This investigation was concerned with the development of numerical measures for the significant geometric-design elements of two-lane highways located in rural areas. A mathematical model for the modulus of geometric aspects was solved to evaluate the ease with which vehicular traffic traverses a highway section with a particular combination of geometric features. These geometric moduli are predicated on the approach speed of the traffic and on the speed reduction produced by the design elements. Solutions to a multiple linear regression equation provided a reasonable estimation of the speed parameters. The geometric-design variables evaluated for two-lane highways were degree of curve, gradient, minimum stopping sight distance, and lane width. In addition, percent of out- of-state cars, percent of truck combinations, number of commercial roadside establishments per mile, and total traffic volume constituted the controls necessary for functional geometric design. The statistical model used for the generation of approach speeds and speed reductions was developed from the multivariate analysis of traffic-flow conditions observed on two-lane, rural highways. Finally, criteria for different levels of balanced geometric design were developed for use by the design engineer. The moduli for geometric characteristics permit the engineer to select various combinations of geometric- design elements that produce the same influence on traffic flow. This technique can also be applied to the evaluation of redesign and to the conduct of various highway planning surveys, such as needs studies and sufficiency ratings. /author/.
Abstract