Operation Stack was put in place along the M20 following a major counter terrorism operation which was carried out at the Port of Dover and the Channel Tunnel.
Kent Police and Border Force staff carried out enhanced checks on every vehicle and passenger crossing the Channel, which created traffic queues of up to 15 miles, and waits of up to nine hours.
Counter Terrorism police requested the enhanced security checks at all UK Ports as part of what they called "ongoing operational activity".
That operation has since been stood down and police say they would like to reassure the public that there is no threat to public safety.
The specific reason for the operation which spanned Tuesday and Wednesday remains unclear.
How does Operation Stack work?
Freight within Operation Stack is separated into two queues on either side of the carriageway (on the hard shoulder and lane 3) – one for tunnel traffic and one for port traffic.
Space is left in the middle of the motorway to allow for emergency vehicles e.g. freight breaking down, medical emergencies etc.
Queues are released when capacity is available at the ports.
You may see stretches of the motorway clear from time to time as groups of freight are moved down the Operation Stack queue in stages when capacity becomes available.
Non-freight traffic is filtered at the first junction that Operation Stack is implemented from and diverted onto the A20 to rejoin the M20 in front of the Operation Stack queue.