When the explosion of condensed materials occurs in a tunnel, the subsequent blast wave reveals two patterns. The region close to the explosive charge exhibits a free-field overpressure decay pattern and the region far from the explosion, which undergoes much less overpressure decay, exhibits a quasi-one-dimensional pattern. Well-known overpressure decay laws that are applicable in each region already exist. In order to assess the validity range of each of these laws, the blast wave clue an explosion inside a typical confined geometry is examined in order to determine the position of the transition zone from the free pattern to the one-dimensional pattern. To this end, the detonation of different quantities of explosive charges was simulated inside a tunnel with a constant cross-sectional area, and the wave aspect was determined for each region. This paper proposes a correlation law that defines the transition distance according to the explosive charge's weight and material and the geometry of the propagating domain. The validity of the proposed correlation law is corroborated by experimental results. In the authors' opinion, this law may be helpful for rapidly and efficiently drawing up the blast wave damage map. (C) 2010 Elsevier B.V. All rights reserved.