Read CHAPTER IV - ROAD DESIGN of American Rural Highways , free online book, by T. R. Agg, on

Necessity for Planning. Sometimes highway improvement is the result of spasmodic and carelessly directed work carried out at odd times on various sections of a road, finally resulting in the worst places being at least temporarily bettered. The grade on the steepest hills is probably reduced somewhat and some of the worst of the low lying sections are filled in and thereby raised. Short sections of surfacing such as gravel or broken stone may be placed here and there. From the standpoint of the responsible official, the road has been “improved,” but too often such work does not produce an improvement that lasts, and sometimes it is not even of any great immediate benefit to those who use the roads. In nearly every instance such work costs more in money and labor that it is worth.

Lasting improvement of public highways can be brought about only through systematic and correlated construction carried on for a series of years. In other words, there must be a road improvement policy which will be made effective through some agency that is so organized that its policies will be perpetuated and is clothed with enough authority to be capable of enforcing the essential features of good design and of securing the proper construction of improvements.

Details of highway construction and design must vary with many local conditions and types of surface. The limits of grades and the many other details of design may properly be adopted for a specific piece of work only after an adequate investigation of the local requirements and in the light of wide experience in supervising road improvement.

New ideas are constantly being injected into the art of road building, but these are disseminated somewhat slowly, so that valuable devices and improvements in methods remain long unknown except to the comparatively few who have the means for informing themselves of all such developments.

It follows then that the logical system of conducting road improvement is through an agency of continuing personnel which will supervise the preparation of suitable plans and direct the construction in accordance with the most recent experience.

Road Plans. The information shown on the plans prepared for road improvement varies somewhat with the design and with the ideas of the engineer as to what constitutes necessary information, but in general the plans show the existing road and the new construction contemplated in an amount of detail depending principally upon the character of the construction. Simple plans suffice for grade reduction or reshaping an earth road surface, while for the construction of paved roads, the plans must be worked out in considerable detail. The essential requirement is that there be given on the plans all information necessary to enable the construction to be carried out according to the intentions of the engineer, that all parts of the work fit together, that the culverts are of the proper size and located at the proper places, ditches drain properly, grades are reduced to the predetermined rate, that excavated material is utilized and that an exact record of the work done is retained. Plans are indispensable to economical road construction and the preparation of the plans is the work of the expert in road design, that is, the highway engineer.

Problem of Design. The problem of road design is to prepare plans for a road improvement with the various details so correlated as to insure in the road constructed in accordance therewith the maximum of safety, convenience and economy to the users thereof. The degree to which the design will be effective will depend to a considerable extent upon the financial limitations imposed upon the engineer, but skill and effort on the plans will do a great deal to offset financial handicap and no pains should be spared in the preparation of the plans. Moreover, the plans must afford all of the information needed by the contractor in preparing a bid for the work.

Preliminary Investigation. The first step in road improvement is to secure an adequate idea of the existing conditions on the road or roads involved. The detail to which this information need go will depend entirely upon the purpose of the preliminary investigation, for before a definite plan is prepared, it may be necessary to choose the best from among several available routes. For this purpose, it is not always necessary to make an actual instrument survey of the several routes. A hasty reconnaissance will usually be sufficient. This is made by walking or riding over the road and noting, in a suitable book or upon prepared blanks, the information needed. The items of information recorded will usually be as follows: distances, grades, type of soil on the road and nature of existing surface, character of drainage, location of bridges and culverts and the type of each with notes as to its condition, location of railway crossings and notes as to type, location of intersecting roads, farm entrances, and all similar features that have a bearing on the choice of routes. These data can be obtained in a comparatively short time by a skilled observer who may drive over the road in a motor car. Sometimes it may be desirable to make a more careful study of some certain sections of road and this may be done by waking over the section in question in order to make a more deliberate survey of the features to be considered than is possible when riding in a motor car.

Factors other than relative lengths of routes will obviously determine the cost of improvement and the comparative merits of the improved roads. Some special characteristic of a road, such as bad railroad crossings or a few bad hills, may eliminate a route, or availability of materials along a route may offset disadvantages of alignment or grade.

In special cases, complete surveys of routes may be required finally to select the best route, but these instances are few in number.

Road Surveys. When a road has been definitely selected for improvement, a careful survey is made to furnish information for the preparation of the plans. This will consist of a transit survey and a level survey.

The transit survey is made by running a line between established corners following the recorded route of the road, or if no records are available or the road is irregular in alignment, by establishing arbitrary reference points and running a line along the center line of the existing road or parallel thereto. The topography is referenced to this line in such completeness that it can be reproduced on the plans. The level survey consists in taking levels on cross sections of the road at one hundred foot intervals, and oftener if there are abrupt changes in grade. Special level determinations are made at streams, railroad crossings, intersecting roads or lanes and wherever it appears some special features of the terrain should be recorded.

From the surveys and such other information as has been assembled relative to the project, a plan is prepared which embodies a design presumed to provide for an improvement in accordance with the best highway practice.


It will be convenient to consider separately the components of a road design, although in the actual design the consideration of these cannot be separated because all parts of the plan must fit together.

Alignment. The alignment of the road is determined to a considerable extent by the existing right-of-way, which may follow section lines, regardless of topography, as is the case with many roads in the prairie states, or it may follow the valleys, ridges, or other favorable location in hilly country. In many places the roads of necessity wind around among the hills in order to avoid excessive grades. In designing an improvement, it is generally desirable to follow the existing right-of-way so far as possible. But the element of safety must not be lost sight of, and curves should not preclude a view ahead for sufficient distance to insure safety to vehicles. The necessary length of clear view ahead is usually assumed to be 250 feet, but probably 200 feet is a satisfactory compromise distance when a greater distance cannot be obtained at reasonable cost. To secure suitable sight distance, the curves must be of long radii, and where possible the right-of-way on the inside of the curve should be cleared of trees or brush that will obstruct the view. Where the topography will not permit a long radius curve and the view is obstructed by an embankment or by growing crops or other growth, it is desirable to separate the tracks around the curve to eliminate the possibility of accidents on the curve. This is readily accomplished if the road is surfaced, but if it is not surfaced, the same end is accomplished by making the earth road of ample width at the curve.

Relocations should be resorted to whenever they shorten distances or reduce grades sufficiently to compensate for the cost.

Intersections. At road intersections, it is always difficult to design a curve that entirely meets the requirements of safety because there is not enough room in the right-of-way, and enough additional right-of-way must be secured to permit the proper design. It is not necessary to provide an intersection that is adapted to high speed traffic, where main roads cross, but, on the contrary, a design that automatically causes traffic to slow up has distinct advantages.

Where a main route, improved with a hard surface, crosses secondary roads, it is satisfactory to continue the paved surface across the intersecting road at normal width and make no provision for the intersecting road traffic other than a properly graded approach at the intersection.

Superelevation. On all curved sections of road, other than intersections, account is taken of the tendency of motor cars to skid toward the outside of the curve. This tendency is counteracted by designing the cross section with superelevation.

Tractive Resistance. The adoption of a policy regarding the grades on a road involves an understanding of the effect of variation in the character of the surface and in rate of grade upon the energy required to transport a load over the highway. The forces that oppose the movement of a horse drawn vehicle are fairly well understood and their magnitude has been measured by several observers, but comparatively little is known about the forces opposing translation of rubber tired self-propelled vehicles.

The resistance to translation of a vehicle is made up of three elements: resistance of the road surface to the rolling wheel, resistance of the air to the movement of the vehicle and internal friction in the vehicle itself.

Rolling Resistance. When the wheel of a vehicle rolls over a road surface, both the wheel and the surface are distorted. If the wheel has steel tires and the road surface is plastic, there will be considerable distortion of the road surface and very little of the wheel. A soft rubber tire will be distorted considerably by a brick road surface. Between these extremes there are innumerable combinations of tire and road surface encountered, but there is always a certain amount of distortion of either road surface or wheel, or of both, which has the same effect upon the force necessary for translation as a slight upward grade. When both the tire and the road surface strongly resist distortion (as steel tires on vitrified brick paving), the resistance to translation is low but the factor of impact is likely to be introduced. Where impact is present, energy is used up in the pounding and grinding of the wheels on the surface, and this factor increases as the speed of translation, and may be a considerable item. Impact is especially significant on rough roads with motor vehicles, particularly trucks, traveling at high speed. These two factors (impact and rolling resistance) combined constitute the major part of the resistance to translation for horse drawn vehicles.

Internal Resistance. For horse drawn vehicles, the internal resistance consists of axle friction, which is small in amount. For self-propelled vehicles, the internal resistance consists of axle friction and friction in the driving mechanism, of which gear friction and the churning of oil in the gear boxes is a large item. Internal friction is of significance in all self-propelled vehicles and especially so at high speeds.

Air Resistance. At slow speeds, the resistance of still air to translation is small, but as the speed increases, the air resistance increases rapidly and at the usual speed of the passenger automobile on the road becomes a very considerable part of the total resistance to translation. This factor has no significance in connection with horse drawn vehicles, but is to be taken into account when dealing with self-propelled vehicles at speeds in excess of five miles per hour.

Many determinations of tractive resistance with horse drawn vehicles have been made from time to time and these show values that are fairly consistent when the inevitable variations in surfaces of the same type are taken into account. Table 4 is a composite made up of values selected from various reliable sources and Table 5 is from experiments by Professor J. B. Davidson on California highways.

Effect of Grades. Grades increase or decrease the resistance to translation due to the fact that there is a component of the weight of the vehicles parallel to the road surface and opposite in direction to the motion when the load is ascending the hill and in the same direction when the vehicle is descending.

Energy Loss on Account of Grades. Whether a vehicle is horse drawn or motor driven, energy has been expended in moving it up a hill. A part of this energy has been required to overcome the various resistances other than grade, and that has been dissipated, but the energy required to translate the vehicle against the resistance due to grade has been transformed into potential energy and can be partially or wholly recovered when the vehicle descends a grade, provided the physical conditions permit its utilization. If the grade is so steep as to cause the vehicle to accelerate rapidly, the brakes must be applied and loss of energy results. The coasting grade is dependent upon the character of the surface and the nature of the vehicle. In the cases discussed in the preceding paragraph, the coasting grades would be five per cent and three per cent respectively. For horse drawn vehicles then the economical grades would be three and five per cent, which again emphasizes the necessity of lower grades on roads that are surfaced than on roads with no wearing surface other than the natural soil.

The theory of grades is somewhat different when motor vehicles are considered, since it is allowable to permit considerably higher speed than with horse drawn vehicles before applying the brakes and the effect of grade can be utilized not only in translating the vehicle down the grade, but also in overcoming resistances due to mechanical friction and the air. On long grades, a speed might be attained that would require the use of the brake or the same condition might apply on very steep short grades. There is at present insufficient data on the tractive resistance and air resistance with motor vehicles to permit the establishing of rules relative to grade, but experience indicates a few general principles that may be accepted.

If a hill is of such rate of grade and of such length that it is not necessary to use the brake it may be assumed that no energy loss results so far as motor vehicles are concerned. Where there is no turn at the bottom of the hill and the physical condition of the road permits speeds up to thirty-five or forty miles per hour grades of five per cent are permissible if the length does not exceed five hundred feet and grades of three per cent one thousand feet long are allowable. It is a rather settled conviction among highway engineers that on trunk line highways the maximum grade should be six per cent, unless a very large amount of grading is necessary to reach that grade.

Undulating Roads. Many hills exist upon highways, the grade of which is much below the maximum permissible. If there are grades ranging from 0 to 4 per cent, with a few hills upon which it is impracticable to reach a grade of less than six per cent, it is questionable economy to reduce the grades that are already lower than the allowable maximum. It is especially unjustifiable to incur expense in reducing a grade from two per cent to one and one-half per cent on a road upon which there are also grades in excess of that amount. The undulating road is not uneconomical unless the grades are above the allowable maximum or are exceptionally long or the alignment follows short radius curves.

