Rules to be Changed for Setting Speed Limits

[martinholmesuk 0]

Anyone know about this?

 

Rules to be Changed for Setting Speed Limits - your chance to register your objections.

 

In November 2004 the Department for Transport (DfT) issued a consultation draft of its proposed new guidance on the setting of local speed limits. The current guidance, Circular Roads 1/93, incorporates the principle that speed limits should be set as close as possible to the 85th percentile speed, i.e. the speed that 85 per cent of drivers would not exceed if there were no speed limit. This principle has been the basis of government advice on how speed limits should be set for over 30 years and is founded on research evidence both in the UK and elsewhere.

The draft revised guidance contains two main proposals that will affect the way speed limits are set. The first is the abandonment of the 85th percentile principle in favour of using the mean speed of traffic for calculating the right speed limit. The second is the classification of roads into two tiers : an upper tier with a through traffic function, e.g. A and B roads, and a lower tier with local access functions, where quality of life considerations take precedence over traffic, so lower speed limits may be allowed.

 

Both these proposals give serious cause for concern, as do the reasons put forward for proposing them. Ditching the 85th percentile principle is proposed because: Practitioners thinking has evolved since then [the publication of the 1993 circular] and many have expressed concern that 85th percentile speed can be heavily influenced by excessive speeds travelled by a minority of drivers & The Department shares this view and therefore recommends that mean speeds be used in future assessments of appropriate speed limits.

 

On the face of it, this appears to show an astonishing lack of understanding by the DfT of basis statistical definitions and methods. To obtain the 85th percentile speed of a sample of vehicles, the individual measured speeds must be ranked in descending order and the top 15 per cent discarded. The highest speed then remaining is the 85th percentile. The mean, on the other hand, is obtained by adding together all the individual speeds and dividing by the number of observations. Consequently, a minority of high speeds will affect the mean but not the 85th percentile.

 

It is hard to believe that the DfT does not understand this - of course it does. In the extract quoted above, the use of the word practitioners is revealing: they mean local authority engineers given the task of implementing the speed limit policies decided by their political masters. Where those policies demand imposing speed limits well below the 85th percentile level, then anyone driving above those predetermined limits is deemed guilty of excessive speed . A minority of such drivers could be up to 49 per cent of the total!

 

It is hardly surprising, therefore, that the practitioners who want to justify low speed limits find it inconvenient trying to reconcile their aims with official guidance based on the 85th percentile principle. The mean speed would suit their ambitions much better, so the DfT is prepared to acquiesce in discarding a method that has served us well for decades.

 

The DfT has sought to play down the impact on speed limits of assessments based on mean speeds. It states that for typical speed distributions on single carriageway rural roads, the 85th percentile speed is about 6mph above the mean where there is a 50mph speed limit, and 8mph above the mean speed where there is a 60mph limit. It concludes that: Setting acceptable mean speeds at or below the limit is therefore consistent with current enforcement thresholds.

 

While there is no information with which to question the DfT s claims about the average differences between mean and 85th percentile speeds on rural roads, there are going to be some roads where the 85th percentile is more than 6 or 8mph above the mean. This will especially apply to those single carriageway roads where it is perfectly safe to travel at the national speed limit of 60mph when traffic flows are light, but where increasing congestion and/or a high proportion of heavy goods vehicles keeps speeds artificially low for part of the day. These roads may also have above average accident rates as a result of the frustration caused by limited chances to overtake.

 

The use of mean speeds for speed limit assessment would almost certainly lead to many more 50mph limits being imposed on this sort of road, as is already happening in counties like Suffolk and Oxfordshire. Far from improving safety, these speed limits increase driver frustration and danger, as well as criminalizing those travelling at perfectly safe speeds when traffic flows are light.

 

The DfT s comment about current speed limit enforcement thresholds also raises an issue of concern. The tendency is for these thresholds to be reduced, so it is more important than ever that speed limits are set at levels that the majority of drivers will respect. In theory, if the new guidance were applied to the letter, some speed limits could actually be increased. However, since the proposed guidance is clearly intended to make it easier for local authorities to justify lower limits, any changes in the opposite direction are likely to be rare.

 

Research shows that drivers who travel at around the 85th percentile speed are the safest and most competent. This is why the 85th percentile speed is recognised by traffic engineers as the optimum level at which to set speed limits. When speed limits are set close to the 85th percentile, it means that the majority of drivers, including the safest, are travelling within the law. If anything, therefore, not only should the 85th percentile principle be retained as the basis of speed limit setting, its use should be strengthened by limiting the amount to which the measured 85th percentile speed can exceed a proposed speed limit.

