I refer to the letter from Philip Jones (NCE 10 October) which offers no evidence that limiting visibility can control traffic speed.
It appears that the recommendations contained in the guidelines are based on inadequate research and are contrary to the Traffic Advisory Leaflet 2/96. This suggests a mean crossing speed of 1218mph for a 75mm flat top hump - ie the maximum having regard to the need to maintain clearance to the underneath of vehicles.
Design Bulletin 32 (second edition) states 'Restricted visibility in the absence of other precautions cannot be considered a safe means of reducing vehicle speed. For safety drivers must be able to see a potential hazard in time to slow down.'
Mr Jones then argues that the 12m stopping distance is more than enough. Indeed, as 9m is the distance travelled in the thinking time it is no surprise that once you have decided to stop it does not take 12m.
In my view these proposals are dangerous and the whole design guide should be withdrawn immediately.
Derek Bissell, Transport in Planning, 107 Worchester Lane, Four Oaks, Sutton Coldfield, B75 5NL