**2. Literature Review**

MaaS has been studied recently and there are numerous published scientific papers. With specific reference to MaaS, this section is divided in two subsections: the first concerns general aspects with particular reference to transport system models, and the second relates to surveys that have been carried out and calibrated models. Section 2.1 contains papers relative to general aspects concerning MaaS, while Section 2.2 focuses on the topic considered in the paper and is relative to the specification, calibration, and validation of demand models relative to MaaS.

#### *2.1. MaaS and Transport System Models*

In relation to general aspects, MaaS has recently been defined and investigated. It can be developed in a short time, and it is expected to be increasingly relevant in the future [2]. The first to introduce MaaS was Hietanen [3]. MaaS is the result of a union between transport systems and ICT technologies. In the context of MaaS systems, one of the major problems is due to the need to model user behavior and the consequences of decision makers' actions [4]. In this section, the state of the art is divided in relation to MaaS into the following: its definition and comparison; packages; experiences; supply models; and demand models.

In relation to MaaS definitions and comparisons, the scientific literature on MaaS is expanding and many different MaaS systems have been activated around the world.


In relation to packages, MaaS includes them for different transport modes with different payment methods [5–7]. In [7], the MaaS literature for defining the common elements that characterize a system, such as the integration of transport modes, fare options (bundle or "pay-as-you-go"), digital platforms, multiple actors, the use of technologies, demand orientation, registration requirement, personalization, and customization (the common elements were obtained from a table reported in [7]) were considered. In [1], the state of the art for an electric MaaS system was analyzed: starting from a MaaS system, they proposed an eMaaS system. In [8], the concept of MaaS, as well as the effects of MaaS for territories and citizens considering "environment, health and well-being, and social inclusion", and the consequences on urban governance were analyzed.

In relation to MaaS experiences, these services have been activated in some areas of Europe (i.e., a feasibility study for a MaaS system in London was developed [9]), while pilot projects have been launched in others. In Italy, a national research project for the study of MaaS was launched, a guideline for the development of MaaS in Italy was proposed, and two proposals have been published under the name of "MaaS for Italy" for pilot MaaS systems in metropolitan cities. Three pilot projects are currently supported through the first proposal in the cities of Milan, Naples, and Rome [2], with second proposals supported in Bari, Florence, and Turin.

In relation to transport supply models, MaaS consists of different components, such as institutional, management, immaterial, and material [9]. MaaS supply barriers relate to public–private cooperation, cybersecurity, business support, and coverage of public transport infrastructure [10,11]. In [9], an overview of transport operators in the city of London (such as fares offered, local coverage, and app availability) was considered.

In relation to transport demand models, some models have been analyzed and classified according to different characteristics: bundling according to modal choice, route choice, or both; RP (revealed preference) or SP (stated preference) surveys; or behavioral models or statistical analyses. In research papers, models can be calibrated [12] and the study area can range from a city to an entire country [12]. In [9], travel demand in London with data obtained from the London Travel Demand Survey and an SP-type survey was analyzed. In [13], a carpooling system to allow a reduction in demand during peak hours was studied, and in [14], movements from point A to point B starting from an itinerary with public transport and replacing some links of the network with carpooling (not dealing with the topic of MaaS) were considered. More details regarding the survey and calibration of demand models are given in Section 2.2.
