Commercial vehicles behaviour in sensitive urban contexts: a lesson from selected Italian cities

Urban freight transportation plays a critical role in ensuring the competitiveness and development of urban economies. Even if congestion levels rise, recent trends in transportation service requirements, such as just-in-time systems, contribute to an increasing proportion of commercial Vehicle Kilometers Travelled (VKT) in urban areas. The necessary replenishment high frequency along with the reluctance of end-users (retailers, private customers, etc.) to receive shipments outside given time-windows, make the distribution operations less efficient. Moreover, when such operations occur in sensitive areas, with premium built environment, very mixed land use and physical constraints (typically hilly morphology and/or narrow streets) distribution may become an additional cause to congestion. An in-depth analysis of current operations in selected cities in northern Italy led to investigate mutual interrelations among typical features: the urban environment, end-users behaviors, operational requirements, and road traffic congestion. More specifically, a database collecting several months of detailed truck activity records from an urban public distribution center located in the outskirt of the cities, allowed creating a taxonomy of recurring tours types in urban operations according to parameters such as percentage of time driving, and distance travel per stop. Moreover, available data led to identify a series of relevant binding constraints imposed by commercial activities and operational requirements on urban vehicle routing. As binding constraints, such as tour length and favorite time slots for deliveries, affect tours design and management, the paper aims at assessing the impact of such constraints and possible consequences on the distribution process efficiency; this leads to analytically model trip chain structure using continuous approximations. The impacts of increased travel times and uncertainty due to travel time variability on urban tour characteristics and efficiency are evaluated accordingly. The resulting vehicle trip generation is mostly driven by the time-sensitivity of the replenishment activity (frequency, route length, drivers’ working time, delivery windows) and congestion levels, as also stressed by the observation that constrains such as truck capacity turn out to be not so binding. In fact, it has been observed that vehicles occupancy rates tend to decrease when the amount of direct deliveries increase and delivery windows shorten. Calculations allowed to assess how VKT and Vehicles Hours Travelled (VHT) may vary according to the different tour types and to appraise the sensitivity of the delivery process to congestion levels (in terms of both increased travel time and travel time variability). The paper outlines relevant implications for data collection efforts: no realistic statements about the properties of urban freight tours in relation to their efficiency and contribution to VKT can be made unless there is disaggregated information on tour routing constraints and their temporal dimension, which suggests a revision of usual methods for truck trips generation based on commodity flows. Moreover, the application of the modeling approach to urban sensitive areas, such as that of the considered cities, can be considered as a step forward compared to those examples applied to less complex urban patterns.