The size of the stations was a response to safety concerns—overcrowding and a lack of exits had been significant factors in the 1987 King's Cross disaster—and an attempt to "future-proof" stations by designing from the start for a high use. Most platforms and halls are full only in a busy rush hour, all provide step-free access, dual exits at either platform ends, ventilation, as well as fireproof lifts. To ease flow extra escalators (at least 3 per station) were installed, totalling 115 over the entire extension, increasing the total number of escalators over the entire Underground network by almost half.