The last simulations only used existing measurements to generate the model. The Department for Transport statistics produces forecasts for teh estimated number of vehicles. This data is available from the following location:

https://www.gov.uk/government/statistical-data-sets/tra99-forecasts-of-traffic

This is the Numbers for the different scenarios

Scenario 1 | 106 | 115 | 124 | 130 | Scenario 2 | 105 | 113 | 120 | 125 | Scenario 3 | 103 | 108 | 113 | 115 | Scenario 4 | 103 | 108 | 115 | 119 | Scenario 5 | 110 | 122 | 133 | 141 |