Travel time is a performance index for measuring users' perception of the quality of service of a roadway. This paper investigated the extent of bus rapid transit (BRT) travel time differentials caused by mixed traffic flow 'with BRT' in Cape Town, South Africa. Travel time differential in the context of this study is simply the difference between the actual time required by both the drivers of BRT buses and other vehicles to traverse a roadway section under mixed traffic flow 'with BRT' and the corresponding travel time under mixed traffic flow 'without BRT'. Based on the hypothesis that a mixed traffic flow involving BRT buses and other vehicles would result in differentials in traffic stream characteristics, this study therefore estimated the extent of BRT travel time differentials between three scenarios namely: BRT dedicated lane scenario, 'without BRT' scenario, and 'with BRT' scenario. Consequently, 'with and without 'BRT impact study was carried out at four selected sites on regional route R27, in Cape Town, South Africa. Traffic characteristics of vehicles such as flow, speed, and density, moving on both the BRT dedicated lanes and their adjoining lanes, were logged using two automatic traffic counters (ATC) per road section for a period of twelve weeks, while flow-density models were calibrated to obtain the free flow speeds. The survey data was supported by design information harvested from the Western Cape Department of Transport and Public Works (WCDTPW), in Cape Town. Subsequently the US Bureau of Public Roads (US-BPR) travel time model equation was used to predict travel time over the roadway segments under the three scenarios and the outcomes compared. Results showed a minor speed reduction of 13.5% under the mixed traffic flow 'with BRT', which triggered a corresponding 11% minor increase in travel time. The paper concluded that mixed traffic flow 'with BRT' affects travel time, caused by speed changes.
