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.
