The paper has two objectives: 1) Presentation of a new enhanced consistency model which considers the geometric features of highways including both horizontal and vertical alignment characteristics; 2) Analysis of the relationship between the consistency result and expected safety level of a planned highway. The proposed consistency model is based on the speed profile characteristics of both cars and trucks. The speed profiles are derived from models developed in Texas for predicting speeds on curves and tangents and on the AASHTO 2001 speed curves for trucks. The speed at any given point along a highway is determined as the minimum speed governed by either the horizontal or vertical alignment features. The consistency is based on the amount of variability of the speed profile. This variability is measured by the deviation of the profile from the average speed and by the standard deviation of all individual speeds. The exponential consistency prediction model that was developed is dependent on these measures of the speed variability and also on the amount of separation between the speed profile of cars and trucks. It is assumed that as this amount of separation increases, the design consistency decreases. The calibrated model provides thresholds to distinguish between good, acceptable and poor consistency. A software package, HSPC, for calculating the consistency of any highway based on its horizontal and vertical alignment has been developed. An analysis of accident rates and design consistency showed that higher consistency values were associated with lower accident rates on rural two-lane highways. This is a significant finding that may be applicable in planning stages, when highways could be evaluated and sorted according to their consistency scores which, as shown, is significantly related to their safety levels.
Samenvatting