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Speed 20mph limit and fuel consumption the facts

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Speed limits and fuel consumption

 

Introduction

 

Some time ago I wrote by email to the Department of Transport (UK) querying what I had read on the Internet claiming as a fact that modern cars are not capable of being driven in high gear at speeds of 20 mph. I received a lengthy reply to my query which is published below along with my letter.

 

Hi

 

My understanding of trying to drive so as to consume less fuel and contribute less CO2 into the environment is to keep my speed down and keep my car’s RPM low as well. However, I have read this:

Slow and Dirty

Another issue is that slower speeds would increase pollution. This may seem counter-intuitive, but any vehicle with an internal combustion engine will need to be driven in a lower gear at 20 mph than at 30 mph. The revolutions per minute will be similar, but the revolutions per mile travelled will be half as much again, leading to an increase in fuel consumption and emissions. In fact, the gearing on typical modern vehicles (such as, for example, a 1.6 litre Ford Focus) is often such that they will cruise at 30 in fourth, but are only comfortable at 20 in second, so their rpm would actually be higher.

http://www.speedlimit.org.uk/twenty.html

The extract is part of an argument to reject the increasing use of 20mph limits in residential areas.  My car a diesel land rover discovery is quite happy to roll along at almost tickover which is not much more than 1000 RPM. The car is an automatic. I don’t think that the sentiments expressed in the extract are correct.

 

The flaw in the first part of the argument is that it would only be valid if the car was forced to travel at a low speed.  The car would, indeed, be forced to travel at a low speed in a restricted area of a residential settlement. But to apply the rationale to driving at lower speeds in general is an error.  For example, in open country going down a hill, in which case the car would “roll” faster again at probably tickover RPM and reach a much higher speed than 20 MPH. My car would easily reach 45 MPH going down a moderate hill. Of course if I was constrained to travel at 20 MPH then either I would be braking and wasting fuel or using a lower gear ratio as a lockup.

 

The flaw, in the second part of the argument is that a “modern” car is well designed when their use is largely in urban areas, rather than open country,  and should surely be designed to run happily at speeds of 20 to 30 MPH.  A modern car only emits CO2 an water, so one can hardly regard that as pollution, except in so far as it is considered that CO2 is damaging to the environment.

 

Whilst for family and work reasons I need a 4x4 and can’t afford to run a small car as well I am concerned to drive and use my car so as to aspire to sustainable Travel. 

 

I would be please to receive a comment on my observations in relation to the above argument.

 

Thank you

 

Geoff Edwards

 

Department for                                            

Transport

Simon Davies

Senior Engineer

CFV3

Department for Transport

Zone 1/34

Great Minster House

76 Marsham Street

London SW1P4DR

Direct Line: 020 7944 2116

Fax: 020 7944 2512

GTN No: 3533 2116

Web Site: www.Oftgov.uk

28 June 2010

 

 

Mr Geoff Edwards by email

 

Dear Mr Edwards.

 

Speed limits and fuel consumption

 

Thank you for your recent enquiry, sent to our "sustainable travel" email address, which has been passed to me as an official in the Division with the most direct interest in vehicle emissions.

 

There are elements of truth in the argument against twenty mile per hour speed limits that you quote, but there are also, as you suggest, a number of flaws. I hope that you will excuse my rehearsing, below, some of the technical background to this argument.

 

I am strongly inclined to agree with your suggestion that modern cars are, in general, designed to run quite happily at thirty miles per hour in fifth gear, rather than in fourth gear as the article suggests. The suggestion, in the article, that it would be necessary to run such a vehicle in second gear if it were travelling at twenty miles per hour is certainly nonsense.

 

Whilst it is true that most of the emissions from a modern car are carbon dioxide and water vapour (plus, of course, nitrogen and any uncombined oxygen from the air that was drawn into the engine), there are still some emissions of air quality pollutants. Although these air quality pollutants form only a small fraction of the total exhaust emissions they are of considerable interest to us because they have significant public hearth implications, particularly in areas where traffic density is high or where there are other sources of pollutants.

 

The air quality exhaust pollutants that are currently regulated are oxides of nitrogen (NO,), carbon monoxide (CO), hydrocarbons (HC)( and particulate matter (PM) which is both visible and invisible smoke. Of these pollutants, it is oxides of nitrogen and particulate matter that are currently of the greatest concern since, partly as a result of the increasingly strict exhaust emissions standards which have been imposed upon manufacturers over the years, the United Kingdom has no exceedences of its air quality targets for atmospheric concentrations of either carbon monoxide or hydrocarbons. Exhaust emissions of lead and lead compounds are controlled by limits on the amount of lead permitted in fuel, and you will be aware that all generally available road transport fuel is now effectively lead-free.

