In our modelling we try to achieve a balance between simplicity and complexity. However for the COVID-19 pandemic we needed to create a specific process to handle it's effects. To do this, we made adjustments to the mileage within the freight or passenger transport sectors for the specified amount of time. This results in a similar proportional bump in activity and energy use as seen in the real world. Importantly this reflected what happened in the real world, where people started travelling less, but didnt sell their cars, change their efficiency or so on.
Please note that we also need to match what the economy said was occuring, even if the data didn't seem likely. For more on the topic of matching what the economy says, see this page.
You can see an example of the effect of COVID-19 on passenger activity below:
And for freight activity (note how much smaller the effect is):
We did the same for non-road as well as international aviation and shipping (which are refered to as bunkers in the model).