Appraisals compare the costs and benefits of doing something - the 'Project Case' (for example, building infrastructure), with a 'Base Case' (or the situation that would have occurred without the initiative, which is not, importantly, the same as a "do nothing" scenario).
Appraisal Summary Table 1 should include a clear and specific explanation of the base case, including reference to the key planning documents and transport plans which inform it.