Bogotá's segregated, four-lane TransMilenio system has a maximum peak-load capacity of 45,000 passengers per direction per hour (ppdph) on its busiest line.  The system uses modular median stations which serve both directions and enable prepaid, multiple-door, level boarding.  The average stop time is 24 seconds. Trunk-line terminals have integrated bicycle parking; the fare card opens a gate to a secure bicycle-parking area. Two lanes in each direction permit "Quickways" (local service on the inside lane combined with express service, skipping four or five stations at a time).  TransMilenio was described as a "model BRT system" in the National Bus Rapid Transit Institute's May 2006 report. It serves Bogotá with high-capacity, articulated, three-door buses. Bi-articulated buses are used on the busiest routes, and a smart card system is used for fare collection. Despite its large capacity, Transmilenio has problems with overcrowding.