When tolls on the Dulles Airport Greenway were increased from $0.50 to $1.50, traffic decreased from 31,000 to 10,000 trips a day.

When tolls on the Dulles Airport Greenway were increased from $0.50 to $1.50, traffic decreased from 31,000 to 10,000 trips a day. Assuming all changes in quantity were due to the change in price, what is the price elasticity of demand for the Dulles Airport Greenway?