ParcelRateRate × frontage onimproved road =assessment unitsAmount ofAssessmentper unit[1]Assessment123451a 5050 × 2640=132,000$0.016655$1558.46b 7575 × 1320=99,0001153.9024040 × 2640=105,6001230.7731010 × 2640=26,400307.6942525 × 1320=33,000384.665[2]8585 × 5280=448,8005230.8861515 × 5280=79,200923.087[2]6565 × 7920=514,8006000.0083535 × 7920=277,2003230.771,716,000$20000.00
[1]The assessment per unit is obtained by dividing the total assessment by the total of column three.
[1]The assessment per unit is obtained by dividing the total assessment by the total of column three.
[2]On these two parcels, it is decided that more than half of the zone rate should apply to the half of the zone toward the improved road, but some modification of the rates adopted might be justified.
[2]On these two parcels, it is decided that more than half of the zone rate should apply to the half of the zone toward the improved road, but some modification of the rates adopted might be justified.
Fig. 1Fig. 1
The assessment of the cost of the east and west one-mile section of road is made up in like manner, and let it be assumed that the portion of the cost of this road that is to be assessed on the area shown is $5500. The assessment area will be one mile wide and each zone one-fourth mile in width and the rates for each zone the same as before.
ParcelRateRate × frontage onimproved road =assessment unitsAmount ofAssessmentper unitAssessment1a 7575 x 1320=99,000$0.010417$1031.25b 1515 x 2640=39,600412.4927575 x 2640=198,0002062.5335050 x 1320=66,000687.514a 2525 x 1320=33,000756.25b 1515 x 2640=39,60051010 x 3300=33,000343.7361010 x 1980=19,800206.24528,0005500.00
It will be noted that the combined assessment for the two sections of road is especially heavy on parcels 1, 2 and 3. In order to prevent unjust charges against such properties, laws usually limit the total assessment against any parcel of land to a fixed percentage of a fair market value or of the assessed value. The assessment on these parcels would be reduced as seemed expedient and the deficit would be distributed over the remainder of the area in the same manner as the original assessment was spread. In practice such re-distribution is ordinarily made by the arbitrary adjustment in accordance with what the authorized officials consider to be fair and equitable. The method outlined is merely a mechanical means of securing distribution and must not be considered as an infallible method of making the assessment. It is always necessary to review the results in the light of the actual benefits to be presumed foreach parcel of land. Nevertheless, the method outlined will prove equitable in a majority of cases.
General Taxation.—There is a general community benefit derived from the construction of good roads in that the actual cost of marketing farm products is lessened with a resulting lowering of the price to the consumer. The benefit also accrues from the greater facility with which all community business may be conducted. The introduction of better opportunities for social, religious and educational activities in the rural districts which results from improved highways is also a community benefit of no mean importance. A part of the cost of road improvement may therefore be equitably paid from funds obtained by general taxation.
A considerable portion of the current expense of maintaining the township and county highway work and at least a part of the cost of maintaining state highway activities is met from funds obtained by general taxation. Likewise, the funds required for the amortization of bond issues are often obtained from general taxation although vehicle license fees are sometimes used for that purpose.
General taxes are levied on all taxable property in a political unit under statutory provisions regulating the amount of the levy and the purpose for which the revenue is to be used. In the aggregate, the road taxes are large but in the township or county the rate is generally small compared to some other taxes, such as the school tax.
Vehicle Taxes.—The great direct benefit derived by those who actually operate vehicles over the roads justifies the policy of requiring a vehicle to pay a license fee in lieu of other taxes, the funds so obtained to be used for the construction and maintenance of public highways. In practice, this method has already been applied to motor vehicles in most states and has proven to be an important source of revenue. Its application to horse-drawn vehicles has not been attempted, due probably to the fact that suchhorse-drawn vehicles as use the public highways are also employed about the farm or in the towns and the determination of an equitable basis for taxation involves many difficulties.
The rate of the fee for motor vehicles should be based on their destructive effect on the road so far as that is possible. The scale of fees should therefore take account of weight and speed of vehicle and if the license is in lieu of all other taxes, it should also be graduated with the cost of the vehicle.
