System for measuring the length of a road section, comprising two on-board devices (2A, 2B) suitable for being installed each on board a respective vehicle (4A, 4B), each on-board device (2A, 2B) comprising: - an overtaking sensor (21A, 21B) configured to provide information indicative of any overtaking between the vehicle (4A, 4B) on board of which said on-board device (2A, 2B) is installed and the other vehicle (4B, 4A) on board of which the other on-board device (2B, 2A) of said system is installed; - a traveled distance meter (20A, 20B) suitable for measuring the distance traveled by said vehicle (4A, 4B); and - a processing unit (22A, 22B) configured to: - when it detects a first overtaking between said vehicle (4A, 4B) and said other vehicle (4B, 4A) on the basis of said information provided by said overtaking sensor (21A, 21B), enable a measurement of the distance that said vehicle (4A, 4B) is traveling along said road section by said traveled distance meter (20A, 20B); and - when it detects a second overtaking opposite to said first overtaking between said vehicle (4A, 4B) and said other vehicle (4B, 4A) on the basis of said information provided by said overtaking sensor (21A, 21B), disable said measurement of the distance that said vehicle (4A, 4B) is traveling along said road section by said traveled distance meter (20A, 20B), wherein said first overtaking is such that said vehicle (4A, 4B) overtakes said other vehicle (4B, 4A) and said second opposite overtaking is such that said vehicle (4A, 4B) is overtaken by said other vehicle (4B, 4A), or wherein said first overtaking is such that said vehicle (4A, 4B) is overtaken by said other vehicle (4B, 4A) and said second opposite overtaking is such that said vehicle (4A, 4B) overtakes said other vehicle (4B, 4A). - an overtaking sensor (21A, 21B) configured to provide information indicative of any overtaking between the vehicle (4A, 4B) on board of which said on-board device (2A, 2B) is installed and the other vehicle (4B, 4A) on board of which the other on-board device (2B, 2A) of said system is installed; - a traveled distance meter (20A, 20B) suitable for measuring the distance traveled by said vehicle (4A, 4B); and - a processing unit (22A, 22B) configured to: - when it detects a first overtaking between said vehicle (4A, 4B) and said other vehicle (4B, 4A) on the basis of said information provided by said overtaking sensor (21A, 21B), enable a measurement of the distance that said vehicle (4A, 4B) is traveling along said road section by said traveled distance meter (20A, 20B); and - when it detects a second overtaking opposite to said first overtaking between said vehicle (4A, 4B) and said other vehicle (4B, 4A) on the basis of said information provided by said overtaking sensor (21A, 21B), disable said measurement of the distance that said vehicle (4A, 4B) is traveling along said road section by said traveled distance meter (20A, 20B), - when it detects a first overtaking between said vehicle (4A, 4B) and said other vehicle (4B, 4A) on the basis of said information provided by said overtaking sensor (21A, 21B), enable a measurement of the distance that said vehicle (4A, 4B) is traveling along said road section by said traveled distance meter (20A, 20B); and - when it detects a second overtaking opposite to said first overtaking between said vehicle (4A, 4B) and said other vehicle (4B, 4A) on the basis of said information provided by said overtaking sensor (21A, 21B), disable said measurement of the distance that said vehicle (4A, 4B) is traveling along said road section by said traveled distance meter (20A, 20B),