The International Civil Aviation Organization defines a runway incursion as any occurrence at an aerodrome involving the incorrect presence of an aircraft, vehicle or person on the protected area of a surface designated for the landing and take off of aircraft. This might be boiled down to incorrect presence on a runway. The Federal Aviation Administration defines a runway incursion as any occurrence in the airport runway environment involving an aircraft, vehicle, person or object on the ground that creates a collision hazard or results in a loss of required separation with an aircraft taking off, intending to take off, landing, or intending to land. Airservices Australia defines a runway incursion as unauthorized entry to an active runway strip by an aircraft, person, animal, vehicle or equipment. The slight differences in definitions may result in differences in numbers of reported incursions; and some harmonization or standardization in the near future is to be hoped for. EuroControl Statistical Reference Area (ERSA) has the following data for the number of runway incursions per million flights: In 1998, the rate of reported incursions was 3 per million flights. That doubled to a rate of six per million flights in 1999, and went of to 10 per million flights in the year 2000. 2001 saw a big jump to a rate of 23 reported incursions per million flights, a number which decreased slightly to 21 per million flights in 2002. For 2003, the last year for which complete data was available, the rate of incursions jumped again to 40 per million flights. These numbers are troubling. Several organizations, including the FAA and EuroControl had similar data on the major cause of reported runway incursions: Pilot Error was determined to be the cause in 51 percent of all reported runway incursions. Vehicle or Pedestrian Error was determined to be the cause in 29 percent of reported incursions. Controller error was determined to be the cause in 20 percent of reported incursions. EuroControl also classified several years of runway incursion reports according to seriousness. Their data was similar to FAA data using a similar classification: About 31% of reported incursions were classified as having No Risk. Another 34% were deemed to involve Some Risk. 30% were classified as having Significant Risk. That totals 95%. The other 5% of reported Incursions were classified as being Extremely Hazardous.1. The rate of reported runway incursions per million flights in 1999 was （ ）.