Highway Research Record

Effect of Lateral Spacing on Overtaking Manoeuvres

Date of Start:August 2004
Indian Institute of Technology Madras (R, C)

Scope and Objective
(i) To study the effect of lateral spacing on overtaking manoeuvres at mid block sections.
(ii) To analyze the role of lateral spacing and overtaking manoeuvres on macroscopic traffic flow characteristics such as speed,      flow, and speed variability under congested mixed traffic conditions at mid block sections.

The scope of this work is limited to the following roadway context:
(i) Urban arterial road with a width of 2-3 lanes under one-way flow.
(ii) The overtaking manoeuvres considered are based on traffic rules, that is, vehicles overtake from the right side only.

Lateral spacing between vehicles is an important factor that can affect traffic flow characteristics. In addition to its effect on headway, lateral spacing can also, affect driver behavior and capacity. The interaction between vehicles is complex and varies across different types of manoeuvres at mid blocks and at intersections. Modeling the overtaking manoeuvres using lane-changing models is unrealistic, because of lack of lane discipline. In this context this study analyses overtaking manoeuvres in mixed flow condition.
In the present study, a multinomial logit model is developed to study the effect of lateral spacing on overtaking manoeuvres at mid-block sections based on the utility maximization principle.
The Haddows road in Chennai is chosen as the study area. Video recording method was chosen as the method for collecting data relating to the traffic flow at the road stretch. The image processing software is used to extract various traffic parameters such as lateral spacing, longitudinal spacing, volume, density, speed etc. The lateral spacing is estimated by using special scaling technique.
The modeling frame work is as follows. The user will select three choice (following, partial overtaking, full overtaking) based on utility of each choice. The utility which depends upon the exploratory variables such as lateral spacing and longitudinal spacing at diverging and merging opportunity, type of following vehicle, speed difference etc..The maximum likelihood method is used to maximize the log-likelihood ratio of the model. The SST (Statistical Software Tool) is used to develop multinomial logit model by maximum likelihood method. The model results give the estimated coefficients and t-statistic of various explanatory variables .The t-test is used to test significance of these variables.
The effect of longitudinal spacing and density on traffic speed and flow have been well investigated, the effect of lateral spacing has received less attention, and is particularly important in mixed flow conditions, since lane-discipline does not necessarily hold. In this context, the present study investigates the following research issues
(i) The effect of lateral spacing on the stream speed and volume
(ii) The effect of overtaking manoeuvres on stream speed, and volume
(iii) The influence of lateral spacing and overtaking manouevres on speed variability in the traffic stream.

Findings and Conclusions
Findings and Conclusions
(i) As per the speed-flow calibrated (regression model) model, free flow speed on the stretch is around 45.61 kmph, which is     consistent with the posted speed limit of 40 kmph and the observed speeds. Results show that as the volume increases,     speeds decrease, but the rate of reduction reduces beyond a certain point. The part of the speed variability is due to     mixed traffic, and partly due to high congestion observed in the study stretch.
(ii) The practical capacity of the stretch is in the vicinity of around 2400 to 2500 pcu/hour/lane, which is significantly higher      than the more conservative IRC recommendation of capacity for 3 lane uni-directional arterial roads.
(iii) The increase in travel time variability appears to affect all vehicle types more or less uniformly. But, among the three      categories of vehicles considered here, it is observed that the autos exhibit the largest variability in speeds (increase from      2.5 kmph to 6 kmph) whereas others exhibit a smaller variability (4-6 kmph)
(iv) The number of full overtaking manoeuvres shows an increasing trend with increasing volume. However, the rate of      increase appears to be slower than partial overtaking. The full overtaking manoeuvres increase by around 50% as volume      increases from 2000 to 2400 pcu/hour/lane, whereas the number of partial overtaking manoeuvres nearly double for a      corresponding volume increase.
(v) The increase in overtaking activity at high volumes also appears to contribute to the high level of variability in average      speeds
(vi) It is observed that as the lateral spacing increases, the average speed increases. Thus, the speed is not only inversely       related to longitudinal density, but also to lateral density.