Scientific data confirms what has previously only been a hunch, that there really are no cabs available for the afternoon rush. From 4 to 5pm, cabs usually change shifts, and the number of active available NYC taxicabs falls by nearly 20 percent compared with an hour before. The Taxi and Limousine Commission conducted a review of GPS records taken from thousands of cab trips over the past year. The trend holds regardless of time of year or day of the week. Taxi officials have never been able to gauge the extent of the shift changeover’s effect on service until this review, and they are still deciding what to do with the results.