If the cost is high, then make the admission charge high enough to pay for the costs. What is appropriate? $20 per car? $50? An appropriate admission charge will reduce the environmental impact as well as covering the costs.

Palo Alto has done everything it could to divert externalities to its neighbors, such as diverting two lanes of east-bound Sand Hill traffic toward Menlo Park, but has kept its Menlo Park neighbors out of its park. Shame!