Influence of Bus Rapid Transit (BRI) on mixed-traffic capacity utilization and its time headway implications.

Abstract

Bus Rapid Transit (BRT) systems are sustainable mobility interventions that provide commuters with fast, safe, and efficient mobility. Unfortunately, they have been characterized by fundamental traffic flow parameter anomalies, especially speed changes, with attendant consequences on capacity utilization, capacity differentials, and time headway implications. Consequently, this study was carried out to determine the influence of Bus Rapid Transit (BRT) on mixed traffic capacity utilization and implications for time headways. The overarching objective was to develop a capacity utilization criteria table for assessing roadway performance and determining the Level of Capacity Utilization (LCU) with and without the influence of BRT. Criteria tables serve as standards by which the performance of a roadways in terms of capacity utilization is decided. Traffic data at peak and off-peak periods were collected for a ‘with and without’ BRT impact study at four selected road segments along route R27, located on BRT major trunk route T02 which connects Atlantis, Table View, and Sunset City. Data were logged for 12 weeks continuously using an Automatic Traffic Counter (ATC). The study assumed that density was a function of speed and flow and hence was not directly impacted by BRT infrastructure based on the conditions at the time of the survey. It suggests that capacity utilization is triggered by variations in speed, and the attendant variations in density, capacity, and time headways were used to estimate capacity utilization rates and determine the LCU. Traffic data on speed, vehicle classes, and volumes were collected, and the results were analyzed. The collected traffic volumes were converted to flow using the South African passenger car equivalent (PCE) values. The results showed speed reductions with attendant differentials in other parameters such as capacity, density, travel time, and time headways. From the developed criteria table, the estimated capacity utilization rates for the BRT dedicated lanes scenario showed poor LCU at E (9% and 36%) across the four sites SS001 to SS004, under steady flow conditions. The capacity utilization rates of the mixed traffic ‘without BRT’ scenario showed fair utilization within the range of 37% and 79%, with average LCU at C across the four sites, whilst the capacity utilization rates of mixed traffic ‘with BRT’ scenario also showed fair utilization (between 40% and 75%), with average LCU C. The time headway implications induced by the capacity utilization and its differentials were also modelled, and the empirical time headway data were fitted to continuous probability distribution models. The Burr continuous probability distribution model, which is known for its compatibility, flexibility and appropriateness in modelling headway data under different traffic conditions, provided the best fit, having emerged with the largest Log-likelihood, and the smallest Akaike Information Criterion (AIC) values at 95% confidence and 0.05 significance levels across the four sites. At sites SS002, SS003, and SS004, it ranked first with the lowest AIC values of 3623.33, 4002.73, 3857.44, and corresponding largest LLH values of -1807.64, -1997.35, and -1924.70, respectively, while the Gaussian distribution performed best at site SS001, with the lowest AIC value of 4356.01 and largest LLH value of -2176.00, closely followed by the Generalized Extreme Value (GEV) distribution with the lowest AIC and largest LLH values of 4368.09 and -2181.03, respectively at off-peak traffic period. The Burr distribution however performed second best at peak traffic with the lowest AIC and largest LLH values of 4352.49 and -2172.23. The P-values, which ranged between 0.65 and 0.81 across the four sites showed the likelihood of the occurrence of the data sets under the null hypothesis. Hence the null hypothesis was accepted. In conclusion, the study showed that mixed traffic operations ‘with BRT’ and its associated minimized time headways, could significantly enhance capacity utilization. In view of the mixed traffic scenarios considered with and without BRT, it is hereby recommended for future research consider analysing traffic capacity through the development and application of microscopic fundamental diagrams, and a simulation of the time headway distribution of the mixed traffic scenario with BRT should also be considered. In traffic engineering practice, the curbside and mixed traffic designs are therefore recommended for implementation in future BRT infrastructure as the way forward for the South African BRT system for enhanced capacity utilization and sustainable mobility.