The best angle for traffic light louvres

Certain traffic signals in the UK are covered with louvres so that the light in the signal is only visible when your eye is aligned with the gap between the louvres.  Not everyone is happy with them.  In an online discussion, I found the following explanation:
The proper use for the louvred aspects on signal heads is to avoid conflicting signals being displayed. For example, if you have a pedestrian crossing within a few yards of a signal controlled junction, if there is a chance that drivers may misread an aspect being shown on the pedestrian signals [green] as the aspect for the junction then the louvres are fitted to ensure that you only see it from a certain distance from the stop line. This is also the case for the pedestrian aspects as well if the phasing is not all the same and you have another button to push in the centre refuge. It may appear 'difficult' to see in some circumstances but it does avoid confusion as you are required to check you are on green before proceeding.
However, not all drivers were happy with the introduction of such louvres, because they were driving trucks and the angle of the louvres was set for the benefit of car drivers, much lower on the ground.  The discussions did not mention the problem for cyclists, whose eye level is also higher than for car drivers.  As a cyclist in Exeter, there are some louvred lights that I cannot see unless I am close to them, because the light is "directed" down towards a car driver.  Geometry says that if the driver is supposed to first see the light when they are 40ft away, a cyclist or truck driver is likely to be about 20ft away when they see it.
 The louvres in this picture are set horizontally, with openings at the base.  Other examples do not have the openings, and have louvres set at an angle ... and as the angle is a number, there is mathematics involved and someone should try and optimise that number.   What criteria to use?  I leave it as an exercise in multiple objective decision-making


  1. Still nothing definitive online. Not even a patent spec.
    NB: The louvre angle is ADJUSTABLE but across Newbury Berks NOT ONE HAS BEEN ADJUSTED.
    This shows the specifying authority and the fitting operatives HAVE NO IDEA WHAT THEY ARE DOING!


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