Drivers from Bulgaria and Romania are using devices that jam traffic police radars in order to speed freely on Greek highways, officers from northern Greece said yesterday. Police in Serres said that they had detected some 150 drivers using the blocking devices over the last few months. Four drivers from Bulgaria were arrested on Thursday on the Serres-Promachona national road after officers found the gadgets in their cars. The devices are legal in the neighboring countries and many drivers do not know they are breaking the law by using them in Greece, traffic police said. Officers also revealed that a Greek man was arrested on Thursday on the Serres-Promachona highway after he was caught driving his Ferrari at 250 kilometers per hour in a 90-kilometer-per-hour zone.