Safety Considerations. On hills it is especially desirable to provide for safety and curves on hills are always more dangerous than on level sections of road. Therefore, it is desirable to provide as flat grades as possible at the curves and to cut away the berm at the side of the road so as to give a view ahead for about three hundred feet. Whether a road be level or on a hill, safety should always be considered and the most important safety precaution is to provide a clear view ahead for a sufficient distance to enable motor vehicle drivers to avoid accidents.

Guard Railing. When a section of road is on an embankment, guard rails are provided at the top of the side slope to serve as warnings of danger, and to prevent vehicles from actually going over the embankment in case of skidding, or if for any reason the driver loses control. These are usually strongly built, but would hardly restrain a vehicle which struck at high speed. But they are adequate for the protection of a driver who uses reasonable care. A typical guard rail is, but many other designs of similar nature are employed. At very dangerous turns a solid plank wall six or eight feet high is sometimes built of such substantial construction as to withstand the severest shock without being displaced.

Trees, shrubs and the berms at the side of the road in cuts are particularly likely to obstruct the view and should be cleared or cut back so far as is necessary to provide the proper sight distance.

Width of Roadway. For roads carrying mixed traffic, 9 feet of width is needed for a single line of vehicles and 18 feet for 2 lines of vehicles. In accordance with the above, secondary roads, carrying perhaps 25 to 50 vehicles per day, may have an available traveled way 18 feet wide. Those more heavily traveled may require room for three vehicles to pass at any place and therefore have an available traveled way 30 feet wide. Greater width is seldom required on rural highways, and 20 feet is the prevailing width for main highways.

Cross Section. The cross section of the road is designed to give the required width of traveled way, and, in addition, provide the drainage channels that may be needed. In regions of small rainfall the side ditches will be of small capacity or may be entirely omitted, but usually some ditch is provided. The transition from the traveled way to ditch should be a gradual slope so as to avoid the danger incident to abrupt change in the shape of the cross section. The depth of ditch may be varied without changing to width or slope of the traveled part of the road.

Control of Erosion. The construction of a highway may be utilized to control general erosion to some extent, particularly when public highways exist every mile or two and are laid out on a gridiron system, as is the case in many of the prairie states. The streams cross the highways at frequent intervals and the culverts can be placed so as effectually to prevent an increase in depth of the stream. This will to some extent limit the erosion above the culvert and if such culverts are built every mile or two along the stream, considerable effect is produced.

Where small streams have their origin a short distance from a culvert under which they pass, it is sometimes advisable to provide tile for carrying the water under the road, instead of the culvert, and, by continuing the tile into the drainage area of the culvert, eliminate the flow of surface water and reclaim considerable areas of land.

Erosion in the ditches along a highway can be prevented by constructing weirs across the ditch at frequent intervals, thus effectually preventing an increase in the depth of the ditch.

Wherever water flows at a velocity sufficient to produce erosion or where the drainage channel changes abruptly from a higher to a lower level, paved gutters, tile or pipe channels should be employed to prevent erosion.

Private Entrances. Entrance to private property along the highway is by means of driveways leading off the main road. These should always be provided for in the design so as to insure easy and convenient access to the property. The driveways will usually cross the side ditch along the road and culverts will be required to carry the water under the driveway. Driveways that cross a gutter by means of a pavement in the gutter are usually unsatisfactory, and to cross the gutter without providing a pavement is to insure stoppage of the flow at the crossing. The culvert at a driveway entrance must be large enough to take the ditch water readily or it will divert the water to the roadway itself. Generally end walls on such culverts are not required as in the case of culverts across a highway.

Aesthetics. Much of the traffic on the public highways is for pleasure and relaxation and anything that tends to increase the attractiveness of the highways is to be encouraged. Usually the roadside is a mass of bloom in the fall, goldenrod, asters and other hardy annuals being especially beautiful. In some states wild roses and other low bushes are planted to serve the two-fold purpose of assisting to prevent erosion and to beautify the roadside. In humid areas trees of any considerable size shade the road surface and are a distinct disadvantage to roads surfaced with the less durable materials such as sand-clay or gravel. It is doubtful if the same is true of paved surfaces, but the trees should be far enough back from the traveled way to afford a clear view ahead. Shrubs are not objectionable from any view-point and are to be encouraged for their beauty, so long as they do not obstruct the view at turns.