Why not ask to see the risk assessment that requires it to be so and challenge it politely?

EN471 has 3 classes. Class 3 being the highest, however your query is probably more to do with The Safety at Street Work and Road Works Code of Practice. The highest level of protection for high visibility safety workwear is EN471 Class 3. This level of Hi Vis protection should be worn on trunk roads and all other roads forming the Primary Route Network (PRN), as well as roads displaying green/blue directional signs and all other roads where the speed limit is 40mph and above. Class 3 is predominantly specified by the amount of background material on show - an unsleeeved gamrent would struggle to reach Class 3 and is almost always listed as Class 2. For intermediate protection, which is also the minimum requirement when working on all roads where the speed limit is less that 40mph, EN471 Class 2 is required. The lowest level of protection, EN471 Class 1, should only be worn in areas of minimal risk, e.g. off road situations such as transport depots. If your drivers breakdown on a motorway and need to leave the vehicle then they should be wearing a sleeved Class 3 garment (this is a legal requirement in many EU countries).

Just to try and help explain the reasons for long sleeves, especially on fast roads. If someone is wearing just a hi vis vest, and is standing sideways on with their arms by their side, the amount of hi viz material is very limited. When with long sleeves, the surface area of hi viz is greatly increased.