When funds are thus derived, every precaution should be taken to insure that the money is used judiciously for construction and especially for maintenance on those roads most useful to motor traffic.
Highway Bonds.—Bond issues for road improvement afford a means of constructing roads and paying for them while they are being used. A very large volume of such bonds are outstanding in the United States. Road bonds should be issued only for durable types of improvement and the life of the bond should be well within the probable useful life of the road surface. It is customary and highly desirable that the general nature and extent of the improvement be established before the bonds are issued. It is desirable that bond issues be subject to approval by referendum before issue and that is provided in every instance.
Highway bonds are of three classes known as Sinking Fund, Annuity and Serial Bonds, respectively. The earlier bonds issued were almost all of the sinking fund class, but in recent years the serial bond has been widely employed and is probably the most satisfactory to administer.
Sinking Fund Bonds.[1]—When this type of bond is employed, the amount of the expenditure for road improvementis determined upon and the length of the period during which tax payments shall be made is settled. To employ a concrete example, it may be assumed that $100,000 is to be expended for road work and is to be paid at the end of ten years. The interest rate on the bonds will vary with the condition of the bond market and the stability of the political unit issuing the bonds, but is usually about 5 per cent. Knowing these factors, the amount to be added to the sinking fund each year is computed. In order to pay the interest on the bonds, a tax of suitable rate is levied, and in order to retire the bonds at the end of the period, a sum is set aside each year which is supposed to be invested and draw interest which will be added to the principle, and the principle and interest comprise the sinking fund. The principle of the sinking fund is obtained by tax levies, a sum being added to the principle of the sinking fund each year.
[1]For a more detailed discussion of highway bonds see Bulletin 136, U. S. Dept. of Agriculture, which is the basis of this discussion.
[1]For a more detailed discussion of highway bonds see Bulletin 136, U. S. Dept. of Agriculture, which is the basis of this discussion.
The success of this method of financing depends upon the proper administration of the sinking fund. It must be invested with fidelity and the fund be kept intact. Usually the sinking fund cannot be invested at as high a rate of interest as the bonds bear and there is some loss as a result. Road bonds bearing 5 per cent interest can usually be sold at par while the sinking fund will usually net about 3 or 3½ per cent interest. The total cost of a bond issue will be greater by the sinking fund method than by either of the other methods described.
Annuity Bonds.—Annuity bonds are drawn in such a manner that the amount of the payment for principle and interest is the same each year during the life of the bond. When the amount of the issue and the rate of interest has been determined and the amount of the desired annual payment has been determined, the number of years the bonds must run is computed.
This method is convenient in that the amount of the tax to be levied each year remains constant.
Serial Bonds.—Serial bonds are drawn so that a uniform amount of the principle is retired each year after retirement starts and the total interest payments decrease each year after the first bonds are retired. The first bond may not be retired for a number of years after the issue of the bonds, but when it once starts retirement proceeds at a constant rate annually.
Comparison of Methods of Issuing Bonds.—The relative costs of financing by either of the three methods depends upon the rate of interest in each case and the net rate secured on the sinking fund provided for retiring sinking fund bonds.
For comparative purposes, some typical examples are given in Table 3. These illustrate the differences in total cost of securing $100,000 by each of the three methods at various interest rates.
AnnualIntereston BondsSinking Fund CompoundedAnnually atAnnuitySerial3 per cent3½ per cent4 per cent4$154,431$150,722$147,163$147,163$142,0004½164,431160,722157,163153,752147,2505174,431170,722167,163160,485152,5005½184,431180,722177,163167,359157,7506194,431190,722187,163174,369163,000
Desirability of Road Bonds.—In theory the bond method of financing enables the highway authorities to construct a large mileage of roads in a few years and spreads the cost over the period during which the public is being benefited. Better prices are obtained on contracts for alarge mileage than for smaller jobs, and the community can receive the benefit more quickly than where construction proceeds piecemeal with current funds. The vital consideration is to insure that the term of the bonds is well within the useful life of the road, and that ample provision is made to maintain the roads during that period. Under proper restrictions the bond method of financing is to be commended. The bonds are an attractive investment and readily marketable on satisfactory terms.
