Improving road-side surveys for a better knowledge of road freight transport

Franois Combes Fabien Leurent Purpose The focus of this paper is to enhance our observation and knowledge of interurban road freight transport. It explores some possibilities for improving roadside freight surveys, usually used to gather origindestination data. Methodology To achieve this, new questions are added to a standard roadside survey form, and the enhanced survey form is tested through two surveys. The new questions relate to currently unobserved variables: the vehicle volume occupied by freight; the method of organisation (double crew, relays) used by the carriers; the existence of specific logistical imperatives; and drivers' compulsory breaks. Results The questions on volume constraint and carrier organisation prove to be the most informative. These two questions are thus two promising areas for improvement in roadside freight surveys. The questions about specific logistical imperatives and driver breaks prove less fruitful. Conclusion The paper concludes that it is possible, through minor modifications in data collection protocols, to significantly enhance observation quality in road freight transport activity. - Theoretical models, simulation models and various kinds of quantitative and qualitative analyses of road freight transport rely on databases describing the activity of road freight transport systems. These databases consist of observations obtained through data collection protocols. Many types of data collection protocols exist; they can be grouped into the following categories [1]: roadside intercepts, telephone interviews, mailout/mail-back, combined telephone and mail-back, personal interviews, internet, focus and stakeholder groups, commercial vehicle trip diaries, Global Positioning System (GPS) vehicle tracking, license plate match, and administrative surveys. Of course, each of these protocols has its strengths and weaknesses, and is therefore useful in different ways. This paper focuses on roadside freight surveys. Its objective is to examine whether general purpose roadside freight surveys can be improved by the widespread or even systematic inclusion of new questions in the survey forms. The paper also examines whether the additional information is worth the additional cost associated with it. Before proceeding to present the methodology, let us briefly discuss the scientific value of such efforts. From a microeconomic perspective, road freight carriers are producers which transform inputs (vehicles, energy, working force) into outputs (transport operations). The relationship between the inputs and the outputs is referred to as the technology, and its structure is most often assumed to be very simple: the amount of inputs necessary to carry a given amount of tons of freight for a certain distance is assumed to be proportional both to the weight of the freight and to the distance to be covered. In other words, the inputs are perfect complements, and it is thus legitimate to describe road transport by a per ton, per kilometer unit cost (with some refinements, e.g. distinction between commodity groups). While this assumption may prove sufficient for many purposes, a more realistic representation of the structure of freight road transport costs is desirable in some cases. This is especially true when addressing issues such as the regulation of truck weight and size , the conversion of commodity flows into traffic in freight transport demand models, and mode choice. Three of the potential difficulties that arise when trying to examine in detail the structure of costs of road freight transport are listed below. First, road freight transport services are produced using an infrastructure network and fixed inputs (vehicles). This has a direct impact on the productivity of freight carriers, and thus on freight rates. For example, on a given origin-destination pair, main haul and back haul freight rates are closely related [2]; these spatial dependencies become increasingly complex when more sophisticated network structures are considered [3]. This is closely related to the issue of empty return [4]. Second, road freight transport operations take place in a logistical context, which is an issue because the microeconomic drivers of logistical decisions are at present only partially understood [5]. In particular, shippers choices about shipment sizes and transport modes are highly dependent on freight rates, and particularly on the relationship between shipment size and freight rates, which is not linear. Conversely, freight rates are influenced by the choices of shipment sizes made by shippers. For a discussion on the microeconomics and econometrics of the shipment size choice, and more generally on the introduction of logistical principles in freight transport modelling, see e.g. [6, 7] Third, carriers optimise their productivity by grouping shipments whenever possible. Assigning shipments to vehicles so as to maximise the average loading factor of those vehicles is known to be a complex problem of operations research (the bin-packing problem), and it has a complex impact on the prices of freight transport [8, 9]. In addition, vehicle capacity is an important instrument of freight transport policy and, as such, has been the object of econometric investigations (see e.g. [10, 11]). Note that vehicle capacity is often measured in tons, whereas the relevant unit varies a lot with the context: it can indeed be tons, but also m3, pallets, etc. The study of these issues requires accurate observation and knowledge of road freight transport. In particular, road freight transport surveys need to be improved whenever possible to provide relevant information, especially as regards the constraints under which road freight carriers operate, and the way these constraints influence their productivity. The additional information obtained is useful from a theoretical perspective, as it allows a better qualitative understanding of road freight transport; from a modelling perspective, since improved data offer an opportunity to improve models; and from a decision support perspective, insofar as the capacity of a stakeholder to assess the effects of a decision is determined by the accuracy and quality of the data at hand. Within the wide range of road freight surveys, the characteristics of roadside freight surveys are particularly interesting. First, they are widely used, and yield a lot of data. Second, given the structure of the costs involved in implementing a roadside survey, the incremental cost of adding a few extra questions to existing forms is fairly small, provided they can be asked and answered quickly. Third, they involve face-to-face contacts between interviewers and truck drivers. As such, they generally obtain excellent response rates, and, more importantly, they offer special opportunities to obtain information on road freight transport operations: truck drivers are of course not freight transport managers, but they know more about the tr (...truncated)