22.36 The marking may be placed on the carriageway only at the request of the police and may be used in as many lanes as required. In order to reduce maintenance costs it should be sited in the middle of the lane.
22.37 Other markings may be used in association with speed cameras, at the highway authority's discretion. They enable the distance a vehicle has moved in the time interval between two photographs to be measured and hence the speed calculated. Such markings are not considered to be traffic signs and are not prescribed in the Regulations.
22.38 The markings must not be laid in the controlled area of pedestrian crossings, or in positions that will interfere with other prescribed markings and cause possible confusion.
VEHICLE SEPARATION MARKINGS
22.39 The chevron markings to diagram 1064 (see figure 22-9) are prescribed for use on motorways only.
22.40 The marking is intended to remind drivers to keep a safe distance from the vehicle in front and has been shown to be beneficial in reducing accidents. Chevrons are placed at intervals of 40 metres, ensuring that a safe distance is kept between vehicles travelling at 70 mph as long as drivers keep a minimum of two chevrons apart.
22.41 The performance of chevrons at very high levels of traffic flow is not known. It is likely that most drivers will reduce their spacing (and ignore the chevrons) as flow levels increase. The average peak hour flow encountered during trials was 4000 vehicles per hour. Caution should therefore be exercised when considering the use of chevrons at sites with higher flow levels.
22.42 The marking consists of a series of chevrons, marked in all lanes at the same 40 metre spacing, as indicated in figure 22-9. They should be laid over a distance of about 4 km, although this may be varied to between 3 km and 6 km.
22.43 The distance between successive series of chevrons should generally be between 40 km and 55 km. This need not be complied with rigidly, as specific site criteria are of greater importance. The start of a series of markings should be at least 1.6 km from the end of a previous entry slip road and should terminate at least 3.2 km before the next slip road. Significant gradients should be avoided. The Overseeing Department should be consulted if it is planned to use them on gradients steeper than 3%.
22.44 The Regulations require at least one sign to diagram 2933 "Keep apart 2 chevrons" to be used with the marking. However, a minimum of three such signs should normally be used, spaced at about
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