The planning assumption is that EU Member States will impose EU third country controls on UK goods at the end of the transition period. The Reasonable Worst Case Scenario (RWCS) is that 40-70% of trucks travelling to the EU might not be ready for new border controls on 1st January 2021.
For the short Channel crossings via Dover and Eurotunnel, 30-50% of trucks might not be border ready. The lack of capacity to hold unready trucks at the French ports, or to turn away freight prior to boarding in the UK, could reduce the flow rate to 60-80% of normal levels. This could lead to maximum queues of 7,000 port bound trucks in Kent and associated maximum delays of up to two days.
The capacity of the traffic flow could be improved within the first three months. A winter spike in COVID-19 could affect the traffic flow as well, as it would suppress freight demand. There will be a risk of queues and delays in the ports where operators have indicated their intention to deny boarding in the UK to freight vehicles that do not have appropriate documentation to enter the EU. Also, there remains a risk of continuing disruption caused by Schengen controls being applied rigorously at the juxtaposed controls at the Port of Dover and Eurotunnel.
 

The RWCS overall assessment of flow constraint is built using a series of sub-assumptions. The first sub-assumption relates to the number of HGVs using the short Strait routes or other Great Britain (GB) to Europe routes. The second sub-assumption relates to the proportion of traders who will have taken the necessary steps to ensure that the right documentation is in place to meet EU requirements – in other words, the level of trader readiness for EU requirements. The next step is to map the levels of trader readiness onto HGV volumes, to give an estimate for the proportion of HGVs which would be arriving at the border unready. The next critical step taken is understanding the UK port and Member State approaches to managing the new customs procedures and how they may treat any freight movements that arrive at the border unready due to missing or erroneous documentation. For example, France has developed a system which segregate cleared vehicles from vehicles requiring further formalities. These vehicles will proceed to specific holding areas, which are constrained by physical limitations at each port and will be hold there until they will be cleared to proceed or until customs officials will complete compliance activity. There will be also checks in the UK where commercial operators have plans to undertake pre-boarding checks on HGVs to ensure compliance with the customs authorities in Member States.
The ranges provided for flow (60-80%) represent an average over the week. Natural variations per ferry or across the day mean that the flow rate could be below or above this range at points in time. 
The worst disruption may not manifest immediately, but after two weeks as demand increases. A winter spike in COVID-19 could suppress freight demand, however other risks such as absenteeism among port or border staff could adversely impact flow.
It is expected that some logistics operators may choose to travel via alternative routes in the event of significant delays in ports.