Experiments and analyses are carried out on the low decelerating and accelerating speed of a flexure crack running in a specimen of a glass plate. The specimen is subjected to a spherical indentation on its upper surface and rests on a knife-edge or ring foundation. The span of the foundation is comparable to the plate thickness so that a three-dimensional stress field is established. The stress distribution is determined analytically in terms of its stretching and bending components. It is shown that the variation of the crack speed depends not on the total surface tensile stress, but on the in-plane stretching stress. The experiment is shown to have produced only a small stretching stress so that the low varying speed of the crack, instead of its high limiting speed, was observed experimentally.