Modelling signalised intersections’ capacity under the impact of minibus public transport in Harare, Zimbabwe.

Abstract

This thesis arises from the need to increase research efforts on the impact of varying driver behaviour on transportation facilities operational performance. Such knowledge is useful in traffic engineering and planning applications when applied particularly to developing cities. The interest in this topic comes from a scholarly clarion call on the need to investigate the impact of varying driver behaviour across regions. The present research is based on a detailed study of minibus (kombi) driver behaviour at six (6) signalised intersections in the Harare CBD. Further the research sought to evaluate the applicability of signalised intersections’ capacity estimation procedures and methods espoused in the US-Highway Capacity Manual (2000) under kombi driver behavioural characteristics. The research design separated two types of minibus driver behaviours, ‘in queue’ and ‘out of queue’ (weak lane discipline) behaviours. In queue lane behaviour was investigated through a rigorous statistical analysis of vehicle headway data gathered through Unmanned Aerial Vehicle technology. Weak lane discipline was investigated through an analysis of the lane creation and lane blockage data obtained through the conventional videography method. Key findings from this study are that kombi traffic generates two conflicting impacts on signalised intersections capacity. At one hand, kombis increase intersection capacity when in queue due to their lower Start Up Lost Time (SULT) values as compared to other passenger cars, higher effective green times and shorter headway values when a kombi is leading another kombi in queue whilst longer headway values were obtained when a kombi leads a passenger car and or vice versa. Evidence from these Measures of Effectiveness supported the hypothesis that kombi traffic increases intersection capacity when in queue. The study recommends for the development of predictive models for capacity estimation based on the relationships between kombi proportions, position in queue and the SULT. On the other hand, results from the weak lane discipline (out of queue) scenario have shown that kombi traffic have a negative impact on lane capacity when they abruptly decelerate and stop to either pick up or drop off passengers along outer lanes. The most outer lane would be significantly affected and the impact reduces from the outer lane into the inner lanes. This finding necessitated iii the development of kombi blockage factor (fkb), through modifying the bus blockage factor (fbb) in the formula for computing the Saturation Flow Rate (SFR) proposed by the HCM (2000). Another significant finding is that the weak lane discipline exhibited by kombi drivers renders computer based analytical methods for signalised intersections capacity estimation less useful. One computer program known as SIDRA INTERSECTION 6 was evaluated for this purpose. From this finding, this thesis recommends more research efforts in the possibilities of combining analytical and simulation models into a single ‘hybrid’ model that factors in driver behavioural parameters. This is envisaged to improve on the understanding of driver behaviour particularly in developing countries for the purposes of developing their own capacity estimation guidelines such as the US-HCM. Existing western developed models proved to be less useful in capturing such behaviour. The major recommendation from this thesis is the need to revisit the long-standing vehicle classification criteria for traffic engineering purposes, including intersection capacity analyses. Currently vehicles classification is based on the two main criteria that is vehicles dynamical (acceleration, speed) and mechanical (vehicle length, width, number of axles) properties, this criteria does not consider driver behavioural parameters as a ‘third’ criteria. Evidence from this thesis shows that it may be more prudent to augment the standard vehicle classification criteria with driver behavioural parameters rather than solely rely from the vehicles dynamical and mechanical properties. Whilst minibus traffic is conventionally classified as a Light Motor Vehicle (LMV) based on its mechanical and dynamical properties, this thesis has proved that classifying minibus traffic as LMV for traffic engineering purposes is problematic due to their unique driver behaviour.