DESIGNING ARIZONA'S HIGHWAYS

ARIZONA HIGHWAYS

Built, for after all the road should be so designed as to assist a car from leaving its natural course. The super-elevation we use as a standard is a simple conclusion arrived at from plotting several existing formulae. In substance it is .01 per foot width of roadway per degree of curve up to and including a 7degree curve. A 6-degree curve, for an example, has a super-elevation of .06 per foot of roadway width. The maximum safe speed for any curve is reached when 12 e+ 12 k f= 0.8 S R where k is assumed as .6 f is co-efficient of friction e is amount of super per foot. s is speed in m.p.h. r is radius of curve. Using the above with .2 as the co-efficient for friction and we have a safe maximum speed of 51 miles per hour on a properly banked 6 degree curve or 59 miles per hour on a 4 degree.

All curves over 6 degrees are widened on the inside to give an added safety factor. Curve widening is important not only for the easement it creates in negotiating the curve but for the added sight distance it may create when in through cuts or "blind" curves. A 6 degree blind curve with no widening has a sight distance on center line of about 350 feet for a 28 foot roadbed.

Assuming that two cars were approaching each other on this 6 degree curve at 40 miles per hour, their speed per second would be 58.7 feet each or in other words the space between the two cars would be reduced 117.4 feet each second and in three seconds they would be passing each other. It is apparent from the above example that maximum sight distance by curve widening or other means is an important safety factor. On paved roads the center strip or traffic lane should be used at all curves both horizontal and vertical where the sight distance is less than 500 feet.

Guard rail as a safety feature is also important, not only as a guard against cars leaving the road when beyond control of their drivers, but from the psychological effect on the average driver. The fact that the guard is there makes a timid driver feel more secure in mountainous country and he will have the tendency to stay near the rail, while without it he will crowd to the center of the road. This has proven itself, for we have in some instances erected only a line of white posts set at 20 feet inter-vals, or placed large white stones at ten foot intervals. Although this practice might be questioned from a safety viewpoint the psychological effect has proven the theory.

Must Consider Maintenance

The maintenance problem must also be considered in the design, for maintenance costs can mount to large proportions due to inadequate design. Deep ditches or borrow pits adjacent to the road are a source of much worry to the maintenance engineer and a minimum berm of 10 feet is hardly sufficient, for in case of future widening these borrow pits must be filled again. Culverts are placed full width of roadbed and sufficient material placed over them to enable the maintenance blade to pass over without lifting.

In the northern part of the state where we have snow the grade line is laid to anticipate the snow removal problem and to provide widened sections for snow storage.

All through the design the matter of cost enters into each problem for after all the engineers' job is to build the most he can with the funds available.

Agreement To Purchase Blythe Bridge Hastens Arizona Road Program

With all agreements signed between Arizona, California and the owners, for the purchase of the toll bridge across the Colorado river at Ehrenberg, approval of the transaction by the United States Bureau of Public Roads only is needed to start the improvement of the highway from Wickenburg to the bridge.

The entire highway has been divided into ten projects by the Arizona highway department. The department contemplates the construction of seven of these projects this year. Of the three remaining projects two now are under state maintenance and the third was constructed in 1927, extending approximately ten miles from the bridge eastward. This section may be oiled during the present year providing funds remain after the improvement of the other sections is made.

The improvement of two of the projects is provided for in the funds of the Continued on page 14

Page Four APRIL, 1931