Tableau makes your life even easier: Your calculated field is not necessary, and you can get the results you want in five mouse clicks.
When you drag a measure pill into a view Tableau will automatically apply the measure's default aggregation (it starts out as SUM), and the computation will be performed in each cell in the view that is formed by the combination of discrete (blue) pills, which are usually (but not always) dimensions. So dragging trips into the view with county on Rows and year on Columns causes Tableau to calculate SUM(trips) for each combination of county & year.
Then, you can click on the right end of the green SUM(trips) pill (or right-click on the pill) and choose Quick Table Calculation->Difference and Tableau will turn that SUM(trips) into a difference from prior table calculation. The default Compute Using for the pill is Table(Down), which computes based on the county, when what you want is the year, so you click once more on the pill to set the Compute Using to year.
sample_county jtd edit.twbx.zip 27.1 KB