Falls are a leading cause of injury for people of all ages, but especially the elderly. While a large percentage of falls occur on stairs and inside buildings or homes, pedestrians walking or jogging on footpaths, stepping off kerbs and crossing roadways also fall. Because falls are largely unreported, there is little data on the magnitude and consequences of pedestrian falls. Falls in public spaces are a prevalent public health problem with significant human and economic consequences.
Because I have tripped so often on uneven surfaces, I don't trust paved paths. I will avoid them because I cannot be sure they are well maintained enough to keep me from tripping.
New Zealand has a safe system approach to road safety, under which road controlling authorities (RCAs) have a responsibility to minimise injury on their road networks, irrespective of whether the injury involves motor vehicles or not. In New Zealand, around 700 pedestrians are admitted to hospital each year as a result of slips, trips and stumbles in the road environment (including the footpath), and so the problem is not insignificant.
The highest proportion of trips and falls (34%) was sustained while stepping over a kerb. A further 18% were caused by irregularities in the path or road surface. Factors that amplified the severity of injuries included the road or path surface, pedestrians' inattention, type of footwear worn, and whether walking or running. Age is also a factor, as the elderly often cannot react quickly with their arms and so hit the pavement with their face or head.
The University of Otago and University of Canterbury’s Geofall project team reviewed St. John data, conducted an online survey exploring elderly persons’ experiences, and reviewed currently available products for reporting and detecting falls. Data was sourced from the Life and Living in Advanced Age study, interrail, hospitals, ACC, and St. John:
Based on an analysis of Ministry of Health data, falls are the leading cause of injury resulting in death amongst people aged 65+. Falls are also the leading cause of hospitalisation amongst nearly all age groups.
The next sections describe three types of falls: slips, trips and stumbles.
All can cause a fall. Commonly reported environmental factors involved in falls in public places include pavement cracks and misalignments, gutters, steps, construction works, uneven ground and slippery surfaces.
Slips are caused by inadequate friction between the foot and the pavement. This can be due to the material and construction of the sole of the shoe, the nature of the pavement surface, the presence of lubricants such as water, any surface treatments such as sealers, and the maintenance of the surface. Polished hard surfaces can become slippery due to the presence of fine dust or grit as well as by water or ice.
A pedestrian’s gait also affects the friction required for stability. Running requires more friction than walking. When people know a surface is slippery they can compensate by taking shorter steps and avoiding sudden movements.
Because of the complex nature of friction measurement and performance, international requirements are not uniform. The New Zealand Building Code DS1/AS1 requires a co-efficient of friction of 0.4 on level surfaces increasing by 0.125 for every percent of gradient. Table 2 of the code provides guidance on the suitability of a variety of materials. Joint Australian/New Zealand standards specify how to measure the friction of new and existing surface materials. For footpath surfaces, the sliding skid resistance of a wet surface is the critical test. This is measured by a pendulum tester using a rubber slider to simulate the sole of a shoe.
Because the amount of friction required depends on the context, the joint Australian/ New Zealand standards have moved away from a single value of required friction. Official guidance for applying these standards is provided in An introductory guide to the slip resistance of pedestrian surface materials HB197:1999 and Slip resistance of pedestrian surfaces–guide to the reduction of slip hazards.
The only matter under the control of those providing the infrastructure is the specification of the surface material and its treatment and maintenance. It is advisable to provide a safety factor by exceeding the requirements of the standards, thereby catering for activities such as running that require more friction.
A pedestrian trips when the surface being walked upon has an abrupt increase in height that is large enough to snag the toe of a shoe and cause the pedestrian to lose balance. A study of people walking shows that the toe is generally the lowest part of the swinging foot (ibid). However, just before ‘initial contact’ the foot pivots so that the heel touches first. The toe is the last part of the foot to lift off at the start of the swing and the heel is first to make contact at the end of the swing. Hence it is most often the toe that makes contact with the obstacle.
The clearance between toe and ground during the ‘swing phase’ is small. This relates to persons walking on an even surface, where the expectation is to place each foot on a surface of the same level as the previous step, as on paved footpaths and roadways. A study by Murray found toe to ground clearance in the range of 1–38mm with a mean of 14mm. Based on this data, a rise in height of 14mm would represent a trip hazard to 50% of the people tested. It is estimated that 10% of those tested would trip if the rise was 6mm. Unfortunately, older pedestrians who are most at risk lift their feet the least and are least likely to recover if they catch their toe on an obstacle.
The relative probability of catching the toe is shown below.
Based on this analysis, 6 mm is the maximum intervention standard for sudden changes in footpath level.
Trips can also occur when a stair riser is taller than expected, or not noticed. This is particularly likely where there is a single step.
Bird, Sowerby and Atkinson analysed the number of third party insurance claims for accidents on footways with respect to the height of footway defect. The exposure of pedestrians to defects of differing heights was also considered. It was found that the probability of an accident occurring increases logarithmically until a defect height of about 40mm, after which the probability remains constant. At higher step heights the defect is more likely to be noticed so the risk does not increase further. This is illustrated in the graph below for varying levels of pedestrian flow.
Stumbles happen when the surface is higher or lower than expected. Stumbles become more likely as undulations in the surface rise above 12mm.