Answer: While the Joinpoint program itself does not do projections/extrapolations, it is not unreasonable to use Joinpoint to project a few years into the future. To do so, one would need to extrapolate the final segment, and compute the standard errors of the projection, using the betas displayed in the output. The Joinpoint Team has had some success using Joinpoint for projections (validated against actual values when the data is rolled back several years so the projected values can be validated against actual values). However, a projection using Joinpoint may or may not be more accurate than other methods (e.g. time series methods), depending on the situation. Long term projections using Joinpoint are not advised. Also, AAPCs, since they are summary measures over an interval, are not as appropriate to use for projections as is the APC of the final segment.
A reference to an article which used Joinpoint for projections is:
Pickle, L., Hao, Y., Jemal, A., et al. (2007) "A new method of estimating United States and state-level cancer incidence counts for the current calendar year" CA: A cancer journal of Clinicians 57:30-42.