Criteria for widening of two-lane rural highways

The purpose of this study was to evaluate flow characteristics on two-lane rural highways and to develop criteria for highway widening. The study is conducted on two tracks: theoretical development of delay models and use of a simulation model to estimate the effect of certain parameters on delay and percent-time-spent-following. Models of delay are presented, as are the regions in which the traffic is stable or unstable. It was also possible from the simulation to obtain the percent-time-spent-following, which is a key parameter in determining level-of-service on two-lane highways.The accrued delay over the usable life of a two-lane highway pavement, assumed to be 20 years, was discounted to present monetary value. This was then compared to four typical construction costs for different terrain types. The threshold average daily traffic volumes were determined at the points where the present value of the accumulated delay was equal to the cost of constructing two more lanes. These threshold values can be used as criteria for widening a two-lane highway and converting it into a four-lane facility. Additional criteria, based on percent-time-spent-following, are also presented.