user: Question: What was the highest lap count for walker racing with a grid larger than 15?
Columns: ['Driver', 'Team', 'Laps', 'Time/Retired', 'Grid', 'Points']
Types: ['text', 'text', 'real', 'text', 'real', 'real']
Sample row: ['Ryan Hunter-Reay', 'Herdez Competition', 250.0, '1:59:12.397', 1.0, 34.0]
SQL:

assistant: SELECT MAX("Laps") FROM "Table" WHERE "Team" = 'walker racing' AND "Grid" > 15.0;