The safety implications of passenger car downsizing is a subject of much current interest. Vehicle-vehicle collisions and vehicle-roadside accidents are the two major accident modes that may be affected by passenger car downsizing. This paper reviews the nature of the current roadside fatal accident problem and the implications of passenger vehicle downsizing on major types of roadside fatal accidents.

Rollover accidents comprise the largest category of current roadside fatalities and the number of these accidents is likely to show a major increase as the size of passenger vehicles in the vehicle fleet decreases. Utility pole collisions, currently accounting for more fatalities than any other manmade roadside object, are expected to increase significantly. Other currently important classes of roadside fatalities such as guardrail and bridge rail collisions may show a much lesser overall change, if any, as cars become smaller;​ however, significant changes in impact behavior are being identified in individual design types.

The quantitative estimates of these various downsizing effects on future fatilities are at best of a very rough and first order nature. The implications from currently available information suggests that the outside of horizontal curves may be locations which justify special attention when decisions are being made to clear obstacles, flatten roadside slopes, or install guardrails.

Guardrail may be cost effective in front of more roadside slope combinations than indicated by past accident research. Adjustments in the guidance given bridge rail designers as to undesirable geometrics and in the maximum desirable breakaway levels of signs and luminaire supports are other implications from current information. Research planned or now underway is intended to further clarify these issues.