Effect of reduced speeds on collisions at junctions

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    Sam Taylor

    Hi, I’m a first time poster. We have a site where a major A road meets a B road at a single lane dualling type junction. The collision analysis is showing a pattern of right turning collisions, from the minor arm on to the A road and from the A road on the minor arm. I’m considering a number of mitigation options, one of which would be to reduce the speed on approach from 50mph to 40mph.

    This would be enforced by average speed cameras or a fixed camera. I’ve read much research on the general benefits of reduced speeds on collisions, but i can’t find anything about collisions at junctions per se. Is anyone aware of any research, or better still any examples of where such an approach may have been used elsewhere? Thank you in advance.

    S Taylor


    One of the main problems with collisions at junctions is that drivers “Look but don’t see” to the right, then look left and pull out.

    Unless drivers focus up the road to the right, the brain can fill in the image with peripheral vision.

    Perhaps a lower speed limit on the approach might help give drivers more time.

    Rod Harrison

    E: chairman@glosiam.org.uk
    T: 07917851706


    Hi there, as you may be aware a quirk of speed limit definitions in UK means the speed limit on the links is 60mph, but at the junction is 70mph because there is a physical median. Drivers also seem to be feel more confident and this may further increase speed compared to ghost island layouts, even thought the conflicts are the same.

    We (Jacobs for Beds CC) treated a pair of staggered SLD junctions here https://www.google.com/maps/@51.8960307,-0.5855933,3a,75y,246.12h,104.29t/data=!3m6!1e1!3m4!1s62KdA2ntmWFhS2xTuaMBTw!2e0!7i13312!8i6656 by reducing limit to 50mph and using speed enforcement on the junctions and the short connecting link, around 2002 I recall. Casualties fell by over 90% and it was Britain’s most-improved road in EuroRAP assessment the following year. technically it was the most-improved two years running and in the top 5 in year 3 but they like to have different schemes apparently so only listed in year 1 with about 97% improvement. the only ‘after’ collision in 3 years was a drunk driver who ran off the road, no-one else involved.

    speed reduction works very well in 2 ways in my view. First the reduced speed limit flags to the driver a need for caution rather than an implied reason to speed up (dual carriageway = faster). Secondly, lower actual speed (if complied with) makes it easier for side road drivers to judge approaching vehicles (right-out errors reduce) and for main road drivers turning right to assess oncoming vehicles (right-in errors reduce). this reduces the likelihood of collision, and resultant severity should one occur, very much in line with the safe system approach. iRAP and safe system resources cover this in general about junction safety. Older drivers and the youngest disproportionately tend to be involved in these collisions for two reasons: they have poorer judgement of speed and position because the pre-frontal cortex develops last, into the mid-20s, and it deteriorates in later life (experience is not relevant to that issue), plus youngest drivers for the same brain-development reason have higher risk acceptance, meaning they judge less well, and are less aware of their shortfall than others. overconfidence is a third overlapping factor. At our A505 scheme, every errant driver was under 21 or over 70!

    It’s worth noting that within 2 years all new cars sold in Europe will have intelligent speed assist and will automatically comply with speed limits. even a small % of cars with this will effectively become mobile traffic calming keeping others to the limit. at very quiet times that won;t constrain speeding but it will help most of the time. Hope this helps!

    Kate Carpenter

    E: kate.carpenter@jacobs.com
    T: 07920143940


    It’s always worth checking out the IRAP Resources:


    This goes into detail on the various countermeasures available.

    I’d say the speed limit at the junction you mention would ‘ideally’ be 40mph but it’s hard to tell without actually carrying out a proper audit.

    Richard Owen

    E: richard.owen@saferroads.org
    T: 01295731811


    Hi interesting that it seems a reduction of speed is one of the first options and probably the more difficult to manage – namely enforcement. Has the trend identified that excess speed (excess for the speed limit). That we have no mention of the seriousness of the incidents, then perhaps they tend towards lower speed than higher.

    As above, have you identified WHY they are happening. Are they rushed due to other road features, is there really sufficient signage on the A road or the approaching junction. Are those turning from A to B rd hitting a vehicle in the B road or being struck by approaching A road vehicle- quite different causes of which only 1 is speed related.

    Can you assist as to what approaches you have made or considered and eliminated before using speed limits. Do we have the causes or are they minor incidents and more reliant upon ‘likely causes rather than as a result of a full causation investigation.

    IMHO, if the speed has already been reduced from 50>40mph, why has it failed – perhaps its because vehicle speed is not the causation.

    Andy Garden

    E: Blueacorns@gmail.com


    Hello, Sam.

    I would be interested in the study which flowed from your road traffic accident analysis, and what other options you are considering. As far as I am aware, there are no studies (of low quality or otherwise) of precisely the situation you describe. I think that has to be my answer to your question.

