Relationships between design consistency and road safety

Geometric design consistency has an important rule in highway design. Identifying and treating any inconsistency on a highway can significantly improve its safety performance. Numerous researches have been undertaken to explore this concept including identifying potential consistency measures and developing models to estimate them. Operating speed is a measure of design consistency. It is the most visible indicator of inconsistencies because when the design of a roadway violates driver expectancy, the driver usually reduces the vehicle’s speed. Operating speed is defined as the speed selected by the drivers in free-flow conditions, and is normally represented by the 85th percentile speed. The speed reduction between two successive elements indicates the inconsistency experienced by drivers when travelling from one element to the next.