Study of the elements of driver`s overtaking sight distance on selected national highways of Bangladesh

Abstract

In case of two-lane two-way highways, drivers' overtaking manocuvrcs has significant bearings on the design and control measures. Parameter related to drivers' overtaking is, therefore, useful to the highway engineers in setting geometric design criteria and implementing traffic control measures on the highways. This research study has been undertaken with the objective of comp~ehensive understanding of the drivers' overtaking process on the national highways, (which arc basically twolane two-way type) of Bangladesh. An empirical approach based on extensive real field data has been adopted for the present study. Both video and manual data on the tramc operation of national highways have been collected for a period of over two months. Thcn the relevant data items have becn transformed in us~-able fonnal. Thc collccted data itcms havc bccn analyscd using di [rcrcnt statistical computer softwares. It has been observed that the whole ovel1aking manoeuvre can be divided into four clearly identifiable distance elements as suggested by earlier studies of AASHTO. These are trailing distance (d,), overtaking distance (d,), clear distance (d,) and lag time distance (d,). However, these clements in the case of national highways of Bangladesh have becn estimated and analysed in this research study. The analysis reveals that all the four components of ovcrtaking process follow n0l111aldistribution model. But there arc variations in the values of these clements depending on the vehicle size (larger/smaller) and type (motorised/non-motorised). Tolal overtaking sight distance required for the drivers' overtaking manoeuvre has been estimated from the four distance elements. This total distance has also been compared with the AASHTO design criteria. It has been found that the total sight distance required for overtaking vehicle on the selected national highway is smaller in comparison to that suggested by AASHTO for the same speed group. Regression analysis of,overtaking distance and speed difference of vehicles involved reveals that there is a linear relationship between the two. Generally, ovcrtaking time and distances dcerease with the increase of speed difference when other conditions remain same. It has also been observed that there is a strong relationship between the overtaking of vehicles and actual pavement width and shoulder condition of the highway. Generally, on wider highways with stable shoulder the number of successful overtakings are higher than the corresponding number in case of narrow pavement with unstable shoulder.