Understanding and Valuing Journey Time Variability
G Copley and P Murphy, FaberMaunsell; D Pearce, Highways Agency, UK
Congestion is an increasingly important issue for road users and the Highways Agency has a specific objective to ?take action to reduce congestion and increase the reliability of journey times?. Reducing travel times is a key issue in economic appraisal. Reducing the variability in travel times has not been treated with the same importance despite evidence from previous studies that variability in journey times is valued more highly by some people than journey time itself. Valuing journey time variability for road users is very difficult given the complexity of the subject. Traditionally researchers have estimated the ratio of parameters for journey time mean and standard deviation or ?reliability ratio?. More recent work carried out in the US has adopted a more behaviourally sound ?scheduling approach? which estimates early and late scheduled delay time and the probability of being late explicitly. This study builds on this work and has undertaken a detailed qualitative assessment of what travel time variability means to people in order to understand the most meaningful method of presenting journey time information. Its methodology has been innovative in presenting journey time information and also including optimisation of departure time choice in the SP experiment.
The study had two key objectives, which were addressed in two phases:
* Explore using qualitative research, what travel time variability means to people and to gain an understanding of the best methods of representing it.
* Measure the value that people place on journey time variability using stated preference techniques.
The qualitative phase consisted of depth interviews and focus groups. Travel Diaries were also used to collect detailed information about journeys made. The main purpose of this phase was to get a understanding of what journey time variability means to people and to explore how it impacts on journey planning. Different types of presentational techniques (linear, clockface based on previous research and histogram developed in this study) were explored to derive a preferred method of presentation for the stated preference experiment. The criteria used by respondents (eg journey time mean, minimum, maximum, range, standard deviation) when faced with a choice between two journey time distributions with different characteristics were explored.
The key findings from the qualitative research were that journey planning does not allow for ?extreme? incidents. Considerable buffers are built into schedules in order to avoid being late. However many business appointments are made with a degree of flexibility - the ?ish?, which recognises the difficulty in predicting journey times and the acceptability of being late for particular appointments. The different presentational methods were generally understood but the importance of different criteria used by travellers varied considerably.
The second quantitative phase consisted of an exploratory stated preference survey with 200 travellers,using the histogram method of presentation. The first stage of the interview consisted of a travel diary. This was followed by a computer aided interview in which respondents were presented with choices between different customised journey time distributions. A key aspect of the computer interview was that respondents were asked to optimise their departure times in the light of the distributions presented. They were then asked to choose between different optimised distributions. The analysis will derive parameters for mean journey time, scheduled delay - early and late time, the probability of being late and also standard deviation. It will be possible to produce separate models for different types of commuter. This will enable analyses using reliability ratios and the more behaviourally sound scheduling approach to be compared.
The results of the exploratory SP survey will provide an insight into how different types of commuters value journey time variability.
Association for European Transport