Traffic guide signs should be settled in a freeway tunnel when there is a short distance between the interchange exit ramp and the tunnel exit in order to provide enough reaction time for drivers. However, there is not enough space for guide sign in a tunnel adopting the same design method as the guide sign along the general segment of the freeway. The maximum dimension of a guide sign in tunnel should be studied firstly. Based on the analysis of the characteristics of the inner outline design of the tunnel and its relationship with the guide sign dimension, the study was classified into different combination conditions: left superelevation and right superelevation under two-, three-, or four-lane freeway tunnels, respectively. The essential elements, the horizontal and vertical clearances, the radius of the tunnel vault circles, the angle of the superelevation, and the allowance vertical dimension for future sign installation were all taken into account to establish the dimension model of the guide sign in the tunnel. The maximum dimensions of the guide signs were proposed under different combination conditions. The results indicated that there is only one set of the width and the height to obtain the maximum area of the guide sign in the freeway tunnel. The height of the guide sign reduces with the increase of its widths, and the area of the guide sign increases and then reduces with the increase of its width under the same grade of superelevation. The changing trend and extent of the dimension of the guide sign under left superelevation condition were different from those under right superelevation.
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