Using mobile eye tracking to record the gaze behaviour of pedestrians suggested that looking towards the oncoming pavement is an important visual task. One reason for doing this is to scan for potential trip hazards.
We detect trip hazards with peripheral vision. Likely hazards are transferred to foveal vision by head and eye movements for detailed inspection, but that is not essential for the gait changes taken to avoid a trip.
We constructed an apparatus to measure how changes in illuminance (0.2, 2.0 and 20 lux) and S/P ratio affect detection of peripheral targets, using younger and older test participants.
The results revealed an important point. At 2.0 lux, there were no significant effects of age or S/P ratio, and no significant benefit to higher light levels. At 0.2 lux, younger people had better detection than older people, and higher S/P ratios improved detection for both age groups.
The experiment was repeated to confirm the results. This used only younger test participants but increased the number of illuminance levels (0.2, 0.63, 2.0, 6.3, 20 lux) to enable the detection vs illuminance relationship to be more precisely characterised.
Given that the detection vs illuminance relationship can be defined, how does this inform design guidance? One approach would be for the local authority to provide sufficient lighting to counter a solicitor’s claims of neglect following a trip incident. We therefore studied the guidance given by solicitors to their potential clients.
A third experiment was conducted, but this time using a larger scale apparatus intended to better represent walking in real environments.
In these trials the pedestrian was walking on a treadmill to simulate the cognitive demand of walking (maintaining balance) and a dynamic fixation task was used to simulate natural gaze behaviour.
It was found that higher illuminances lead to increased detection probability with this effect reaching a ceiling in the region of 2.0 lux. Observer age and light source S/P ratio affected detection only at the lowest illuminance used in this experiment (0.63 lux).
One purpose of the dynamic fixation task in the third experiment was to promote foveal fixation on fixation mark, and hence that the target was in peripheral vision. To test this we used eye tracking to record gaze behaviour during detection.
We reviewed the eye tracking records to find out how we scan for trip hazards. This is an interesting analysis because eye tracking identifies foveal fixation but hazard detection is peripheral vision task. Our estimate is that hazards are typically detected when 3.4 m ahead.
We used accident data and foot clearance data to determine the critical height of a pavement hazard – the minimum size likely to cause a trip if not seen. This was found to be 10 mm. It is smaller than the 25 mm rule-of-thumb previously used; it is therefore more difficult to see, which has an impact on the lighting needed after dark.
To detect a 10mm obstacle at a distance ahead of 3.4m, results from the third experiment suggest horizontal illuminances in the range from 0.22 lux to 0.93 lux are required, depending on the S/P ratio of the lamp and the age of the observer.
Consideration of these results alongside those of Boyce (1985) suggests that a minimum photopic illuminance of 1.0 lux is sufficient light for pedestrians of all ages to safely detect and avoid trip hazards under any type of lamp.
In the experiments we have conducted previously, trip hazards were represented by raised objects. However, surface depressions (e.g. potholes) can also lead to trips, stumbles and falls. In this experiment we compared the detection in peripheral vision of raised and depressed pavement surface irregularities. In terms of detection, the results did not suggest a significant difference.
Fotios S, Mao Y, Uttley J, Cheal C. Road lighting for pedestrians: Effects of luminaire position on the detection of raised and lowered trip hazards. Lighting Research and Technology 2020; 52(1): 79-93.
Following from the key tasks proposal of Caminada and van Bommel, work in pedestrian lighting has focused on the detection of trip hazards. However, slip hazards may be an equally prevalent problem. This brief item was written to raise awareness of slip hazards, which are rarely considered.
Experiments are ongoing to test the proposal than 1.0 lux is sufficient:
1) By considering depressed hazards (pot holes) in addition to raised hazards.
2) By considering variations in the position of the hazard relative to the light source.
3) By considering the effects of prominence and parallel cognitive activity by conducting trials mixed with facial emotion recognition trials.
4) By considering the impact of glare.