Workload surrogates commonly used in Emergency Medical Services (EMS) to evaluate the efforts of crewmembers throughout their workday have not been validated. This study investigated the relationship between workload assessments and surrogate metrics that can be calculated using conventional EMS dispatch data systems. Workload was assessed at random points in time during random shifts using the Visual, Auditory, Cognitive, Psychomotor (VACP) approach. Direct observation was used to assess the VACP scores of individual tasks commonly performed by ambulance crewmembers. Dispatch data was mapped to VACP score profiles through a trace-based simulation approach that adds random samples of tasks sequences and durations to the timestamps available in the EMS data. Pearson correlation and linear regression were used to quantify the relationship between the average time-weighted VACP workload and EMS metrics. Overall utilization and call response utilization explained time-weighted VACP the best, with strong positive correlations (0.88 and 0.89, respectively) and significant linear regression models (with and respectively). Call volume yielded the lowest correlation (0.61) and values (0.41) of the metrics studied, challenging its validity in fairly representing workload.