 

The proposed two-tier system of rural roads for the purpose of speed limit setting shows that the DfT is still wedded to the assumption that lower speeds mean fewer accidents. It is proposing an assessment framework for choosing appropriate speed limits within each tier, based on how accident rates (injury accidents per 100 million vehicle-kilometres) compare with nationally-derived threshold levels. If the accident rate for a particular road exceeds one of these thresholds, a lower speed limit may be considered.

 

The recommended speed limits for single carriageways in the upper tier of A and B roads are:

 

· 60mph on high quality roads with fewer than 35 injury accidents per 100 million vehicle-kilometres.

 

· 50mph where there are more than 35 injury accidents per 100 million vehicle-kilometres and/or the mean speed is already below 50mph.

 

· 40mph where there is substantial development or there are considerable numbers of vulnerable road users.

 

· 30mph in villages.

(Dual carriageways are not included in the assessment framework and the default national speed limit of 70mph will continue to apply.)

 

Note the alternative criteria for justifying a 50mph speed limit: there is no need for the accident rate to be high if mean speeds are below 50mph. This reinforces the concern that the use of mean speeds could lead to the widespread imposition of 50mph speed limits on main roads. Also, TRL s guide on accident analysis suggests that accident numbers, as well as rates, should be considered in deciding the routes to prioritise for investigation. This is undoubtedly the reason why many lower speed limits have been imposed on rural main roads in recent years: a handful of well publicised accidents gives the impression that there is a serious safety problem, when a dispassionate analysis might show that the accident rate is not high at all.

 

For the lower tier of rural roads, where quality of life considerations are to be given greater weight, the recommended speed limits are:

 

· 60mph on roads with a mixed function, i.e. where providing a route for through traffic is still of some importance.

 

· 50mph on roads with a local or access function and with fewer than 60 injury accidents per 100 million vehicle-kilometres.

 

· 40mph on roads with a local or access function and with more than 60 injury accidents per 100 million vehicle-kilometres, and/or where they form part of a recognised route for vulnerable road users.

 

· 30mph in villages.

Under these recommendations a 50mph speed limit could be imposed simply because a road does not carry much through traffic, even though there are hardly any houses along it, it is rarely used by pedestrians, cyclists or horse riders, it does not have an excessive accident rate, and its alignment might be perfectly suitable for higher speeds. Also, the alternative criteria for justifying a 40mph speed limit allow one to be imposed that operates at all times, just because there might be a handful of pedestrians or cyclists using it for a few hours per week.

 

For roads in either tier of the hierarchy, the recommended speed limit for villages is 30mph. This might seem reasonable, until you discover that the DfT has defined a village for speed limit setting purposes as 20 or more houses along a frontage of at least 600 metres. Plenty of scope here for justifying 30mph speed limits through low-density developments where a 40mph limit might be expected.

 

The closing date for responses to the consultation draft is 18 February. The ABD will be sending a robust response to all the issues raised in the draft circular. That response will be available on the website in due course. The consultation document is available on-line at

http://www.dft.gov.uk/stellent/groups/d ... 32869.hcsp if you would like to respond individually. You may also wish to bring the issues to the attention of your MP, as the proposals and are likely to have a significant effect on drivers throughout the country.

 

http://www.dft.gov.uk/stellent/groups/d ... 32869.hcsp

[mrbeige 0]

Just trying to work out what difference it makes? Does it just mean that they will be introducing more 40mph and 50mph speed limits in accident black spots?

[Timo. 0]

hope we get to do more than 70mph on the motorway!!

 

T.

[stevemac 0]

I would say that's a "complete load of bollocks", aimed at extracting more speeding fines from safe drivers who travel at speeds appropriate to the road conditions. :mad:

[Jim 2]

I reckon 80-90 on the motorway to be honest. If the car can go and stop well enough like some of the big BMW's, I dont see why they can not be allowed to do it.. its not like we are still driving old bloody Fords with drum brakes and stuff any more which is where these rules seem to hark back to.

[Furkz 0]

I reckon 80-90 on the motorway to be honest. If the car can go and stop well enough like some of the big BMW's, I dont see why they can not be allowed to do it.. its not like we are still driving old bloody Fords with drum brakes and stuff any more which is where these rules seem to hark back to.

 

i think they will prob loook into the amount of accidents on motorways rather than technology improved on cars tbh.... you and me my share similiar views and that prob why they will do something completely diff to upset the ppl who use the roads....... :roll:

 

why dont they have lanes based on diff bhp of cars

 

lane 1.... under 100bhp

lane 2.... 100bhp-200bhp

lane 3.... 200bhp +

lane 4.... (if there is one) for twatts in rep cars... coz they like the right hand side lanes A LOT

 

i think they couldnt do much worse :lol: