Why does the tunneling time of a particle or wave packet become independent of barrier width for thick enough barriers? The origin of the Hartman effect had been a mystery for decades. If the tunneling time becomes independent of barrier width, the implication is that the wave packet speeds up as the barrier is made longer. Not only does it speed up, but it speeds up by just the right amount to traverse the increased distance in the same amount of time. In 2002 Herbert Winful showed that the group delay for a photonic bandgap structure is identical to the dwell time which is proportional to the stored energy in the barrier. In fact, the dwell time is the stored energy divided by the input power. In the stop band, the electric field is an exponentially decaying function of distance. The stored energy is proportional to the integral of the square of the field. This integral, the area under a decaying exponential, becomes independent of length for a long enough barrier. The group delay saturates because the stored energy saturates. He redefined the group delay in tunneling as the lifetime of stored energy escaping through both ends. This interpretation of group delay as a lifetime also explains why the transmission and reflection group delays are equal for a symmetric barrier. He pointed out that the tunnelling time is not a propagation delay and "should not be linked to a velocity since evanescent waves do not propagate". In other papers Winful extended his analysis to quantum (as opposed to electromagnetic) tunneling and showed that the group delay is equal to the dwell time plus a self-interference delay, both of which are proportional to the integrated probability density and hence saturate with barrier length.