The Necessity for Drainage.—The importance of drainage for all roads subject to the effects of storm or underground water has always been recognized by road builders, but during recent years constantly increasing attention has been given to this phase of road construction. It is unfortunate that there has in the past been some tendency to consider elaborate drainage provisions less necessary where rigid types of surfaces were employed. It has become apparent, from the nature of the defects observed in all sorts of road surfaces, that to neglect or minimize the importance of drainage in connection with either earth roads or any class of surfaced roads is to invite rapid deterioration of some sections of the roadway surface and to add to maintenance costs.
The degree to which lack of drainage provisions affect the serviceability of the road surface varies with the amount of precipitation in the locality and the manner in which it is distributed throughout the year. In the humid areas of the United States, which are, roughly, those portions east of a north and south line passing through Omaha and Kansas City, together with the northern part of the Pacific slope, precipitation is generally in excess of 30 inches per year and fairly well distributed throughout the year, but with seasonal variations in rate. In these areas, the effect of the precipitation, both as regards its tendency to lower the stability of soils and as an eroding agent, must be carefully provided against in highway design.
Outside of the areas mentioned above, the precipitation is much less than 30 inches per year and its effect as an agent of erosion is of greatest significance, although in restricted areas there may be short periods when the soil is made unstable by ground water.
Importance of Design.—The drainage system for a proposed road improvement ought to be designed with as much care as any other element, and, to do so, a study must be made of all factors that have any bearing on the drainage requirements and the probable effectiveness of the proposed drainage system. The well established principles of land drainage should be followed so far as applicable.
The basic principle of road drainage is to minimize the effect of water to such an extent that there will always be a layer of comparatively dry soil of appreciable thickness under the traveled part of the road. This layer should probably never be less than two feet thick and for soils of a structure favorable to capillary action it should be at least three feet thick. The means employed to accomplish the requisite drainage will be as various as the conditions encountered.
Surface Drainage.—The drainage method which is by far the most nearly general in application is that which utilizes open ditches, and the system which employs these ditches is usually referred to as surface drainage. The full possibilities of this method of minimizing the effects of storm water are rarely fully utilized in road construction. Very frequently, deterioration of a road surface is directly attributable to failure to provide adequately for the removal of the storm water or water from the melting of snow that has fallen on the road, or water that flows to the road from land adjacent thereto. Surface water can usually most cheaply and expeditiously be carried away in open ditches, although special conditions are occasionally encountered which require supplementary tile drains.The cross section commonly adopted for roads lends itself naturally to the construction of drainage ditches at the sides of the traveled way, and these are usually the principal dependence for the disposal of storm water.
Run-off.—The capacity required of side ditches to insure satisfactory surface drainage will be affected by the amount and nature of the precipitation in the region where the road is built. The annual rainfall in a region may amount to several feet, but may be well distributed throughout the year with an absence of excessive rainfall for short periods, that is, flood conditions may rarely occur. In other areas, the annual rainfall may be comparatively small but the precipitation occurs at a very high rate, that is, flood conditions may be common, or it may be at a low rate extending over a considerable period. These peculiarities must be known before an adequate drainage system can be planned.
It is almost universally true in the United States that precipitation at a very high rate will be for a relatively short duration, and during these short periods, which usually do not exceed thirty minutes, a portion of the water that falls on the areas adjacent to the road and that drains to the road ditches will soak into the soil and therefore not reach the ditches along the road. The extent to which the water is taken up by the soil will vary with the porosity and slope of the land and the character of the growth thereon. Cultivated land will absorb nearly all of the water from showers up to fifteen or twenty minutes duration; grass land a somewhat smaller percentage; and hard baked or other impervious soil will absorb a comparatively small amount. Rocky ground and steep slopes will absorb very little storm water.
The surface of the road is designed to turn water rapidly to the ditches, but when the material is the natural soil, there is always considerable absorption of storm water. Surfaces such as sandclay, gravel and macadam do notabsorb to exceed 10 per cent of the precipitation during short showers. Bituminous surfaces, brick and concrete pavements, do not absorb an appreciable amount of storm water.
Generally it is best to assume that if a rain lasts for forty-five minutes or more, all of the water will run off, as the soil will reach a state of saturation in that time. This is not true of deep sand, but is for nearly all other soils.
The ditch capacity needed will therefore depend upon the area drained, the character of the soil, the slopes and the rainfall characteristics of the region, and upon the nature of the road surface.
For a required capacity, the cross section area of the ditch will vary inversely as the grade, because the velocity of flow increases with an increase in the grade of the ditch. If the surface water must be carried along the road for distances exceeding five or six hundred feet, the ditch must be constructed of increasing capacity toward the outlet in order to accommodate the accumulated volume of water.
The velocity of flow varies not only with the grade, but with the shape of the cross section, cleanness of the channel, the depth of the water in the channel, alignment of the channel and the kind of material in which the channel is formed. It is not necessary to go to great refinement in the design of the side ditches for the ordinary case where the water is carried along the road for only a few hundred feet. The ditches are made of ample capacity by using the commonly accepted cross section for a road, which will be discussed in a later paragraph. But where large areas must be drained by the road ditches, it is desirable carefully to design the side ditches. The basis for that design is too lengthy to be included herein, and reference should be made to a standard treatise on the subject.
Ordinary Design of Ditches.—For grades of one per cent or less on roads in the humid area, the bottom of the ditch should be at least three and one-half feet lower thanthe traveled surface of the road, except for very sandy soil. For grades greater than one per cent, this depth may be decreased one foot, and for grades of four per cent and upward, the depth may be still less. These general rules for depth are susceptible of variation but are believed to be the minimum except in arid or semi-arid climates. It is far better to be too liberal in ditch allowance than to be too conservative. In arid or semi-arid regions, the ditch design will be based on the necessity of providing for flood flow and preventing damage through erosion. Ordinary drainage requirements will be satisfactory with the ditch about one foot deep.
If the topography is such that it is evident considerable storm water will flow from the adjacent land to the road ditches, the design must be modified to take this into account. Sometimes such water can be diverted by ditches well back from the road, and thus prevented from flowing into the side ditches along the roadway. It is especially desirable to divert water, which would otherwise flow down the slope of a cut, by means of a ditch on the hill-side above the upper edge of the slope of the cut.
Ditches are not effective unless they afford a free flow throughout their length and have an outlet to a drainage channel of ample capacity. Therefore, ditch grades should be established by survey, especially if the gradient is less than one per cent, and the construction work should be checked to insure that the ditch is actually constructed as planned. A few high places in the ditch will greatly reduce the effectiveness, although these may appear at the time of construction to be slight. Constricted places, such as might be due to a small amount of loose earth left in the ditch, are always to be avoided.
Where the side ditch passes from a cut to the berm alongside a fill, the ditch should be excavated throughout in the undisturbed natural soil, five feet or more from the toe of the slope of the fill, and along the filled portion of theroad there should be a berm of three or four feet between the toe of the slope of the fill and the near edge of the ditch.
Underground Water.—In a preceding paragraph, mention was made of the fact that only a part of the storm water runs off over the surface of the ground, the larger part being absorbed by the soil. The water thus absorbed flows downward through the pores in the soil until it is deflected laterally by some physical characteristic of the soil structure. The movement of underground water is affected by many circumstances, but only two conditions need be discussed herein.
Underground water, like surface water, tends to attain a level surface, but in so doing it may need to flow long distances through the pores of the soil, and to overcome the resistance incident to so doing some head will be required. That is to say, the water will be higher at some places than at others. If a cut is made in grading the road, the road surface may actually be lower than the ground water level in the land adjoining the road. As a result, the water will seep out of the side slopes in the cut and keep the ditches wet, or even furnish enough water to occasion a flow in the ditch. Similarly, the higher head of the underground water near the top of a hill may result in ground water coming quite close to the surface some distance down the hill. The remedy in both cases is tile underdrains alongside the road to lower the ground water level so that it cannot affect the road surface.
