Empty trips, what are the drivers? – a trip chain approach
Ole Kveiborg and Megersa Abate, DTU Transport
Freight transport is mostly analysed in rather aggregate terms. Descriptive statistics is on total amounts for certain defined geographical areas. The statistics are normally distinguished on vehicle types, goods types; describing the loads, the empty trips, the utilisation of vehicles. Model analyses are similarly in rather aggregate terms indicating overall patterns.
Another branch of analysis focuses on the logistics operations and the organisation of the supply chain, but do not combine this with the transport patterns described by general statistics.
The link between the two branches of analyses and the actual transport in a network is linked to the use of vehicles. The knowledge about what the heavy vehicles carry and how they operate has not really been investigated. Hence, we do not know much about the potential wasted traffic related to e.g. empty running.
The number of empty trip kilometres within Denmark has only increased slightly over the past decade, but total (national) traffic has declined in the same period. The qualitative explanations for changes in the number of empty trips (in the UK) are well understood and described (e.g. by McKinnon and Ge, 2006), but quantification and relation to transport demand has not been established. Holguin-Veras and Thorsen (2003) describes different methodological ways of modelling empty trips and relate them to loaded trips, but the estimated model is based on limited information.
The present paper bridges the gap between aggregate statistics and the logistic understanding of empty trips and capacity utilisation as well as link to empty trip modelling. The analysis is based on national trip diaries that are filled out for approximately 1200 vehicles every year and covering all trips undertaken during one week of operation. These data are used by Statistics Denmark to calculate national transport using heavy vehicles (above 6 ton gross weight). In the current analysis, we use the individual diaries to describe the different trip chains undertaken by the vehicles. The analysis identifies whether the trip chain patterns are the same within vehicle type and/or type of commodity. This type of analysis is to some extent similar to the analyses made on passenger transport using e.g. the national travel survey (“Transportvaneundersøgelsen”), where the detailed analysis of trip chains and individual’s behaviour give valuable insights into areas, where policies may have an impacts as well as quantifications of different transport behaviour patterns. The truck travel diaries provide similar possibilities in analysing trip chains and the “behaviour” of these vehicles. This detailed trip chain approach is unparalleled in other studies and enables us to link the aggregate statistics to the actual patterns of utilisation of the vehicles. We will discuss the relation to the decision makers who makes decisions that influence the individual vehicle, but focuses on the commodity supply chain rather than the vehicle.
A specific focus is on the empty trips. The paper analyses certain patterns of empty trips that can be identified and possibly be reduced. This is by definition a way of reducing traffic and thus energy consumption as well as CO2 emissions.
One area where the findings become highly relevant is in freight transport demand modelling. In particular it is intended to use them in strategic modelling. The model approach will be in line with the approach by Holguin-Veras and Thorsen (2003), but the investigation will contain more information as the specific trip chains are used for the analysis and modelling.
The paper will address and outline how this modelling can be undertaken, but the actual model will not be developed and/or estimated in the present paper as this is the next step.