The curvature of tunnels introduces an extra loss in the wave propagation. A simulation and measurement study are performed on the straight and the curved tunnels to investigate the extra loss in the curved tunnels in comparison with the straight tunnels at a frequency of 2.4 GHz. This study suggests the existence of two wave propagation mechanisms in the curved tunnel; <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">enhanced waveguiding mechanism</i> induced by rich multipath components from the curved tunnels and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">degraded waveguiding mechanism</i> due to the blockage from the curved tunnel walls. For efficient radio planning, a new propagation model with curvature dependent break point is proposed. The proposed break point indicates the end of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">enhanced waveguiding mechanism</i> and the beginning of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">degraded waveguiding mechanism</i> . A two-slope radio wave propagation model is then proposed for radio communications inside curved tunnels by using the determined break point, with performance evaluation.