## 2 Shortcomings of Urban Cold Transportation  
As it is, road transportation is responsible for 15% of global greenhouse gas (GHG) emissions, with road freight accounting for nearly 6% of global GHG emissions; this is more than the emissions produced by all aviation, shipping, and rail traffic combined, which amount to around 4.6% of global emissions [3]. There is a lack of literature estimating the GHG emissions of refrigerated transportation specifically, as required environmental data on companies' operations is scarce, though some sources mention figures: the US government [4], for example, estimates that cold transport vehicles consume around 20% more fuel than standard road vehicles, and a literature review on cold chain management [5] asserts that cold transport accounts for around 1% of global GHG emissions, noting that in developed countries, the share is even higher; for the United Kingdom, the figure is 3.5%. Though various figures are thrown around, there is a consensus in the field that refrigerated vehicles, still mostly relying on diesel engines to power their refrigerated units (RU), consume significantly more fuel than their non-refrigerated counterparts. Increased fuel consumption means that inefficiencies in urban road traffic--mainly congestion and idling in traffic--are also worse when it comes to refrigerated transportation. Similarly, the noise and air pollution caused by refrigerated transportation in urban areas can be greater than for other traffic, as the added fuel consumption leads to more exhaust fumes and the generators often used to power RUs are very loud. Also problematic from an optimization point of view is that nighttime is often the most fuel-efficient time of day to make deliveries as traffic is low; however, noise pollution at this time is at its most harmful as it can negatively impact people's sleep quality. In addition to the vehicles themselves, there are some other environmental considerations to make relating to refrigeration within the cold chain. A Chinese research paper [6] modeling different scenarios regarding the development of GHG emissions in the Chinese cold chain found that, in all scenarios, warehousing accounted for over 85% of cold chain operations' emissions due to the high energy consumption of refrigerated storage. This notion makes clear that it is also essential to consider the wider energy infrastructure surrounding cold transportation, as switching to renewable energy alone could eliminate most of the emissions of cold chain operations. Refrigeration in cold transport vehicles could also be improved significantly regarding sustainability. The refrigerants used in cold transport RUs often have high global warming potential (GWP), which means their lifetime environmental impact is high even though their in-use impact might be low. Additionally, as the cargo space of a cold transport vehicle is often just a single cooled space, loading and unloading can waste significant amounts of energy in repeatedly exposing the cooled space to warm outside air. Exposure to outside temperatures, as well as the breakdown or malfunction of a cold transport vehicle or its RU, cause quality degradation or the spoilage of the perishable goods being transported. Today, 13% of all globally produced food or about 720 million tons is lost during transportation each year [7]. In 2018, the production of this lost food alone was responsible for nearly 4% of global GHG emissions [8]. A 2021 study [9] found that studies and data collection about food loss during supply chains are still somewhat underdeveloped. It is thus difficult to estimate how much food is lost during cold transportation specifically, but a wider estimate states that more than a fifth of all food wastage can be attributed to loss due to failures in, or the lack of, cold chains [10]. Medicines similarly degrade and spoil in the absence of proper refrigeration, but they often have significantly higher production costs than food, and their wastage can deprive people of healthcare.