 

Whilst emissions of carbon monoxide, of hydrocarbons, and of particulate matter are, in general, the result of incomplete combustion of the fuel (or of lubricating oil drawn into the cylinder), oxides of nitrogen are formed when oxygen and nitrogen from the air are brought together at high temperatures. This means that emissions of oxides of nitrogen from the engine itself (though, because of aftertreatment systems such as catalysts, not necessarily from the exhaust pipe) tend to be higher when the engine is working most efficiently, whilst emissions of the other pollutants tend to be higher when the engine is working less efficiently.

 

It is certainly true that changing to a lower gear at any particular speed will generally increase fuel consumption, and will be liable to increase emissions of some air quality pollutants from the engine. Whilst the relationship between fuel consumption and emissions of carbon dioxide is quite simple (with one litre of petrol producing 2.3 kilogrammes and one litre of diesel producing 2.64 kilogrammes of carbon dioxide) the relationship between fuel consumption and emissions of air quality pollutants either from the engine itself or from the exhaust pipe is quite a complex one.

 

The question of fuel consumption comes down, at a simple level, of course, to one about the amount of energy expended in moving the vehicle. This amount of energy may be calculated simply by multiplying the force required to move the vehicle by the distance over which the vehicle is moved. A particular fuel, meanwhile, will have a characteristic calorific value, and hence a characteristic energy content per litre or per kilogramme, or per gallon or per pound mass if you are working in Imperial units. (For petrol, this energy content is approximately 32.2 Megajoules per litre, and for diesel fuel it is about 35.9 Megajoules per litre.)

 

It follows from the argument above that the amount of fuel used in moving a vehicle will be directly proportional, in a simple theory, to the distance travelled and the resistance to motion experienced by the vehicle. Consideration will have to be given, in working out how much fuel wilt actually be consumed, to the efficiency with which the engine converts the chemical energy in the fuel into mechanical work. A figure of about thirty percent for this efficiency of conversion is a reasonable rule of thumb for a petrol engine, but the actual figure will depend upon engine speed and load and will vary somewhat from one engine model to another.

 

The resistance to motion experienced by the vehicle on a level road is made up of two elements. These two elements are simple rolling resistance, and aerodynamic drag.

 

The simple rolling resistance is due to such things as flexing of the tyres, scrub of the rubber on the road, and friction in the wheel bearings. The simple rolling resistance tends to be proportional to the vehicle weight, and tends not to rise very much with increasing speed. For cars, the simple rolling resistance usually lies in the region of one percent of the vehicle weight (in Newtons, for the purposes of calculations in SI units, where a mass of one kilogramme involves a weight of 9.81 Newtons) and this figure can reasonably safely, for rough working, be assumed to be the same at all speeds.

 

Aerodynamics is a complex science, but, because compressibility effects and variations in Reynold's number can be safely ignored over the range of speeds that are of interest, the approximate aerodynamic drag on a road vehicle may be calculated using the formula;

 

Fd = 0.5CdAp V2

 

where;

   Fd  is the drag force in Newtons.

    Cd is the drag coefficient for the moving vehicle.

    A is the cross sectional area of the vehicle presented to the airflow, in square metres.

    p is the density of the air (1.2 kg/m3).

    V is the speed of movement, in metres per second.

 

Modern motor cars tend to have drag coefficients a little above 0.3 {so that the aerodynamic drag on them is about one third of what it would be upon a sheet of plywood presenting the same frontal area) although the drag coefficient of a car like a Land Rover would be rather higher.  Examination of the formula shows that, if the other factors remain constant, the aerodynamic drag force is proportional to the square of the speed. This means that doubling the speed results in four times the aerodynamic drag.  It means that a vehicle is experiencing sixteen times more aerodynamic drag at eighty miles per hour than it was at twenty miles per hour.  It follows that decreasing the speed to fifty from seventy miles per hour approximately halves the aerodynamic drag.

 

By a speed of about fifty miles per hour the aerodynamic drag is becoming the dominant element in the resistance to motion experienced by the average car. if we chose to say that aerodynamic drag and simple rolling resistance were equal at fifty miles per hour, then our calculations would suggest that a vehicle travelling at seventy miles per hour would be experiencing about one and a half times the total resistance to motion, and so be using fuel at about one and a half times the rate, that it was at fifty miles per hour.

 

Although there is a very rough rule of thumb that says that, of the energy released by burning the fuel in a piston engine, one third goes to the coolant and is lost as heat from the radiator, one third is lost as heat in the exhaust gases, and one third produces useful work, in practice, for a variety of reasons, the thermal efficiency of an internal combustion engine does vary, as I mentioned above, over its operating envelope.

