Travel Time Variability versus Freeway Characteristics

Travel times on freeways are variable due to variations both in demand and in capacity. For example, the volume of traffic which a road can handle on one day may cause severe congestion on the next due to for example adverse weather affecting capacity. One group of factors affecting capacity are geometrical characteristics of freeways, such as the number of ramps (on-ramp or off-ramp) and weaving sections per unit road length and their physical characteristics. It is expected that these characteristics also influence travel time variability. Empirical analysis on the basis of a large database of travel times on various freeways shows that travel time variability is strongly affected by these geometrical aspects. Above a certain threshold of ramps per unit road length, travel time variability increases sharply as their number increases. It is also found that too short acceleration, deceleration lanes and weaving sections increase travel time variability significantly. As a result of this study, there are thresholds found for designing reliable infrastructure networks with respect to putting clear constraints on the number of ramps and weaving sections and their physical properties