Tony H. Reinhardt-Rutland, Reader in Psychology,
September 26, 2011

Ackery et al (1) show that risk to cyclists in collisions with motor- vehicles increases with the size of the motor-vehicle. This evidence may generalize to other types of collisions: consistent with Ackery et al are studies concerning different sizes of automobile with pedestrians (2,4) and collisions concerning different sizes of automobiles in general (3). One can infer that a smaller entity - both in terms of linear dimensions and mass - will likely come off worse than a larger entity. I use the term "entity" to refer to the road-user in conjunction where appropriate with her/his means of conveyance. Thus, the smallest entities are pedestrians and cyclists: larger entities refer to automobiles, pick-up trucks and articulated trucks.

The study of visual perception provides a number of factors regarding size. Time-to-collision is determined from visual expansion of the viewed entity, supplemented by factors such as physical size and the viewer's expectations: a smaller entity entails reduced visual expansion, so would be at risk of overestimated time-to-collision. Also important may be height in the visual field: for example, the driver's height above the road will be greater in a SUV than in a small hatchback. Height affects the upper extent of visual expansion generated by the viewed entity; this is reduced as height increases, so a SUV presents a greater risk than most other private automobiles (4,5).

A second issue concerns the potential power of the entity. A major division follows that for size: the slow speeds and accelerations of self- powered travel contrast with the speeds and accelerations of motor-powered travel. One obvious consequence concerns kinetic energy reflecting both mass and speed: there is a mathematically fourth-power relationship between speed and survivability (6).

The issues of size and power are hardly rocket-science. Yet they often have meagre effects in shaping road safety policy. One example from the UK: any attempts to reduce SUV ownership in urban and suburban areas where the size and power of SUVs are unnecessary in relation to their function have been ineffectual, even given the savings that can be made in a time of increasing fuel prices.


1. Achery AD, McLellan BA, Redelmeier DA. Bicyclist deaths and striking vehicles in the USA. Inj Prev 2011; 10.1136/injuryprev-1011- 04066l

2. Simms C, O'Neill D. Sports utility vehicles and older pedestrians. BMJ 2005;331:787-788.

3. Eberts RE, MacMillan AG. Misperception of small cars. In RE Eberts, CG Eberts (eds). Trends in Ergonomics/Human Factors II. North Holland: Elsevier 1985;33-40.

4. Stewart D, Cudworth CJ, Lishman JR. Misperception of time-to- collision by drivers in pedestrian accidents. Perception 1993:22:1227- 1244.

5. Cavallo V, Berthelon C, Mestre D, et al. Visual information and perceptual style in time-to-collision estimation. Vision in Vehicles VI. North Holland: Elsevier 1992;81-89.

6. Finch DJ, Kompfner P, Lockwood CR, et al. Speed, speed limits and accidents. Project Report 58. Crowthorne UK: TRL 1994.

Conflict of Interest:

None declared

Conflict of Interest

None declared