 

The graph that I have included below, and have added to the attached Microsoft Word document so that you will be able to view it in colour, shows a somewhat idealised version of the fuelling map for a fairly typical petrol engine. The overarching curve that forms the upper boundary to the map in both cases is the full-throttle torque curve for the engine, and so represents the maximum torque that can be extracted from the engine at any particular engine speed. Maps of this kind are produced either on a test bed, with the engine loaded using a dynamometer in order to record its fuel consumption both on the full-throttle torque curve and at a very large number of speed and toque points beneath it, or with the aid of very sophisticated modelling software. The contour lines on the map would normally be labelled as lines of equal specific fuel consumption (in grammes of fuel per kilowatt hour of engine output energy, or some equivalent unit) but the Dutch researchers to whom I am indebted for the idealised map have converted the contours in their map to lines of equal thermal efficiency.

 

A feature of the fuelling map is that there is an "eye," just below the maximum torque speed, where the engine is most fuel efficient The map shows that the efficiency of the engine is quite poor when it is running lightly loaded at any speed, and so underlines the importance of purchasing a vehicle with an engine which has an appropriate, and not an excessive, nominal power output.

 

Fuelling map typical petrol driven car

  

Fuel map

 

The two points marked 'A' and 'B' on the fuelling map were originally added for another purpose, but serve to illustrate the value, in fuel efficiency terms, of driving in the highest gear that the engine will comfortably permit.

 

The two marked points actually lie on a constant power curve, and are both points where the engine is producing about five kilowatts. I think that the figure of five kilowatts (6.7 horsepower) was chosen as representing approximately the power required to drive an average car at a steady speed of about thirty miles (fifty kilometres) per hour.

 

At point 'A' the vehicle is being driven in a high gear where the engine is running at hardly more than tickover speed, whilst at point 'B' it is being driven at the same road speed in a lower gear. It is clear from examination of the fuelling map that driving in the lower gear, although it makes the engine run at a speed closer to the maximum torque speed, involves running the engine at an operating point where it is less efficient because it is more lightly loaded. (The situation is, in fact, rather worse than the fuelling map suggests, since lower gears and higher engine speeds are usually associated with higher gearbox losses and higher parasitic losses from engine auxiliaries.)

 

Although the analysis above supports an argument that changing to a lower gear unnecessarily at any particular speed is likely to be a mistake from an environmental impact point of view, it does not necessarily support an argument that slowing down to such an extent as to require changing to a lower gear is also a mistake. Whilst changing to the lower gear in the diagram moves the engine operating point from point 'A' to the less efficient point 'B', for instance, and slowing the vehicle would then move the operating point leftwards and downwards towards an even less efficient operating point, it has to be remembered that diagrams of this sort are expressed in "specific" terms, and that using more fuel per unit of energy required at the wheels will not require more fuel in absolute terms if the energy required at the wheels goes down at the same time.

 

I suspect that, in reality, if travelling at steady speeds were the only thing that cars actually did, then slowing a typical saloon car from a steady thirty to a steady twenty miles per hour, and changing to a lower gear, might actually be environmentally disadvantageous. This is because the change in resistance to motion between the two speeds would be small for a car with a low drag coefficient. It does not follow, however, either that slowing from, say, 3 steady fifty miles per hour to a steady twenty miles per hour and changing to a lower gear would also be environmentally disadvantageous, or that slowing from thirty to twenty miles per hour would be environmentally disadvantageous in a vehicle with a higher drag coefficient.

 

Some of the claims that are made to the effect that lower speeds lead to increased fuel consumption and increased emissions are based upon a misreading of the data on emissions that is presented on the websites of the Department and of its agencies. This is because the data shows that emissions in grammes per kilometre travelled rise steeply at low speeds. The data refers, however, not to vehicles travelling at steady speeds but to vehicles in traffic streams travelling at the average speeds given. The emissions rise at lower speeds in these cases because it is assumed that a vehicle in a traffic stream travelling at an average speed of ten miles per hour, for instance, is having to stop and start far more frequently than one in a traffic stream travelling at an average speed of thirty miles per hour. The data presented is not intended to reflect what is possible or what can be achieved by the application of skill on the part of an individual driver, but what currently happens to the average vehicle in a real traffic stream.

