Community A doesn't take out a loan. The roads were built with exogenous funds. Starting in 30 years, Community A has to start paying X dollars a year in maintenance on it's roads. Community A taxes itself $X/2 per year and pays for a fire department with it.
In Year 30, Community B is built with exogenous funds. Starting in 30 years, Community B has to start paying $X a year in maintenance on it's roads. Community A has to raise taxes to $X, half of which goes to maintenance on it's roads and half of which goes to A's fire department, and Community B starts paying $X in taxes every year, half of which goes to maintenance on A's roads and half of which goes to B's fire department.
In Year 60, Community A and Community B each have to come up with an additional $X/2 per year in taxes or cut their fire departments.
In Year 30, Community B is built with exogenous funds. Starting in 30 years, Community B has to start paying $X a year in maintenance on it's roads. Community A has to raise taxes to $X, half of which goes to maintenance on it's roads and half of which goes to A's fire department, and Community B starts paying $X in taxes every year, half of which goes to maintenance on A's roads and half of which goes to B's fire department.
In Year 60, Community A and Community B each have to come up with an additional $X/2 per year in taxes or cut their fire departments.