To increase channel capacity, MIMO techniques are often used, and now, with the development of the 5G technique, massive transmitting arrays become to be widely implemented. For vehicular communication, the dedicated frequency band is around 5.9 GHz, and, to our knowledge, there are very few contributions to massive MIMO in tunnels and especially in road tunnels. In this paper, the case of a highway rectangular tunnel is treated, being its width much larger than its height. The Tx array is a square array of 64 elements and the correlation between array elements, deduced from simulation and measurements, is first presented. Due to the shape of the tunnel cross-section, it appears that the correlation between vertically aligned elements is much greater than that occurring between elements situated on a horizontal line, and, as an example, the correlation between elements 7.5 cm apart, can be equal to 0.9 and 0.7, respectively. Consequently, the way of partitioning the massive array is studied to find the best compromise between channel capacity and complexity of the transmission scheme, taking the number of radio frequency chains into account. Illustrations are given for transmission techniques based either on beamforming or singular values decomposition of the transfer matrix. Lastly, for a MIMO transmission, the precoding matrices must be periodically updated when the mobile moves along the tunnel, due to changes in the transfer matrix. The longitudinal correlation distance and the channel stationarity are thus calculated for correlation values equal to 0.7 or 0.9.