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40 each other are available only 50 percent of the time, if the time required for the completion of the passing maneuver is 10 seconds. While it is believed that 10 seconds is entirely reasonable, reduction of the time required to 8 seconds would still leave only 55 percent of the time available for passing. It is believed, therefore, that a volume of 400 vehicles per hour with traffic approximately evenly divided in the two directions is a maximum which it is safe to assume may be carried over a two-lane road without inconvenience at some time during the year. Plate 24 shows that this maximum volume of 400 vehicles per hour corresponds to an average daily volume of 1,500 vehicles. Sections of the selected system on which the average daily traffic volume is expected to exceed 1,500 in 1960 are, therefore, planned to be more than two lanes in width.

THREE-LANE PAVEMENTS UNDESIRABLE

Neither in theory not in practice is there complete agreement as to the width of road to build when more than two lanes are required. Were adequate accident records available, the proper width for highways wider than two lanes could be based entirely on the factor of safety. In no State, however, are accident data sufficiently complete to permit a reliable analysis of the accident rate on roads of various widths. In theory, a three-lane road has a definite place where the traffic in one direction is much heavier than that in the other direction and where the direction of heavy traffic flow reverses as, for example, between morning and evening. In the case of toll highways, however, it is not anticipated that there will be any such marked difference in the volume of traffic in the two directions and therefore this theoretical advantage of the three-lane highway is not present. Furthermore, many studies of vehicular movement on three-lane highways definitely show a reluctance of drivers to utilize the center lane to the maximum advantage.

From the point of view of driver behavior, the three-lane highway suffers a psychological disadvantage which might well result in an abnormally high accident rate. On a two-lane road, a driver engaged in a passing Maneuver must encroach upon the left lane, the lane which is definitely reserved for traffic in the opposite direction, and he does it with full realization that his passing is accomplished only in the face of the superior rights of drivers in the opposing lane. In the case of the three-lane road, particularly with traffic evenly divided in the two directions, there is no clear-cut right-of-way distinction. A vehicle moving in one direction has as much right in the center lane as one moving in the other direction, and passings may involve much greater traffic hazards.

Furthermore, for the selected routes the three-lane pavement could be regarded only as an expedient. Its traffic capacity is somewhat higher than that of a two-lane pavement, but four lanes will permit, without inconvenience, traffic volumes several times as high as those which may be accommodated on a two-lane road. Inasmuch as the three-lane road does not lend itself readily to remodeling as a four-lane divided highway, which is believed to be the ultimately desirable type of construction, the three-lane road has no place on the selected routes. Although the available accident figures are not sufficiently reliable or voluminous to be employed as a criterion, it is interesting that in one