  We present an advance towards accurately predicting the arrivals of coronal
mass ejections (CMEs) at the terrestrial planets, including Earth. For the
first time, we are able to assess a CME prediction model using data over 2/3 of
a solar cycle of observations with the Heliophysics System Observatory. We
validate modeling results of 1337 CMEs observed with the Solar Terrestrial
Relations Observatory (STEREO) heliospheric imagers (HI) (science data) from 8
years of observations by 5 in situ observing spacecraft. We use the
self-similar expansion model for CME fronts assuming 60 degree longitudinal
width, constant speed and constant propagation direction. With these
assumptions we find that 23%-35% of all CMEs that were predicted to hit a
certain spacecraft lead to clear in situ signatures, so that for 1 correct
prediction, 2 to 3 false alarms would have been issued. In addition, we find
that the prediction accuracy does not degrade with the HI longitudinal
separation from Earth. Predicted arrival times are on average within 2.6 +/-
16.6 hours difference of the in situ arrival time, similar to analytical and
numerical modeling, and a true skill statistic of 0.21. We also discuss various
factors that may improve the accuracy of space weather forecasting using
wide-angle heliospheric imager observations. These results form a first order
approximated baseline of the prediction accuracy that is possible with HI and
other methods used for data by an operational space weather mission at the
Sun-Earth L5 point.
