I. OVERVIEW

The structure of SimMobility Freight follows the three temporal dimensions of SimMobility. The long-term (LT) model simulates commodity contracts and goods vehicle overnight parking based on the data of establishments, goods vehicles, and the “overnight” parking supply. The mid-term (MT) model has three components, simulating “pre-day” logistics planning, “within-day” goods vehicle operations, and traffic movements (mesoscopic traffic simulation), respectively, based on commodity contracts and goods vehicle population. The simulation system of the short-term (ST) model is the same for both passenger and freight vehicle movements; it conducts the micro-simulation of vehicle behaviors on the road network, with detailed replication of them.

Figure 1. SimMobility Freight Conceptual Framework

II. COMMODITY CONTRACT ESTIMATION IN LONG-TERM MODEL

A commodity contract defines the origin and destination of a commodity flow, commodity type, contract size, and shipment size and frequency. Commodity contracts determine commodity flows and are the main inputs for simulating goods vehicle behaviors. The estimation of commodity contracts consists of three steps: freight generation, supplier selection, and shipment size/frequency choice. Three modules are developed for the three steps: Freight Generation Module (FGM), Shipper Selection Module (SSM), and Size & Frequency Module (SFM) (Figure. 1). The main input to the simulation is the list of establishments located in the study area, with the details of employment, floor area, industry type, and function type. SimMobility LT model has a functionality that constructs the synthetic population of establishments based on various statistics available to public agencies.

The model considers three levels of units for commodity flows. The simulation starts from the annual quantity, i.e. the annual total amount for an establishment to ship (or supply) or receive. The annual quantities shipped and received are called, “production” and “consumption” respectively, following the commonly used definitions in freight simulations. This annual quantity can be translated into one or more contracts. Each “contract” is associated with a shipper-receiver pair. Once established, a contract can be translated into “shipments”, the smallest unit, by estimating the size of shipments. A shipment is the fundamental dimension and represents the amount of goods that is transported together at the same time between a shipper and a receiver. The three modules in the commodity contract estimation correspond to these levels of units, respectively.

III. MID-TERM MODEL

The overview of the three components, “pre-day” logistics planning, “within-day” goods vehicle operations, and a mesoscopic traffic simulation (Figure 2) is provided as follows.

Figure 2. Process of the Mid-term simulation

“Pre-day” logistics planning

The objective of logistics planning module is to convert the shipments derived from commodity contracts to “vehicle operations plans”, which define the movements (i.e. tours and trips) of goods vehicles, including stop locations, activities associated with the stop locations (pick-up, delivery, overnight parking, etc.), and expected arrival and departure times. The logistics planning process includes modules for carrier selection and vehicle operations planning. The carrier selection module selects carrier(s) for each “outsourcing shipper”, which requires third-party carriers for its shipments, based on the distance from the shipper and goods vehicle availability. The vehicle operations planning module matches shipments with goods vehicles, makes an order of pickups/deliveries and generates the vehicle operations plans. The links between vehicle operations plans and shipment records are preserved throughout a simulation, and therefore, the relationships between vehicle trips and shipments are identifiable. Ongoing enhancements include the considerations of receiver time windows and collaborative urban consolidation schemes among others.

“Within-day” goods vehicle operations and mesoscopic traffic simulation

The models of goods vehicle operations bridge the vehicle operations plans to a mesoscopic traffic simulation. The route choice model predicts the route a driver takes for each trip (i.e. a movement from one location to another), given traffic conditions. The delivery/pickup parking choice model, on the other hand, predicts the parking choice depending on parking infrastructure availability and/or suitability and conditions such as waiting time.

The “supply” mesoscopic traffic simulator simulates traffic movements on a road network, which has a hierarchical structure with the information of links, segments (each link connects two nodes and consists of multiple segments), and lanes. By considering the two types of traffic status, moving and queuing, as well as the details of road and parking infrastructures (e.g. bus stops and loading/unloading bays), the simulator allows of a detailed and realistic simulation of traffic movements.