If I understand you correctly, I would recommend that you create a new field to accommodate this data. Therefore;
[Mode of Transport],
If(Match([Mode of Transport], 'Run', 'Walk',)>0, [Mode of Transport], 'Transport') as [Sum. Mode of Transport],
This will give you the flexibility to have either field displayed (or indeed both) and it will not require processing on the front end.
It would be useful to create a Cyclical Group of these two fields, so a user can quickly change between the two should they require a breakdown.
I hope this helps, let me know if I've missed anything.
All the best