Sometimes the ground water encounters an impervious stratum as it flows downward through the soil, or one that is less pervious than the surface soil. When such is the case, the water will follow along this stratum, and should there be an outcrop of the dense stratum, a spring will be found at that place. This may be on a highway. The impervious stratum may not actually outcrop but may lie only a few feet under the surface of the road, in whichcase, the road surface will be so water soaked as to be unstable. The so-called "seepy places" so often noted along a road are generally the result of this condition. This condition can be corrected by tile laid so as to intercept the flow at a depth that precludes damage to the road. Commonly, the tile will be laid diagonally across the road some distance above the section where the effect of the water is noted, and will be turned parallel to the road at the ditch line and carried under one of the side ditches to an outlet.
Tile Drains.—Where the soil and climatic conditions are such that the roadway at times becomes unstable because of underground water rising to a level not far below the road surface, the ground water level is lowered by means of tile underdrains. The function of the tile drains in such cases is precisely the same as when employed in land drainage; to lower the ground water level.
Laying Tile.—The tile lines are usually laid in trenches parallel to the center line of the road near the ditch line and at least 4 feet deep so as to keep the ground water level well down. They must be carefully laid to line and grade. A good outlet must be provided and the last few joints of pipe should be bell-and-spigot sewer pipe with the joints filled with cement mortar. The opening of the tile should be covered with a coarse screen to prevent animals from nesting in the tile.
It is frequently necessary to lay a line of tile at the toe of the slope in cuts to intercept water that will percolate under the road from the banks at the sides. In some cases, it is desirable to back-fill the tile trench with gravel or broken stone to insure rapid penetration of surface water to the tile. In other instances, it is advantageous to place catch basins about every three or four hundred feet. These may be of concrete or of tile placed on end or may be blind catch basins formed by filling a section of the trench with broken stone. When a blind catch basin is used, the topshould be built up into a mound, and for a tile or concrete catch basin, a grating of the beehive type should be used, so that flow to the tile will not be obstructed by weeds and other trash that is carried to the catch basin.
Culverts.—Culverts and bridges are a part of the drainage system and the distinction between the two is merely a matter of size. Generally, structures of spans less than about eight feet are classed as culverts, but the practice is not uniform. In this discussion culverts will be defined as of spans of 8 feet or less.
Numerous culverts are required to afford passage for storm water and small streams crosswise of the road, and their aggregate cost is a large item in the cost of road improvement. The size of the waterway of a culvert required in any location will be estimated by an inspection of the stream and existing structure, and by determining the extent and physical characteristics of the drainage area. Sometimes there is sufficient evidence at the site to indicate quite closely the size required, but this should always be checked by run-off computations. The drainage area contributing water to the stream passing through the culvert under consideration is computed from contour maps or from a survey of the ground, and the size of culvert determined by one of the empirical formulas applicable to that purpose. In these formulas, the solution depends upon the proper selection of a factor "C" which varies in accordance with the nature of the drainage area. Two of these that are quite widely used are as follows:
Myers' Formula: a = CA
Wherea= area of cross section of culvert in square feet.A= area in acres of the drainage area above culvert.Ca factor varying from 1 for flat country to 4 for mountainous country or rocky soil, the exact value to be selected after an inspection of the drainage area.
Talbot's Formula: Area of waterway in square feet =
C=√(Drainage area in acres)3
Cbeing variable according to circumstances thus:
"For steep and rocky groundCvaries from 2/3 to 1. For rolling agricultural country, subject to floods at times of melting snow, and with length of valley three or four times its width,Cis about 1/3, and if stream is longer in proportion to the area, decreaseC. In districts not affected by accumulated snow, and where the length of valley is several times its width, 1/5 or 1/6 or even less may be used.Cshould be increased for steep side slopes, especially if the upper part of the valley has a much greater fall than the channel at the culvert. The value ofCto be used in any case is determined after an inspection of the drainage area."
