Welcome to the board.
This feature used to be present in older versions of Streets & Trips, but was removed about 3-4 years ago, probably at the behest of the lawyers. This was around the same time that real-time driving directions were added to the product, and the big warning dialog is also the result of the lawyers.
It is unlikely that this feature will ever be restored, unless and until it appears in competing programs. Unfortunately, we live in a litigious society, and companies have to do silly things to reduce the likelihood of litigation.
(Speaking of speed -- your GPS allows you to determine the bias in your car's speedometer. This deliberate error is also introduced for legal reasons.)