The focus of this paper is on safety-critical inter-vehicular communications in strings circulating on highways and main roads. The bounded channel access delay (BCAD) and the time-bounded message acknowledgment (TBMA) problems are specified. We present a collision-free deterministic directional MAC protocol that solves both problems. At the same time, that protocol minimizes string-wide message dissemination delays, in the presence of message and acknowledgement losses. Analytical expressions of various time bounds are given and illustrated with numerical examples.