 

Whilst the effect of steady speed upon fuel consumption is significant, the choice of a steady cruising speed under circumstances where a steady speed can be maintained is only one of the aspects of driver behaviour that affect fuel consumption on real journeys. Research shows that a driver who thinks ahead, and drives more smoothly and steadily, is likely to complete the same journey, in the same time, for rather less fuel consumed than is his or her less skilled counterpart who (often under the mistaken, if understandable, impression that a higher average speed will necessarily be achieved by always driving as fast as possible) engages in a great deal of unnecessary acceleration and braking. Unnecessary braking simply turns the kinetic energy stored in the moving vehicle into heat and into brake dust and then leads, inevitably, to unnecessary acceleration. The more highly skilled driver achieves an identical journey time whilst using less fuel simply by matching the speed of his or her vehicle more closely to the average speed of the traffic.

 

Although a vehicle with four-wheel drive will necessarily be somewhat heavier, and will be likely to have slightly higher transmission losses, than an equivalent two-wheel drive vehicle (as a consequence of having the transmission components required to get the drive to the additional wheels) the simple fact of a vehicle having four-wheel drive is not really of any environmental significance. In environmental terms the aspects of a vehicle that are important are largely the ones such as mass, aerodynamic drag coefficient, nominal power to weight ratio, and the unnecessary use of power-hungry ancillaries like air conditioning, which impinge upon fuel consumption. Cars with conventional automatic transmissions, as you will be aware, tend to have higher transmission losses than those with manual transmissions. The largest single factor in the fuel consumption and environmental impact of a car, however, is, as you suggest, the behaviour of the driver.

 

I hope that this is of assistance.

 

Yours sincerely,

Simon Davies

 

 

Cats killed on Freshford's roads

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Freshford six cats killed on the roads in 2012 despite a 20mph speed limit

It is alarming that last year (2012) six cats died after being run over by vehicles in Freshford. This carnage is happening in a village with a 20 mph speed limit. Some of these cats died on a Sunday so this issue is not entirely due to speeding motorists going to work.  I suppose that what we are dealing with is drivers who are not concerned about other living creatures. Perhaps, as is more likely, these drivers are unaware of what they are doing when they drive their vehicles, driving has become automatic.

Freshford motorists should be ashamed of themselves

However, it is a shame, I repeat a shame that families are unwilling to own a cat because they are afraid they will lose it to drivers who do not care. Or should I say, seem to be unaware of the danger they pose to wildlife when they drive their vehicle.  I wonder why these drivers live in the country?

Not only animals are at risk in Freshford

I was nearly hit as a cyclist going down The Hill by a motorist (the road was wet) who said she was not going more than 20mph. Did she really think that she was not going too fast because she was not going more than 20mph? Twenty miles per hour up hill on a wet road around a bend, The Hill is not straight. She skidded, how can someone in a Mercedes car have to skid while driving up hill, supposedly at no more than twenty miles per hour?  Perhaps she was looking at her speedometer and failed to see me until she had to brake, to brake heavily enough to make a modern car with ABS braking system skid whilst going up hill!  However, twenty miles per hour is a little less than the speed of the World record holder runner over a kilometer distance and if one were to ride a bike at 20mph one would realist that 20 mph is fast- try it.

Speed limits and village quality of life in Westwood, Freshford and Limpley Stoke

We have twenty mile per hour speed limits though Westwood, Freshford and Limpley Stoke. Some, if not all, of these speed limits have yet to be approved.  Unfortunately, whilst speed limits supported by a TRO (Traffic regulation order) can be enforced by police much of the problem seems to be centred around drivers who drive at the speed limit. To a pedestrian, cyclist and equestian vehicles do travel too fast. As recent measurements in Westwood indicate, often the vehcles are not travelling as fast as people on foot think. But in terms of safety, being in a car travelling at 20mph or more is like sitting watching TV compared to being passed on the road as a pedestrian by a motor vehicle: even a small car weighs 500kg or half a ton. The majority of drivers, both Freshford drivers and other local drivers drive with considerable consideration for other road users the problem lies, as always, with those that do not.

We really need the speed limits to be approved by TROs and must insist that police do regular speed checks, otherwise the limits will be ignored as perhaps they are now.

Housing in the countryside the consequences

There has been considerable building in the countryside and will be more. The countryside being land which surrounds a city, town or village. There is no demand for this housing other than it is cheaper than living in town (eg. Bath) as there is little opportunity for work. Recent expansion in Bradford on Avon, for example, is not matched by local work opportunities.  One of the consequences of this new building is that more and more peope are travelling comsiderably distances to work, mostly by car. Unfortunately, I do not feel that these commuters are in the mood to obey speed limits let alone have regard for someone's pet. There is no neccesity for a driver to notify the police if he or she has an accident involving a cat although there is with a dog. Or this is incorrect see below:

Road accidents that must be reported to police

If you hit and injure an animal (apart from a bird), you are required by law to do whatever you reasonably can to ease its pain. If it’s not a wild animal then the injury must be reported to the police or the animal’s owner. This is from an Australian website but one shouold think that it should apply to the UK.

http://www.lawlink.nsw.gov.au/lawlink/lawaccess/ll_lawassist.nsf/pages/lawassist_accidents_involving_animals

Anyone witnessing an accident involving a cat should take the registration number of the car. 