    I share Andy Garden’s concern, however, that you do not appear to have ascertained why the accidents are happening, and why you think reducing the speed limit would help.

    On the face of it, the junction needs to be replaced by a roundabout, which would simplify matters for all drivers, and does not require yet more aggressive “enforcement” measures in a road environment which is probably quite incompatible with the limit you propose. (My understanding was that the English police forces had said that they would not “enforce” in such circumstances – an unusually enlightened view.)

    Have you considered vehicle activated junction warning signs? These may have the effect of raising a main road driver’s state of alertness, and they don’t involve arbitrary speed limits and all the (rarely discussed) disadvantages of speed cameras. See:


    You may have gathered from the above that I disapprove strongly of the camera approach. I do – I think it has had a very damaging effect on our aims. I was therefore pleased to see Kate Carpenter’s reminder that ISA is now well on the way. However, while this should rid the country of the disgrace of speed cameras, it still may not solve your particular problem – whatever it really is.

    I hope this is of some help.


    Andrew Fraser

    E: andmarg@hotmail.co.uk


    It would be interesting to know what proportion of the collisions involved LGV (large goods vehicles), and indeed PCVs, emerging from the side roads: A high or disproportionate percentage of collisions involving large vehicles may indicate that they have trouble emerging onto a fast main road. For example, do the side roads provide access to an industrial estate?

    The performance of cars and vans has improved incredibly over the last 20 years and modern “ordinary” family cars have acceleration which would only have been possible with high end sports cars in the past. The performance of trucks has also improved but the effort has been concentrated on reducing fuel consumption and emissions, not on providing additional acceleration. The effect of this is that emerging onto a main road, or even a roundabout with a large truck has become much more difficult as it takes a lot longer to get an LGV moving.

    The effect is markedly worse if the roads are busy as congestion means cars travel much closer together and the gaps, and time available to emerge, reduces.

    Edward Handley

    E: ed_handley@hotmail.co.uk
    T: 01189883600


    I would say that speed is the main issue, or rather the different speeds or velocities of the different streams of traffic. Some vehicles travelling through the junction on the main road may be travelling at 60mph or more. Vehicles turning into or out of the side road are perhaps starting up from 0mph to merge or cut across the main stream. Speed may be misjudged for a number of reasons. There is always a potential for collisions where traffic streams are in conflict, but the speed differential is likely to increase the risk.

    Motorways avoid this type of junction.

    In Sunderland, where I use to work, we had a slightly staggered cross roads with a wide central median which attempted to accommodate the turning manoeuvres; a junction on the A690 between Sunderland and Durham. We proposed a roundabout, but as a temporary measure reduced the speed limit to 50mph. Actually both Sunderland City Council and Durham County Council reduced the speed limit of most of the road to 50mph over time.

    As I understand it, the severity of collisions reduced, but not necessarily the number. Since then the highway authority has changed the layout to prohibit right turns out of both side roads. This seems to have reduced the number of collisions, but whether it has moved them to the nearest roundabout in both main road directions, I couldn’t say.

    Charles Thompson

    E: charles.thompson@novo55.com
    T: 07883495768


    Interesting to read the different comments and views on this topic. In Scotland, we have similar issues at a number of priority junctions on our trunk road network where right turn movements are the dominant factor in injury accidents or collisions.

    We have avoided the use of a reduced speed limit at sites, primarily based on the guidance relating to speed limit assessments which suggests that; reducing a speed limit at an individual site to treat an accident problem should be considered as a last resort after all other traditional accident reduction measures have been tried. There is also the matter that there may be a number of similar junctions on a route so should these all be considered for consistency or would the public using them expect a reduced limit to improve safety or at least perceived safety.

    We have used junction warning vehicle activated signs which Andy mentioned, which have proved successful at a number of locations across the network.

    One final comment which I think was mentioned, by reducing the speed limit would likely result in vehicles on the main line travelling at closer spacing which would reduce the opportunity for a vehicle to turn right in or out of the junction and this could result in driver frustration leading to increased risk of a conflict.

    I hope this is helpful.

    Derek Williamson

    E: derek.williamson@transport.gov.scot


    DfT Circular “Setting Local Speed Limits” is very clear on this;

    “Speed limits should not be used to attempt to solve the problem of isolated
    hazards, such as a single road junction or reduced forward visibility, e.g. at a

    Simon Barker

    E: simon.barker@cheshireeasthighways.org


    It would be interesting to know a little more detail about the collisions which are occuring at the junction.

    For example, are the vehicles which are turning right out of the minor arm being struck by vehicles approaching from the right or from the left? Similarly are those vehicles turning right into the minor arm struck by vehicles approaching along the mainline or by vehicles coming out of the minor road wanting to occupy the same waiting space?

    E: andrew.russ@sweco.co.uk

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