Travel Time Variability of a Group of Car Commuters in North London

This working paper describes a study of the variability of journey times of a panel of fifteen commuters using one north London radial corridor, over a total of seven weeks in the spring and summer of 1987. The objectives were to gain preliminary information on the extent of journey time variability and on the commuters* perception of and responses to the variability they experienced. The procedure for selecting the panel and contacting them is described, together with the data collection method, which consisted principally of panellists using tape recorders in their cars to record the time at which fixed points along the route were passed on each day's journey to work. Early or late arrival and its consequences in relation to the day's tasks were recorded at the destination. Analyses are presented of the distributions of times of complete and partial journeys, of the relationship between journey duration and variability and of the effects of diversion in response to perceived traffic conditions. The safety margins needed to ensure given percentages of punctual arrivals were estimated and contrasted with those from previous studies. Further analyses are presented indicating the general unimportance of punctuality for these panellists, the effects of significant lateness when it did occur, and of the panellists' ability to predict their arrival times in advance. A study of highway link travel time variability and its causes was also carried out for cars at the a same time in the same corridor. The methodology, surveys and data processing of this parallel study are set out in ITS Working - Paper 278, and the results and analysis in ITS Working Paper 279.