Using the speed of professional truck drivers, the effectiveness of speed limit signs was compared for 1) a temporary variable-message sign that uses LED technology and displays the current speed limit (hereinafter “VSL sign”) and 2) a normal vertical road sign with retroreflective sheeting (hereinafter “ordinary sign”). Measurements were made at two locations in the Czech Republic. Each location provided a different scenario with a different displayed speed limit on both types of signs (60 kph or 80kph). Similar measurements were made in a driving simulation. Drivers were more likely to slow down in the presence of a VSL sign than in the presence of an ordinary sign on a real road, however, no differences were found in the driving simulation. The overall results indicate that VSL signs seem to be respected more than ordinary signs in reducing professional truck drivers’ speed. Therefore, VSL signs may be beneficial as a part of a telematic system and in places with high accident risk.