There to be seen - Expert Witness & Investigations Services

You are driving down a residential road at dusk, your eyes adjusting to the fading daylight, when suddenly, an oncoming vehicle blinds you with its headlights on main beam. This triggers your pupils to constrict and leaves you temporarily unable to fully perceive the surroundings. The traffic lights ahead have changed to red, but you do not see them and you cross into the junction, colliding with another vehicle, causing injuries to the driver. What could or should you have done differently? Invariably this will be the focus of an investigation and that investigation will need to consider how your vision contributed to the collision. This article explains how the eye works and its relevance in an investigation.

Sight is a remarkable sense, dealing not only with daytime conditions and complex visual environments for which, as diurnal creatures, we are ‘designed for’, but also lower lighting levels and to some extent the night-time nocturnal lighting conditions. However, unlike nocturnal animals, which have tapetal cells to reflect light within the eye, to maximise lighting on their retinas, we don’t. Hence our night‑time limitations. Daytime (photopic) conditions allow us to resolve complex scenes, detail and high colour rendition, defining myriad colour shades and tones.

Night-time lighting

Night-time low lighting levels finds our sight reverting to monochrome shading and tones, to the greater extent without any colour rendition. However, the change from colour lighting conditions to monochrome conditions is not like a switching on and off of one system to another, but is a melding of the two systems addressed by the cones (colour cells) and the rods (monochrome) cells on the retina of the eye, with neither system functioning optimally.

Broadly speaking the distribution of the retinal cells is not even, with most colour cells (cones) located toward our central field of view and with the monochromatic cells (rods) located around the periphery.

illustration of the operation of human retina

Illustration of the dispersal of cones and rods on the human retina

Dawn and dusk

A problem comes when we meet with mesopic lighting conditions, where at dawn and dusk the lighting is such that neither system functions fully – colours are bland and monochromatic tones are also not fully clear.

So how does this apply to driving? Good street lighting assists in raising the lighting intensity within a street environment toward full colour rendition, inviting our photopic colour cells to function in what is more akin to our favoured daylight conditions.

The irony of artificial lighting in an open environment is that this tends to produce and replicate the ‘mesopic’ conditions of twilight – neither acting efficiently upon our scotopic or photopic sensitivities.

The weakness comes with lower levels of street lighting, or before the street lighting comes on at the end of the day. This transitional condition finds the eye moving more toward one system before falling more toward the other system, with neither system functioning effectively.

Daytime/bright lighting

In order to deal with high levels of lighting, such as very bright sunny conditions, or headlights in a dark environment, the pupils of the eye constrict (miosis), to limit light entering the eye and temporarily bleaching the retina (transitory retinal blindness). Most of us will be familiar with having at some time inadvertently looked into an electric light, or caught the sunlight, and experienced the ‘orange spot’ effect that residually can last for several minutes on some occasions. That is, dependent upon the strength of the light source and the time of exposure.

The eye loses sensitivity to objects and scenes which fall in the foreground and which might present in lower contrast. In isolation the foreground in typical daylight, but without direct sun light in the field of view, allows good detection and detailed resolution, but where miosis occurs due to the sun position, or from strong lighting sources, contrast detection and resolution often suffer.

Whereas the pupil constricts very quickly, within milliseconds, the reversal process with the pupil dilating (mydriasis) occurs slowly and in a matter of minutes. This can be the difference between sufficient light entering the eye to resolve objects and not being able to do so effectively.

Mydriasis and miosis of the pupil. © Ophthalmology Breaking News.

An example of this is on an unlit road during darkness and when an opposing vehicle comes toward us with their lights on. The eye, in a state of mydriasis, with a wide pupil accommodating as much light as possible, then ‘shutters down’ the pupil to protect from the opposing strong light source. It is thereafter, for a notable period of time that the eye is not functioning fully for the otherwise dark environment. It is what is then not detected because of the light-limiting effect of the pupil that is of concern.

The Highway Code offers somewhat trite advice at Rule 237:

Keep your vehicle well-ventilated to avoid drowsiness. Be aware that the road surface may become soft or if it rains after a dry spell it may become slippery. These conditions could affect your steering and braking. If you are dazzled by bright sunlight, slow down and if necessary, stop.

Although as road users we cannot be expected to stop every time we are dazzled by opposing lights, or by the sun, perhaps by understanding a little more about how our sight functions might assist with accommodating its weaknesses under certain lighting conditions, we can adapt more readily when on the roads.