The majority of drivers, both Freshford drivers and other local drivers drive with considerable consideration for other road users the problem lies, as always, with those that do not.

 

Most rural roads have wrong speed limit

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Rural roads not safe at posted speed limit

The extract below from the European Road Assessment Programme - EuroRAP's website summarises the current standard of road safety in a report on England's roads which can be downloaded from the website. I note this: two-thirds of English trunk road single carriageways rate 2-stars.

John Dawson, Chairman of EuroRAP says: "If a driver is belted, sober and obeying the speed limit, then the risk of death and injury in a 4-star car on a 4-star road is small. But most rural roads in Europe are not safe at the posted speed limit. Most deaths happen on busy 1- or 2-star main single carriageway roads that need urgent investment in affordable safety line markings, safety fencing and junction layouts."

Dawson continues: "Road crashes cost the British economy 1.5 per cent of GDP, some GBP18 billion, annually. A fortune is being spent on emergency services, hospitalisation and long term care of victims when we could make road travel as safe as rail or air. UK roads can be the safest in the world over the next decade if the same high return investment in safe road design is made as other leading nations."

European Road Assessment Programme

 

UK speed Limit system request 5 mph increments

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Sensible speed limits: we need 5 mph increments!

With local arguments raging about whether traffic will comply with the introduction of the trial 20 mph speed limits to be introduced in Limpley Stoke and especially Westwood there has arisen a query as to why we can't have 25 mph speed limits for roads that are regarded as too fast for a 30 mph limit and too slow for a 20 mph  limit.

The USA has at its disposal a speed limit system that may use increments of 5 mph. This system allows for highway engineers to set speed limits that are matched to the road and its associated geometry. So, why can't our highway engineers have the option of finely tuning our speed limits?

A Google search does not result in references to why the UK does not have the option of using speed limits that are posted in multiples of 5 mph, other than some possible legal issue in relation to motor vehicle speedometer design variations. Surely, we can think of speed limits other than in multiples of ten? Anticipating that I might be suggesting more road signs to clutter up the countryside and distract drivers, I would like to suggest that I am not proposing more road signs, only that in suitable locations the existing limits may be changed. The proposed changes might be an increase in the existing speed limit or a reduction but they would be sensible changes to what at present are often speed limits that are inappropriate.  Probably, the majority of speed limits are set correctly, but there are others that are not.

Highways authorities tend to respond to requests to lower speed limits only when accident figures confirm that a road is dangerous. They ignore the intimidation that non motorists are subject to. They have coined the term Traffic Intimidation and have even held courses to help people come to terms with their fears. Nonsense, what we want is the freedom to walk, or cycle, or ride a horse on our local roads within or close to our villages without feeling that our lives are at risk.

Wiltshire County Council has responded to the increasing demand for action by frightened and angry residents by agreeing to run trial 20 mph speed limits in Limpley Stoke and Westwood and I compliment them for doing so. However, in relation to Lower Westwood Road in Westwood my opinion is that a 25 mph speed limit would be more appropriate.  But then that's my opinion, if the Highway authority agrees with me, we are back to the issues of this article that a 25 mph speed limit is not yet legal.

On paper, what speed limits mean is that the posted speed limit is the absolute maximum speed that a vehicle might be driven at in ideal conditions ie. with due regard for road conditions: weather, pedestrians, cyclists, motorcyclists and horse riders and pot holes! In which case the question one could pose is can any of these roads be driven at even for short distances at a speed of for example, 30 mph?  If there are stretches of road where 30 mph under ideal conditions is appropriate then fair enough keep them. Fair enough, provided that that's what drivers think and are prepared to respond to, unfortunately, who can argue that motorists tend to drive at the maximum speed limit rather than regard the posted speed limit as the maximum? 

 

25 mph speed limit is illegal so raise it to 30 mph!

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20 mph too slow, 25 mph illegal, so raise the 25 mph limit to 30 mph!

A case to show the absurdity of the UK system of speed limits. Location town or parish: Weston-Super-Mare and Kewstoke. Thre is no mention of road users other than motorists.

Quote:

With regards to the accident history on this road, during the 3 years up to April 2004 when the last fatal accident occurred there had been 12 personal injury accidents. During the following 18-month period when the various safety measures have been installed there have not been any accidents. Whilst it is still too early to be definitive, it appears that whilst the safety measures have not significantly reduced speeds on the straight sections of road, driver behaviour has been influenced and safety along the road has improved significantly.

