## 1 Introduction  
In 2019, the net income of the global courier, express, and parcel market was over 330.4 billion EUR, with an almost 10% percent increase from the previous year [1]. Operators such as DHL, TNT, and FedEx primarily provide air, road, or combined express parcel delivery services of packages for various customers [2]. Although many modes of transport are used for parcel transportation, the packages travel a significant portion of the total transportation period in small vehicles like vans that collect from and deliver to senders and receivers. The shipments of courier packages are usually small in size, light in weight, and time-sensitive products. The transport environment of these shipments produces physical events that can affect the integrity of the products and can be the primary source of damage [3]. The most common physical events include vibration, impacts (shocks), and compression [4]. These physical conditions are random, so it is important to measure and analyze them in order for packaging engineers to gain a better understanding and design suitable protective packaging systems. It is very important to avoid those damages during distribution that can cause losses by making the product unmarketable at its destination. On the other hand, the cost-effectivenessissue is also important for engineers to design optimal packaging that simultaneously serves to protect the goods and reduce environmental impact. The vibration environment during distribution can be based on factors such as the vehicle body structure, type of vehicle suspension and tires, road roughness, vehicle speed, and actual payload [4]. Previous studies have measured and analyzed vibration levels for parcel delivery vehicles and CEP (Courier Express Parcel) modes of transportation [3; 5; 6; 7]. Factors involving changes in vibration levels, including payload, suspension (leaf spring vs. air ride), and road condition, for small parcel delivery vehicles have also been previously investigated [8; 9; 10]. Former results have shown that significant vibration levels increase when a small vehicle such as a van travels on poor roads, with a higher speed, and with less payload. All these studies have only measured the vibration levels on the platform of the vehicle, but did not take into consideration that most of these parcels are delivered in a stacked state due to the relatively limited capacity of small vehicles and the large quantity of goods. In this context, the vibration levels in stacked loads (in layers) will be completely different from the direct input that can be experienced on the platform. Moreover, these shipments do not act as a palletized unit as they are not fixed to each other; furthermore, they are loaded into the vehicle with free movement spaces around them. This phenomenon can be seen in Figure 1a,b. Nowadays, the above-mentioned circumstance can be often observed in parcel delivery methods due to the very intensive flow of goods in periods where customers' orders occur via online stores, such as in situations like COVID-19, when personal purchase of goods is strictly limited, or during festive seasons when the malls are overcrowded. However, we could not find any published field research that measured and analyzed the vibration levels in stacked units for parcel delivery methods. Previous results have mainly focused on vehicle vibration as an input to normal cargo, but in reality, there is not enough knowledge to estimate the conditions of stacked parcels, which could suffer more significant damage than parcels in normal transport solutions. Therefore, this study presents new measured and analyzed data that can help packaging engineers gain a better understanding of this distribution environment and design appropriate protective packaging for CEP packages. The main goal of this paper was to measure and analyze the vibration levels in a three-layer unit in van shipments for average delivery routes using tri-axial data recorders.