snaking oscillations and reducing settling time. But this does not necessarily mean that it makes the coupled combination any safer, i.e. increase the snaking speed. Theory states that this type of damping has little effect on the snaking speed, and tests have shown no evidence to suggest otherwise.


11.1 Practical advice for caravan owners & designers

Nothing fundamentally new was discovered in this study, but theoretical factors suggested in the past have been substantiated by road-tested evidence. The theoretical advice that is now known to be significant in increasing stability includes the following points.

For owners:

Make sure car is suitable for the size of caravan

Load heaviest items in the car if possible

Locate other heavy items in the centre of the caravan, just forward of its axle

Make sure tow ball load is adequately high

Avoid sharp steer inputs at high speed

Never exceed 60mph


I hope that the above extract will encourage more reading of the Bath Research, but please note that when making your tow ball load as great as possible you should not exceed the tow car manufacture’s maximum allowed load.

See also


Paragraphs 31a and 36

for further details of above Bath University research.




Peter W Jones MInstP

Scroll to the end of this section

for hgv accident statistics

I have noticed that monitoring for wind speed at critical points on our major roads is usually done with pre ww2 wind socks. Every sailing cruiser only slightly larger than my Jaguar 21 (with 2 crew weight = 1000kg ) seems to be equipped with an anemometer giving a digital read out of wind speed and direction in front of the person steering.

We need these devices on our major roads linked to traffic control centres so that Police can give out on the indicator boards actual wind speeds so that drivers can act on their own initiative as appropriate for their own vehicles and trailers, instead of just being told (for instance) that there is a speed restriction of 40 mph due to strong winds which is meant to restrict all vehicles irrespective of weight and other characteristics.

My rough estimate of critical head wind air speed for one type of HGV trailer is given further below.

My small 4m body length caravan weighing 1000kg is susceptible to snaking when its air speed exceeds 50 mph and the side wind component exceeds 30 mph. I need to know therefore if the side wind at a particularly exposed section of road exceeds 30 mph. I have already explained in other parts of my blogs how I arrived at these values, partly using my experience gained on open waters with sailing cruisers.

When looking at weather forecasts it is better to seek out the speed of the strongest gusts forecast before deciding whether it is safe to be on the road.

Notices on the back of a number of HGV’s urge the public to phone and comment on the driving standards. However we do not know the standards prescribed. The DfT should consider updating their advice on safety when it is windy particularly on matters that relate to the safety of HGV’s especially high aspect trailers, caravans and below 3.5 tonne trailers.

I have previously pointed out that vehicle manufacturers ( as with air craft manufacturers) should be required to establish critical wind speeds with some degree of accuracy for each type of vehicle sold.

I have observed that road users are increasingly prepared to pay for extra safety devices such as electronic brakes and air bags, and feel certain that when they understand why knowledge of air speeds and side wind components is important for safety reasons, they will be prepared to pay extra for vehicles that are sold with the relevant information in the owner’s handbook.

As I have never driven an HGV or sailed a boat larger than my daughter’s 29ft Westerly (about 4 tonnes), my estimates for HGV critical air speeds and side wind components are not going to be be very accurate. I am therefore pointing out how to make estimates so that those with relevant towing experience can do this for themselves.

I can be quite accurate concerning the fact that in the USA (for instance) when a Hurricane force wind is expected those able to do so move out of the immediate coastal area until the danger has passed. The remainder of the population go into buildings constructed so that they can withstand Hurricane force winds.

According to the Beaufort Wind Scale for Mariners a Hurricane is Force 12 ( about 78 mph) and above. If you are driving at an indicated 55 mph it will only take a head wind of 23 mph to bring the air speed that the vehicle/trailer is subjected to up to the bottom end of the dreaded Hurricane strength. The aerodynamics of trailers in particular needs to be taken very seriously.

In 1999 Bath University published “Towed Vehicle Aerodynamics” by Standen; the wind tunnel tests had proved that aerofoils to create down force as speed increases improved the stability of a caravan with a (approximately) central axle.

Aerodynamically a HGV trailer with approximately central axles/axle is similar to a caravan.

Only a minority of HGV’s tow trailers with central axles, but there will also be the evidence of tyre marks on the road from all snaking accidents which may be similar to those which have been “leaked” to me concerning snaking caravan accidents.

The above Bath university results clearly stated that some trailer snaking is caused by the wind and explained how this happens.

As I feel that I have established that the maximum safe air speed for my small caravan is about 50 mph, and this is the stalling speed for a typical light aircraft of the same weight, I feel that I am justified in saying that the maximum safe air speed for a central axle HGV trailer is VERY ROUGHLY the same as the stalling speed of an aircraft of the same weight. In the absence at the moment of accurate figures from people with access to instrumentation that can measure the amount of lift created by the air around the vehicles, I suggest that HGV drivers cultivate the acquaintance of an airline pilot and try and obtain some confidential advice concerning stalling speeds to compare with their own experiences of the air speeds and side wind speeds when snaking/jack knifing has taken place. Starting about 30 years ago I always managed to avoid wind induced snaking by obtaining information from Radio 4 shipping forecasts and estimating the likely speeds inland. More recently I have been able to get a very good wind speed forecast from the BBC weather forecast on the web, but we really need to go direct to the met office so that we obtain the maximum speed of wind gusts.