Fig. 2. Design of Pipe Culvert and BulkheadFig. 2. Design of Pipe Culvert and Bulkhead
Length of Culvert.—The clear length between end walls on a culvert should be at least equal to the width of the roadway between ditches. This is a minimum of 20 feet for secondary roads and ranges from 24 to 30 feet for main roads. The headwall to the culvert should not be a monument, but should be no higher than needed to prevent vehicles from leaving the roadway at the culvert.
Farm Entrance Culverts.—At farm entrances, culverts are required to carry the farm driveway across the sideditch of the road. These culverts are usually about 16 feet along, and should be of a size adequate to take the flow of the side ditch. The farm entrance culvert should be of such design that it can be easily removed to permit cleaning out the ditches with a road grader.
Culverts constructed of concrete and poured in place are called box culverts because of the rectangular form of the cross section. Culverts of pre-cast pipe are known as pipe culverts. Several forms of pipe culvert are in general use.
Fig. 3.—Typical Concrete Box CulvertFig. 3.—Typical Concrete Box Culvert
Metal Pipe.—These may be of cast iron, steel or wrought iron. The cast iron pipe is very durable but expensive and heavy to handle and is not widely used in highway construction. Steel pipe has been employed to a limited extent but its durability is questioned. At least it is known that the pipe made from uncoated, light sheet steel is not very durable. Sheet iron and sheets madefrom alloy iron coated with spelter have been extensively used and seem to be durable, especially when laid deep enough to eliminate possibility of damage from heavy loads. To insure reasonable resistance to corrosion, the metal sheets should be coated with at least one and one-half ounces of spelter per square foot of sheet and the sheets should not be lighter than 16 gauge for small sizes and should be heavier for the larger sizes.
Clay and Cement Concrete Pipe.—The ordinary burned clay bell and spigot pipe that is employed for sewer construction is sometimes used for culverts. It must be very carefully bedded, preferably on a concrete cradle and the joints filled with cement mortar. Culverts of this type have a tendency to break under unusual loads, such as traction engines or trucks. They may be damaged by the pressure from freezing water, particularly when successive freezing and thawing results in the culvert filling with mushy snow, which subsequently freezes.
Concrete Pipe.—Reinforced concrete pipe is a satisfactory material for culverts, if the pipe is properly designed. The pipe should be carefully laid on a firm earth bed with earth carefully back-filled and tamped around the pipe. The joints in the pipe should be filled with cement mortar, or should be of a design that will be tight.
Endwalls for Culverts.—A substantial retaining wall is placed at each end of the culvert barrel, whatever the type. This is to prevent the end of the culvert from becoming choked with earth and to retain the roadway at the culvert. It also indicates to the drivers the location of the end of the culvert. The endwall extends a foot or more below the floor of the culvert to prevent water from cutting under the barrel. Plain concrete or stone masonry are most commonly used for culvert endwalls.
Fig. 4.—Two Types of Drop Inlet CulvertFig. 4.—Two Types of Drop Inlet Culvert
Reinforced Concrete Box Culverts.—The pipe culvert is limited in application to the smaller waterways. Reinforced concrete is extensively used for culverts of all sizes,but especially for the larger ones. These are usually constructed with endwalls integral with the barrel of the culvert. Culverts of this type must be designed for the loads anticipated to insure suitable strength and stability, and must be constructed of a good quality of concrete. Figs. 2 and 3 show designs for pipe and box culverts.
Fig. 5.—Drop Inlet CulvertFig. 5.—Drop Inlet Culvert
Drop Inlet Culverts.—In some locations erosion has begun in the fields adjacent to a culvert and it will probably continue until the stream above the culvert has eroded to about the level of the floor of the culvert. This is a reason for placing the culvert as high as the roadway will permit, so long as the area above the culvert will be properly drained. Considerable reclamation of land is possible if the culvert is constructed with a box at the inlet and as shown in Fig. 4. The area up-stream from the culvert will not erode below the level of the top of the box at the inlet end.
Where the stream crossing the road has eroded to considerable depth or has considerable fall, as would sometimes be the case on side hill roads, the culvert barrel would follow the general slope of the ditch but should have a drop inlet. This type of culvert is shown in Fig. 5.
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 andin 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 variousdetails 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 theconsideration 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 securedto 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.