Strobed Road Studs (Cats eyes)

Rev Adrian Kennard made this Freedom of Information request to Highways Agency

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The request was successful.

Rev Adrian Kennard

Dear Highways Agency,

I have noticed some sections of road now have strobed road studs, presumably solar powered. I observed a section of M40 south with these north of High Wycombe. These are very distracting and annoying.

Please advise :-

1. Where these are deployed
2. What frequency and mark/space ratio they are strobed
3. What studies have been done on the level of annoyance and distraction to drivers
4. What studies have been done regarding epilepsy related to these lights

Yours faithfully,

Rev Adrian Kennard

Thank you for contacting the Highways Agency Information Line on 21st November 2011 with your Freedom of Information (FOI) request. Your request has been forwarded to the relevant team who will respond within 20 working days under ref 13235536

In the meantime, please take a look at the following link: for more information regarding the Highways Agency's process for FOI requests

Katherine Partridge
Highways Agency | National Traffic Control Centre | 3 Ridgeway, Quinton Bus. Park | Birmingham | B32 1AF

Safe roads, Reliable journeys, Informed travellers
Highways Agency, an Executive Agency of the Department for Transport.

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1 Attachment

Dear Rev Kennard

Thank you for your enquiry to the Highways Agency Information Line on 21
November about road studs.

Most road studs are passive and rely on retroreflected light from
vehicleÂ*s headlights to provide visual guidance. In some situations the
road geometry is such that insufficient light from headlight fall on the
stud for it to be visible to most road users at the required distances
(approximately 60 to 80m for a high speed road). In these situations road
studs with active elements are required to provide the required
visibility; expect for a limited number of situations, mainly in tunnels,
active studs use solar powered LEDs.

It is not lawful for a road stud to incorporate a deliberate strobe
effect. It is however acceptable for road studs to show an intermittent
light source; but not at a rate that would constitute a strobe effect and
must appear as a continuous light to all drivers who meet the minimum
medical requirements to be granted a licence.

All road studs incorporating an active element must have type approval
from the Secretary of State for Transport for aspects not covered by
existing British Standards (BS EN 1436 Parts 1 and 2). This requirement is
stated in Directions 57 and 58 of the Road Traffic Regulations and General
Directions 2002 (S.I. 3113:2002). The European standard for active road
studs (EN 1463 Part 3) is currently being drafted; when this is published
the requirement for compliant products to be type approved will be

Previously active studs were type approved with intermittent flash rates
of 27hz, 54hz and 54hz with a double peak. The type approval for these
products has been revoked as provided for in Direction 58 in the above
mentioned Statutory Instrument. The current minimum requirement for type
approval is 100hz.

In 2007, in response to concerns raised about 54hz active road studs on
the A12 in Essex, the Highways Agency let a contract to the consultant
Mott Macdonald to investigate potential problems with Photo Sensitive
Epilepsy (PSE) as well as general distraction issues. As part of this work
two leading UK specialists in the field of PSE were consulted
(ProfessorÂ*s Seri and Harding of Aston University).

Due to the limited amount of background Neurophysiology data available
from either the UK or worldwide it was not possible complete all the tasks
relating to distraction. However, sufficient information on PSE was
available to provide a definitive assessment:-

Â*The majority of people with PSE are sensitive to flickering around a
frequency of 16-25Hz (1Hz is the equivalent of 1 flash per second),
although some people may be sensitive to rates as low as 3Hz and as high
as 60Hz. Figure 3.3 shows an adaptation of the results of an experiment
carried out by The Electrodiagnostic Centre (Harding and Jeavons (1994))
regarding the proportion of photosensitive epileptics responding to a
range of various frequencies. The stimuli were delivered using a Grass PS
22 photo stimulator, fitted with a diffuser and a grid (or quadrille)
pattern in which the spacing between lines occupied 20° min visual angle.
The photo stimulator was placed 30 cm from the eyes and subtended a visual
angle of 24°. The stimulus intensity was 1,363 cd/m2.Â*

Proportion of photosensitive epileptics responding to a range of

Â*People with photosensitive epilepsy usually develop the condition before
the age of 20, with it being most common between the ages of seven and 19.
More females than males seem to have photosensitive epilepsy.Â*

In response to the above findings the performance requirement for active
road studs increased to a minimum of 100hz.

Due to the lack of background information mentioned above it was not
possible to adequately identify if general concurs about distraction could
be proven or actively disproven. However, there was no evidence to show
that these roads studs, while easily identified by some drivers,
constituted a specific safety hazard. Due to inability to complete the
project the original Project Sponsor did not require or obtain a published
final report.

Further work in this are is being undertaken by the working group drafting
the European standard on behalf of CEN (European Committee for
Standardisation). The provided and input into the drafting process via the
British Standards Institution.

Active road studs have been deployed at the following locations:

South East


·         A3 Guildford to Ripley - MP 23/0 - 22/2.

·         A31 Stag Gate - extending approximately 180 metres either side
of the gate.

·         M25 studs between J6 through to the end of the easterly slip
roads at Clackets Lane Services on both carriageways.

·         A2 and A20 in areas that are prone to fog

·         Top end of the A21 

·         A27 exits from Southwick Tunnel for 1km Eastbound and Westbound

·         A23 Northbound between Pease Pottage towards M23 Junction 10a
for approx 2km.

·         M20 London Bound between Junction 5 and 4 for 1km to 2km


East Midlands


·         A5 at Wibtoft




·         A12 at Witham and on M40 j3-4 and 5-6 eastbound and westbound


West Midlands


·         M50 j1-2 westbound and eastbound


Yorkshire and North East


·         Westmoor junction on A1(T), south of Felton


North/South West


·         no active roadstuds installed


This list is correct as far as we are able to ascertain.  However, the use
of active studs was only required to be recorded from 2007.  For the areas
installed  prior to 2007 we do not have detailed records of the type of
road studs installed.  In order to get this information we would have to
install traffic management and deploy resources to inspect the locations.
 This would involve booking roadspace and would incur significant expense
and time.


Thank you again for your e-mail.  I hope that you are happy that this
response answers all of your questions.



Victoria Webber
Highways Agency | Federated House | London Road | Dorking | RH4 1SZ
Tel: +44 (0) 1306 878142
Web: [1]
GTN: 3904 8142

Safe roads, Reliable journeys, Informed travellers
Highways Agency, an Executive Agency of the Department for Transport.

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