The primary object of the work described was to develop and validate a method of measuring the airborne sound insulation in situ so that it would be possible to carry out a roadside survey of barriers to determine the nature and extent of any sound leakage. The in situ measurement of airborne sound insulation involved the determination of the incident and transmitted sound intensity using the traffic as the source of noise. Incident sound intensity on the traffic face was estimated using microphones attached to a reflective panel and transmitted intensity was measured using a sound intensity probe. Measurements were made on four timber barrier panels at two sites on the M25 London orbital motorway. The panels were removed and their sound insulation properties tested according to the international standard ISO 140/3. In general, there was good agreement between the methods which suggests that the in situ technique is an adequate method of assessing sound leakage through timber barriers.