Estimation of critical length in ascending grades of two-lane rural roads

Abstract

The critical length of grade on two-lane rural roads is used as criterion to assess the need of ascending lanes for heavy vehicles. To estimate it, a speed profile is needed, which depends on the road’s grade and length, and on the physical, dynamic and mechanical characteristics of heavy vehicles. The Chilean standard uses a speed profile for a design truck that have a weight-to-power ratio of 120 kg/kW and an entrance speed of 88 km/h. This unique model prevents from studying cases in which the entrance speed can be different or in which the design vehicle is significantly different to the design speed of standards due to local conditions and consequently, does not allow estimating realistic critical lengths of grades. In this work, speed profiles models based on acceleration and force equilibrium were simulated, considering weight-to-power ratios between 120 and 263 kg/kW, entrance speeds between 80 and 100 km/h. Several values of critical lengths of grades were estimated, which offers engineers more options when designing ascending slopes.