State the reported accidents on undivided roadways of two-, three- and four-lane width show but a small difference in the accident expectancy on the basis of accidents per million vehicle-miles. Very limited data from another State in which a modern four-lane highway has recently been opened to traffic show a remarkable reduction in accidents on the four-lane divided section as compared with those of an adjacent fourlane highway. There was 0.31 accident per million vehicle-miles on the undivided highway as compared to 0.13 accident per million vehicle-miles on the four-lane divided section. Although both figures are low, they are fairly comparable, and the advantage of the divided highway is striking.
DIVIDED FOUR-LANE HIGHWAYS RECOMMENDED FOR HEAVY TRAFFIC
It is recommended that when the traffic volume becomes too greatto be reasonably accomodated by a two-lane highway, a four-lane divided highway should be provided. All the cost estimates have been based upon this recommendation. The dividing line between two- and four-lane pavements has been taken at an average traffic of 1,500 vehicles per day in 1960. It is unwise to recommend for general use a capacity limit for four-lane highways in terms of total traffic since it is felt that local conditions producing traffic volumes sufficient to justify more than four lanes will be so individual in their characteristics that each case will require special study. Serious congestion on four-lane pavements would occur only in proximity to large cities, where in the periods of heavy volume, the traffic is not evenly divided between the two directions. Accordingly, the volume justifying an increase in width beyond four lanes is based on the traffic in one direction only, rather than on the total in both directions.
Figures show that the same freedom of movement possible on a two-lane road with a total hourly volume of 400 vehicles will be found on a four-lane divided highway with an hourly volume of 2,600 vehicles, or 1,300 vehicles in each direction. Using the ratio between the maximum hourly volume and the average daily volume throughout the year found on two-lane roads, 1,300 vehicles per hour represent a daily average of 5,000 vehicles in each direction. Typical heavily traveled roads show that in periods of heavy volume, the volume in one direction is seldom more than double that in the other. Therefore, an average of 7,500 vehicles per day in both directions is considered a conservative figure for the capacity of a four-lane divided highway without inconvenience at any time. This figure is offered only as a rough indication, and should not be used where it is possible to study and analyze traffic characteristics. Traffic volumes far higher than 7,500 vehicles per day may be accomodated on four-lane surfaces with slight inconvenience at certain times, but still at relatively high average speeds. The 1,300 vehicles per hour in each direction could travel at an average speed of 45 miles per hour, but the traffic may be increased to 3,400 vehicles per hour in each direction with an average speed of 40 miles per hour. Each of these figures represents an average speed, and with the distribution of speeds normally encountered, some vehicles must travel at 60 miles per hour or faster to offset those which travel under 40 miles per hour even in light volumes.
