Perhaps its a bug but snapping doesnt work with the measurement tool until after a first point has been selected. So it is not possible to start a measurement exactly from an existing feature. This needs to be corrected.
A related request is to enable the "constrain to perpendicuar" mode for use with the measurement tool. This way I can (for example) measure the exact width of a road. As is I cannot snap the first measurement point to the road edge (previous paragraph) and I cannot constrain the measurement to be perpendicular to the first road edge.