As I started towing on the road when I purchased my first sailing cruiser I was always very aware of the power of the wind and those who doubt this should talk to people who sail some of the types of boat I list below. These sailors will be only too pleased to help as the relevant information concerning sailing cruisers is not confidential. I want the sailing community to tell people the area of sail they use to sail their boats in gale force winds (about 44mph) in sheltered waters where the sea is calm. If HGV drivers then compare the relevant sail area with the area of the side of their trailer of the same weight as a sailing cruiser, they will then understand why something needs to be done to improve road safety by using electronic braking systems and aerofoils to create down force on all trailers as well as paying greater attention to weather forecasts and “critical” speeds.

Aerofoils for HGV trailers would have to be quite large, and as this matter was not allowed for when bridges were built engineers will have a difficult problem to solve when designing the aerofoils for large trailers. For those who can afford the million euro VW sports car this problem has been solved by a computer controlled aerofoil over the rear wheels ( but I would not suggest that the latter is a good tow car!) Formula 1 racing cars, after years of development, now have two fixed aerofoils, one between the front wheels and one over the rear wheels. Rally cars are beginning to use fixed aerofoils and when the latter are above the level of the car roof they must be very effective.

Addendum Nov 2008

As I took a particular interest in Louis Hamiltons’ performance in F1 racing this year I could not fail to notice that they have now added extra aerofoils.

A few Statistics:-


Nicholson 55 (Weight 17.3 metric tonnes; area of main sail = 48.3 square metres ); Nicholson 42 (wt 10.16 tonnes)

Bowman 57 ( wt 19.05 tonnes); Ocean 71 (29.06 tonnes);

Source:- Boating World Guide to Sailing Cruisers; 1976


Boeing 737; weight 45.36 tonnes; take off air speed 150 mph.

Source of information:-


My estimate of stalling air speed = 120 mph.

Fairchild Aviation C123B

Weight, 27 tonnes. Stalling air speed, 96mph

Based on the above I would therefore estimate that a 27 tonne HGV trailer (if such a trailer is legal and exists) with approximately central axles would become liable to snaking when the air speed reached 96mph. The driver would be aware of the air speed if he/she had an appropriate indicator.

At 55mph road speed one only needs a head wind component of 41 mph (gale force is about 44 mph) to reach the estimated critical speed.

Tiger Moth Biplane.

As one of my main recreational activties over the last 32 years has been estuary and coastal sailing, I have never had the usual experience of flying off to wamer climates for my holidays.

I have only taken to the air twice, and that was as a passenger in a Tiger Moth Biplane in 1951. During the Summer holidays HMG “called me up” again to do an additional two weeks National Service in the RAF. During my 18 months national service I was an “Audiometrician,” so it was deemed appropriate that I did some flying.

In 2003 when I started enquiring about the reasons for caravans snaking I recalled that the Tiger Moth take off speed was about 60 mph and that it must have weighed about the same as my small caravan.

You will see from my blogs that my memory had not failed as similar sized modern light aircraft have comparable statistics.



(The official record of



Road Accidents (HGVs)

Mr. Lilley:

To ask the Secretary of State for the Environment, Transport and the Regions if he will list the number of road accidents involving a heavy goods vehicle for each month since January 1994. [78481]

Dr. Reid: The number of road accidents involving a Heavy Goods Vehicle which resulted in personal injury is shown in the following table:

The number of road accidents involving a Heavy Goods Vehicle which resulted in personal injury is shown in the following table:

Road accidents per month involving at least one Heavy Goods

Vehicle (1): 1994-97 Number of accidents
















(1) Accidents may involve more than one Heavy Goods Vehicle


I show these statistics to draw attention to the serious disruption caused to our road transport system by hgv’s, the vast majority of which tow trailers.

It can be seen that there are on average 247 accidents per week and each one of these will involve enormous disruption due to the size of the vehicles involved. The very considerable cost to the UK economy of having many thousands ( in a great many cases when the accident is on a motor way) of people stuck in a traffic jam for some time until the police have finished their investigations and the accident scene has been cleared up is not being taken in to account. This applies to viability of road v rail traffic as well as very large hgv’s v large hgv’s.

This is of course in addition to the great loss of life and serious injuries sustained.


The Department for Transport only keep records of accidents which involve fatalities or serious injury

Trailer accidents in particular would be greatly reduced if we knew the side wind speeds at which snaking/jack knifing was set off in particular vehicles; the maximum safe air speed for individual trailers; and had anemometers on tow vehicles plus more road/rail side anemometers linked to remote electronic warning signs.

The air craft industry already does something similar.

There has been an overall slight drop in road traffic accidents since the above figures were produced. I shall up date this asap.


The only similar statistics that I have been able to find are for a period from 1990 to 1994. Mrs Dunwoody had asked a similar question to Mr Lilley and the answer gives an average of 262 HGV accidents per week.


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