Speeding in motorway work zones lowers the perceived safety of road workers and increases the accident risks of traffic participants. In a work zone, traffic participants share the road with road workers. Road workers are often vulnerable and feel threatened by vehicles passing with high speed. Traffic participants are also at risk since driving with high speeds in work zones increases the severity of risky events. This study aims at designing and testing a new speed management measure for a typical overnight Dutch motorway work zone. To design a new solution for the problem, a design method is chosen which incorporates human behavioural factors and system engineering. This design method is called Human System Integration (HSI). The current system is lacking flexibility and therefore the work zone and speed limit are sometimes interpreted as not credible. Also, some work phases are more dangerous than others and information about these phases may increase the awareness of the work zone with drivers. The designed solution aims to optimise the credibility of the total work zone system. To optimise credibility, the work zone should meet the drivers’ expectations. In this study a portable changeable message sign (PCMS) is added to the design to set driver expectations about the work zone. The possible displayed messages depending on the work zone phase are: ‘Placement of traffic measures’, ‘No visible work activity’, ‘Work on crash barrier’ and ‘Removal of traffic measures’. Furthermore, if there is no visible work activity in the work zone, the speed limit is increased from 70 km/h to 90 km/h. The design is tested on the A12 motorway in one work zone and compared to two different work zones on the A12. For the evaluation, minute aggregated loop detector data is available. To incorporate these spatial and temporal factors like on-ramps and truck share a Multiple Linear Regression Model (MLRM) is estimated. The PCMS has a small but significant effect of -1 km/h. The spatial impact of the PCMS is significant up till 2.7 km downstream of the sign location. The evaluation of the increased speed limit shows that speed limit compliance is higher during the 90 km/h speed limit in the ‘no visible work activity’ phase. Based on the field trial, the verification of the design requirements and validation of the designs performance, it is concluded that the design looks promising. However, it has not been researched what are the most important factors in the credibility of work zones. It can still be studied which kinds of messages improve the credibility of work zones. And while this field trial uses roadside systems, in-car technology may be even more effective in providing this kind of information. The HSI design method could be applied and tested in a more complex technical project. In most traffic systems and in the development of automated vehicles, human factors are important. It is thus recommended to study if the HSI design framework can be useful in this industry as well.