 The options available are:

            a) Retain the existing non-enforceable 25 mph limit (i.e. “do nothing”),

b) Introduce a statutory 20 mph limit,

c) Introduce a statutory 30 mph limit.

Option a) would have no effect on existing speeds, and would not allow action against drivers on the basis of speed alone. It is considered that an enforceable speed limit must be installed.

 Option b) does not comply with the advice in the Circular, being too far below the prevailing speeds. A 20 mph limit is considered to be unrealistically low, with virtually all drivers currently exceeding this speed, and about 80% exceeding the enforcement threshold of 24 mph. Studies show that speed limits on their own have little impact on speeds, so signs would need to be supplemented by further traffic calming measures.

Option c) is consistent with the existing measured speeds, but would allow enforcement against the small number of drivers who drive significantly faster. ACPO guidelines suggest enforcement above (speed limit + 10% + 2 mph), i.e. above 35 mph in a 30 mph limit. The Safety Camera Partnership has indicated that they could enforce this legal limit as a “Community Concern Site”.

 Concern has been expressed that replacing the existing 25 mph limit with one of 30 mph, albeit an enforceable one, may “send the wrong message to drivers”. Any change should be accompanied by publicity reminding drivers that the speed limit is the maximum speed, not a “target” speed, and that they should respond to prevailing weather and road conditions.

 The roads at both ends of the Toll Road have existing 30 mph speed limits, by virtue of being lit. The existing changes of speed limit are marked by larger “terminal” signs. These would have to be removed if the 30 mph limit is made continuous, but “gateway” features could be installed by providing “30” roundel markings on the road surface at the change from Public Highway to private road. The Toll Road itself is unlit, and would therefore require “30” repeater signs (to replace the existing “25” repeaters).

 Accordingly, it is recommended that a formal 30mph speed limit is installed on Kewstoke Road.

http://www.n-somerset.gov.uk/cairo/docs/doc11507.htm 

One wonders what the safety measures that were installed are, and how much effective enforcement costs.

"Concern has been expressed that replacing the existing 25 mph limit with one of 30 mph, albeit an enforceable one, may “send the wrong message to drivers”. Any change should be accompanied by publicity reminding drivers that the speed limit is the maximum speed, not a “target” speed, and that they should respond to prevailing weather and road conditions."

I suppose in this case nearly all driveres can be reached by publicity? Or do they mean a sign that lights up reminding drivers that the speed limit is the absolute maximum speed?  The fact remains that it is surely nonsense to have to reduce a speed limit which has together with safety measures eliminated accidents because the 25 mph cannot be enforced.

 

 

So how fast is the 20 mph speed limit?

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So just how fast is a 20mph speed limit?

Whilst city and town dwellers argue for lowering the speed limit from 30 mph to 20 mph because they are very concerned about the safety of themselves and particularly their families, motorists complain that they will be driving at a snails pace.  The reality is that none of these motorists could run faster over 100M than the current World Record Holder Usain Bolt, who managed an average speed of 23.35 mph. No, 20 mph is not slow, try it on a bike.

Locally, Limpley Stoke and Westwood villages have been selected for 20 mph trial speed limits. Any experienced drivers will suggest that there are parts of both villages where depending on the time of day and other factors 30 mph would be a safe driving speed.  The crucial word is "parts".

I drove through Westwood the other night very late and I drove at 20 mph. As I left the village a black and white cat darted across the road in front of me. Possibly, if I had been going faster I would have run it over.  I don't won't to be responsible for killing anyone's cat let alone intimidating women and children by not driving at a reasonable speed when there are people about, so congratulations to both villages for campaigning for the 20 mph limit.

If motorist don't like it that is just too bad, they have only themselves to blame as the Highway Code states quite clearly that the posted speed limit is the ABSOLUTE MAXIMUM speed that a vehicle may be driven at, and that speed limit does not mean it is safe to drive at that speed.  Unfortunately, many drivers do not heed that advice and tend to drive at the maximum speed limit and even tailgate drivers in front who are going slower! Other road users including: pedestrians, cyclists and horse riders, as a consequence of inconsiderate driving feel intimidated, and often angry, and they have a right to feel that way.

Another issue is the nonsense that speed limits have to be in multiple increments of 10 mph. Yes, it's either 30 mph or 20 mph, when it could be 20, 25 or 30 mph.  In some areas there are 40 mph speed limits when 35 mph would be more appropriate, unlike the USA which has 5 mph increments, the UK does not at present have an incremental speed limit system.

