I REFER to Mr Kevin Lam's letter headlined, ''Open another tunnel toll booth'' (South China Morning Post, November 27). He did not specify the day or time of his journey, therefore, I can only reply in general to his comments. The Aberdeen Tunnel normally maintains two AutoToll lanes and three manual toll lanes in each direction. If traffic demands, a fourth manual toll lane is manned. During peak hours, however, the sliproad leading to Wong Nai Chung Road becomes congested and to avoid stationary traffic inside the northbound tube, a periodical stoppage at the toll booth area is necessary. Increasing the number of available toll booths will not help to ease the congestion under such circumstances. M. K. W. LIU Traffic Manager The Cross Harbour Tunnel Co. Ltd.