Optimal trigger speeds for vehicle activated signs were not considered in previous studies. The main aim of this paper is to summarise the findings of optimum trigger speed for vehicle activated signs. A secondary aim is to be able to build and report a dynamic trigger speed based on an accurate predictive model to be able to trigger operation of vehicle activated signs. A data based calibration method for the radar used in the experiment has been developed and evaluated. Results from the study indicate that the optimal trigger speed should be primarily aimed at lowering the standard deviation. Results also indicate that the optimal trigger speed should be set near the 85th percentile speed, to be able to lower the standard deviation. A comparative study investigating the use of several predictive models showed that random forest is an appropriate model to dynamically predict trigger speeds.