 

National Speed Limit for Villages

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National speed limit for villages

I think that this extract says it all. Too many people think that they should drive at the speed limit and to hell with every other road user. This is what the Highway Code has to say: The speed limit is the absolute maximum and does not mean it is safe to drive at that speed  (the relevant section in the code is republished below)

Speed limits: Road Safety Bill

Mr. Chope: I had not intended to participate in the debate, but I think that the Minister and the hon. Member for Caithness, Sutherland and Easter Ross have missed the main point, which is surely that all drivers should be driving at the speed that is appropriate in the circumstances. The village where I live used to have the national speed limit, although most people drove at about 20 mph because the road was single track and there were bends. If we impose a speed limit that is not regarded as the maximum in ideal circumstances, it will not command respect.

In the less populated rural areas, in particular, it is surely important that all motorists go at an appropriate speed. During the day, when there are children around, that speed may be different from what it would be at the night time or during the early hours of a summer morning. I am worried that there would be speed limit signs in every village in the countryside and that they would add to the rural clutter instead of reinforcing the message that too many people are driving too fast in particular circumstances, in relation to their own safety and the safety of others. That is what is emphasised in the highway code.

Too many people think that they should drive up to the speed limit. To finish my example, a blanket 40 mph speed limit zone was introduced in the whole of the New Forest, but that was too fast for our village, so another limitation on driving in our village had to be introduced, because it seemed implicit that people would drive at 40 mph. Surely, without the need for a lot of clutter and a lot more regulation, we should re-emphasise the need for people to drive at an appropriate speed, irrespective of what the speed limit sign says.

Hansard: You are here: Publications and Records > Commons Publications > Committees > Standing Committee on Bills New clause 7

National speed limit for villages

http://www.publications.parliament.uk/pa/cm200405/cmstand/a/st050203/pm/50203s04.htm

The Highway Code and speed limits

Speed limits

124

You MUST NOT exceed the maximum speed limits for the road and for your vehicle (see the table above). The presence of street lights generally means that there is a 30 mph (48 km/h) speed limit unless otherwise specified.

[Law RTRA sects 81, 86, 89 & sch 6]

125

The speed limit is the absolute maximum and does not mean it is safe to drive at that speed irrespective of conditions. Driving at speeds too fast for the road and traffic conditions is dangerous. You should always reduce your speed when

  • the road layout or condition presents hazards, such as bends
  • sharing the road with pedestrians, cyclists and horse riders, particularly children, and motorcyclists
  • weather conditions make it safer to do so
  • driving at night as it is more difficult to see other road users
 

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Stopping Distances

Drive at a speed that will allow you to stop well within the distance you can see to be clear. You should

  • leave enough space between you and the vehicle in front so that you can pull up safely if it suddenly slows down or stops. The safe rule is never to get closer than the overall stopping distance (see Typical Stopping Distances PDF below)
  • allow at least a two-second gap between you and the vehicle in front on roads carrying faster-moving traffic and in tunnels where visibility is reduced. The gap should be at least doubled on wet roads and increased still further on icy roads
  • remember, large vehicles and motorcycles need a greater distance to stop. If driving a large vehicle in a tunnel, you should allow a four-second gap between you and the vehicle in front

If you have to stop in a tunnel, leave at least a 5-metre gap between you and the vehicle in front.

 

This means that if I travel at  20mph a following driver should be driving at a distance of no less than 40 feet behind me and at 30 mph a driver behind should maintain a gap of 75 feet, yes and pigs can fly!  

speed Limits

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Speed Limits and health

The following extracts are from a government publication. The report reveals the public health implications of the speed of vehicles

http://www.publications.parliament.uk/pa/cm200102/cmselect/cmtlgr/557/557ap05.htm

 <quote> Excessive speed is a major cause of deaths and injuries, especially in children 

 Speeding is dangerous for the driver (for whom it is a self-imposed risk), passengers and other motorists, but it is especially dangerous for vulnerable road users such as cyclists and pedestrians, particularly children and older people.

   Trauma is the most common cause of death among children, and road traffic injuries account for half of these. Two thirds of deaths and serious injuries among children involve child pedestrians injured in road crashes.

   The death rate from road traffic injuries for children in the UK is twice the European average. Most of these injuries occur in urban areas, and excessive speed is the single most important factor in such crashes.

   Even apparently low levels of speeding pose significant risks. For each 1 per cent increase in speed there is a 5 per cent increase in mortality; in many urban and residential situations travelling at the legal speed limit may be too fast.

