secting roads exceeded 2,000 vehicles in 1948, with at least 500 vehicles on the intersecting road of least traffic density. Under these conditions the standards require separation of the grades of the intersecting roads by the construction of overpass or underpass bridges.
In rural areas
It is apparent that the need for this type of improvement on rural sections of the system is not great, and it is encouraging to compare the remaining need of only 118 structures with the 341 similar grade separations already existing.
In urban areas
It is in the urban areas, however, where the greatest need for separation of grade intersection with other streets or highways will exist. This will usually be accomplished by the elevation or depression of the interstate highway for considerable distances. Application of the traffic-volume criterion to the heavy volumes normally present at city intersections would require this in any case, but even where, at particular street intersections, the cross movement may be less than 500 vehicles per hour, the need for greater freedom of flow on the heavy-traffic interstate route is such as to require elimination of the grade intersections anyhow, and in this case the desired end will usually be attained by closure of the unimportant cross streets and diversion of their traffic to nearby separated intersections.
Improvement other than separation
It should be added here, that while on rural sections of the system no great need is found for further separation of highway grade intersections, there is widespread need for the improvement of intersections by other means, such as the lengthening of sight distance, the construction of deceleration and acceleration lanes on the interstate highway, and the construction of channelizing islands to regularize intersecting traffic movements.
DEFICIENCY OF BRIDGES AND TUNNELS
Deficiency of bridge load capacity
The extent of bridge insufficiency has already been generally indicated in the foregoing description of the existing bridges. The defined standards call for load capacity equivalent to that provided by H20-S16 design. The weighing of vehicles now using routes of the system has shown definitely that this standard is none too high for normal civil usage, and it is definitely required in anticipation of possible military needs.
When, at all weighing points, the average weight of all maximum loads occurring with a frequency of at least once daily is found to be 30 tons, and at frequent points throughout the system once-daily-loads reach maxima averaging nearly 62 tons, it is apparent that bridges safely designed for the 36-ton gross vehicle weight on a 28-foot wheel base that is accommodated by the H20-S16 design are a necessity on the system.
On the entire system there are at present only 1,607 bridges of H20-S16 design or rating, of which 1,388 are in rural areas and 219 in urban areas. All other bridges on the system, 8,662 in rura! areas and 1,779 in urban areas, are deficient in some degree in load-carrying