Physical inactivity is a major public health problem

  Across the UK physical inactivity now has a greater absolute effect on levels of coronary heart disease than smoking, and the problem is increasing, with dramatic increases in overweight and obesity; this is particularly worrying among children. One of the main reasons for reduced activity levels is the decline of walking and cycling resulting from perceptions of danger from fast traffic. </quote>

 Road safety

There are many drivers who do drive considerately but there are also many who do not. For those that do not drive with due care and consideration there are speed limits. Speed limits should help to enforce safer driving. However, unless the police are able to use speed traps on a regular basis there is unlikely to be a great change in driver behaviour. Until drivers slow down, pedestrians will continue to feel intimidated and will either keep off the roads or reduce their excursions.

What can we do

We can help to reduce the speed of vehicles travelling through our towns and villages and the countryside by driving slower. There are many roads that are too narrow to allow for overtaking so speeding motorists will find that their speed is controlled by the vehicle in front.  Be that vehicle in front :)

Speed Limits and Sat-nav in the UK

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Speed Limit

But first what is a speed limit? What better resource is there than Wikipedia? http://en.wikipedia.org/wiki/Speed_limit It is a very, very interesting article. And I mean very interesting. The article needs about fifteen minutes to read and digest, but it is well worthwhile! Let's take,as an example, the first person to be convicted of breaking a speed limit. The first person to be convicted of speeding in the UK was Walter Arnold of East Peckham, Kent. On January 28, 1896 he was fined for speeding at 8 mph (13 km/h), thus exceeding the contemporary speed limit of 2 mph (3.2 km/h). He was fined 1 shilling plus costs (from the Wikipedia article) Now, doesn't that sound absurd? Yes , yes, yes! How on earth can someone be accused of speeding at that speed, slower than someone can walk, except by some balmy legal system?  I am not going to answer that, but I don't really suppose that the authority was balmy, they had their reasons. But today, in 2010 I can drive my Land Rover Discovery weight 2 tons through most villages and towns at speeds up to 30 mph. Most of these towns and certainly villages do not really enforce the speed limit. The police might occasionally organise a speed trap, but they need to choose a location that is safe not only for the officers but also where there is space to pull the driver over.  I don't think that the authorities that set these speed limits are balmy. But, I do challenge the wisdom of many of them when they set speed limits, and when they review them. Wiltshire has just published a review of speed limits on the A and B roads. the C and D roads, many of which go through villages will be reviewed later. I am not sure whether I am allowed to publish the recommendations which are subject to review.

Maximum speed limits

I will leave for another  article, the discussion of what is an appropriate speed limit. My understanding of what a speed limit is for, is that it is for informing road users that they are entering a zone where a speed limit is in operation. The local authorities have determined that vehicles should not exceed the posted speed limit for the safety of the public. There are maximum speeds associated with certain types of road in the UK: 70 mph on motorways and dual carriageways and 60 mph on all other roads. sisty mph is the National Speed Limit. There are also additional speed limits to reduce the speed of motorists. The reason why there are speed limits is because to exceed the posted limit would endanger the driver and other road users. A speed limit sign is an important guide to road users that they should limit their speed for one reason or another. The sign is also, a warning for that those who exceed the speed limit that they will be breaking the law and this can result in a fine. This means that speed limit signs should not be disregarded. The government link Speed: know your limits is a PDF document. It is interesting to note that The Times Online has an article on a government proposal to cut the National Speed Limit from 60 to 50 mph, except for certain A class roads. Cut Speed to 50 

Scrap speed limits?

Whilst local people will understand that the speed limit is appropriate, and may not really need to have a speed limit, strangers will not have that local knowledge. There is also an important role that road markings and signs have that should contribute information to road users that they should modify their speed. These signs can negate the necessity to have a speed limit. Whilst the majority of the public who drive on the roads do so sensibly and use road signage to guide them in selecting an appropriate speed, unfortunately there are others who do not. It is therefore, necessary to have speed limits. The existence of a speed limit does mean that the police could be used to control motorists speed. This is especially effective where there are radar traps that are in operation most of the time. However, the use of speed limits, it could be argued, is not necessary. There is a danger that drivers ignore speed limits that appear to be unnecessary, especially where it is unlikely that the limit will be enforced.

Sat-nav

With the introduction of sat-nav navigation systems motorists are directed to take routes that they would not have been aware of before the advent of sat-nav. This has meant an increase in the number of road users in rural areas. These road users are ignorant of local road conditions and a proportion of these drivers are impatient to get to their destination, and will thus be travelling faster than they should. It is therefore, very important that both road signage and speed limits need to be very carefully selected.

In another article I will discuss some roads in Wiltshire and Somerset that are subject to inappropriate speed limits. These roads are examples of a lack of wisdom locally, and I am sure that examples could be found elsewhere. 

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