ROAD DESIGN
=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.
THE PROBLEM OF DESIGN
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.
[Illustration: Fig. 6]
In Fig. 6, _F_ represents the tangential force that tends to cause
skidding. _W_ represents the weight of the vehicle in pounds, THETA
= the angle of superelevated surface _c-d_, with the horizontal _c-a_.
_R_ represents the radius of the curve upon which the vehicle is
moving. _w_ is the component of the weight parallel to the surface
_c-d_, _v_ = velocity of the vehicle in feet per second. _m_ = mass
of vehicle = _W/g THETA_
_w_ = _W_ tan _THETA_
_mv^2_ _wv^2_
_F_ = ------- = ------
_R_ _gR_
If _F_ = _w_ there will be no tendency to skid; hence the rate of
superelevation necessary in any case is as follows:
_Wv^2_
_W_ tan _THETA_ = -------
_gR_
_v^2_
tan _THETA_ = -------
_gR_
The amount of superelevation required, therefore, varies as the square
of the velocity and inversely as the radius of the curve.
Theoretically, the amount of the superelevation should increase with a
decrease in the radius of the curve and should also increase as the
square of the speed of the vehicle. On account of the variation in
speeds of the vehicles, the superelevation for curves on a highway can
only be designed to suit the average speed. At turns approaching
ninety degrees, the curve is likely to be of such short radius that it
is impossible to maintain the ordinary road speed around the curve,
even with the maximum superelevation permissible. It is good practice
to provide the theoretical superelevation on all curves having radii
greater than 300 feet for vehicle speeds of the maximum allowed by
law, which is generally about 25 miles per hour. Where the radii are
less than 300 feet, the theoretical superelevation for the maximum
vehicle speeds gives a superelevation too great for motor trucks and
horse drawn vehicles and generally no charge is made in superelevation
for radii less than 300 feet, but all such curves are constructed with
the same superelevation as the curve with 300 foot radius.
The diagram in Fig. 7 shows the theoretical superelevation for various
curve radii.
[Illustration: Fig. 7. Curves showing Theoretical Superelevation for
Various Degrees of Curve for Various Speeds of Vehicle]
At the intersection of important highways, the problem is complicated
by the necessity for providing for through traffic in both directions
and for traffic which may turn in either direction and the engineer
must provide safe roadways for each class of traffic.
=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.
TABLE 4
AVERAGE TRACTIVE RESISTANCE OF ROAD SURFACES TO STEEL TIRED
VEHICLES
Surface Tractive force per ton
Earth packed and dry 100
Earth dusty 106
Earth muddy 190
Sand loose 320
Gravel good 51
Gravel loose 147
Cinders well-packed 92
Oiled road--dry 61
Oiled road--wet 108
Macadam--very good 38
Macadam--average 46
Sheet asphalt 38
Asphaltic concrete 40
Vitrified brick--new 56
Wood block--good 33
Wood block--poor 42
Cobblestone 54
Granite tramway 27
Asphalt block 52
Granite block 47
TABLE 5
TRACTIVE RESISTANCES TO STEEL TIRED VEHICLES[1]
----------+-----------------+-----------------+-----------+-----------
| | Condition | Tractive | Resistance
Test No. | Kind of Road | of Road | Total lb. | per ton lb.
----------+-----------------+-----------------+-----------+-----------
29-30-31 | Concrete |Good, excellent | 83.0 | 27.6
| (unsurfaced) | | |
[2]11-12 | Concrete |Good, excellent | 90.0 | 30.0
| (unsurfaced) | | |
26-27-28 | Concrete 3/8-in.|Good, excellent | 147.6 | 49.2
| surface | | |
| asphaltic oil | | |
| and screenings| | |
13-14 | Concrete 3/8-in.|Good, excellent | 155.0 | 51.6
| surface | | |
| asphaltic oil | | |
| and screenings| | |
9-10 | Macadam, |Good, excellent | 193.0 | 64.3
| water-bound | | |
22-23 | Topeka on |Good, excellent | 205.5 | 68.5
| concrete | | |
8 | Gravel |Compact, good | 225.0 | 75.0
| | condition | |
[3]45-48 | Oil macadam |Good, new | 234.5 | 78.2
[4]46-47 | Oil macadam |Good, new | 244.0 | 81.3
38 | Gravel |Packed, in | 247.0 | 82.3
| | good condition | |
18-19-20 | Topeka on plank |Good condition, | 265.0 | 88.3
| | soft, wagon | |
| | left marks | |
34 | Earth road |Firm, 1-1/2-in. | 276.0 | 92.0
| | fine loose dust| |
24-25 | Topeka on plank |Good condition, | 278.0 | 92.6
| | but soft | |
1-2-5 | Earth road |Dust 3/4 to 2 in.| 298.0 | 99.3
3-3 | Earth |Mud, stiff, firm | 654.0 | 218.0
| | underneath | |
6-7 | Gravel |Loose, not packed| 789.0 | 263.0
----------+-----------------+-----------------+-----------+-----------
[1] Prof. J. B. Davidson in _Engineering News-Record_, August 17,
1918.
[2] Graphic record indicates that the load was being accelerated
when test was started.
[3] Drawn with motor truck at 2-1/2 miles per hour.
[4] Drawn with motor truck at 5 miles per hour.
Comparatively few data are available showing the tractive resistance
of motor vehicles, but the following tables are based on sufficient
data to serve to illustrate the general trend.
These data on the tractive resistances of an electric truck with solid
rubber tires on asphalt and bitulithic, wood, brick and granite block,
water-bonded and tar macadam, cinder and gravel road surfaces were
obtained by A. E. Kennelly and O. R. Schurig in the research division
of the electrical engineering department of the Massachusetts
Institute of Technology, and are published in Bulletin No. 10 of the
division.
An electric truck was run over measured sections, ranging from 400 to
2600 feet in length, surfaced with these various materials, at certain
speeds per hour, ranging from about 8 to about 15.5 miles per hour.
The result of the observations of speeds, tractive resistances,
conditions of surfaces, etc., were collected and studied in various
combinations.
TABLE 6
----------------------+-----------------------+-----------+----------
| | Tractive | Tractive
| |Resistance |Resistance
Type of Surface | Condition of Surface | in lbs. | in lbs.
| | per ton | per ton
| | 10 miles |12.4 miles
| | per hr. | per hr.
----------------------+-----------------------+-----------+----------
Asphalt | Good | 20.4 |
Asphalt | Poor | 22.6 | 25.5
Wood block | Good | 24.2 | 25.3
Brick block | Good | 24.6 | 26.6
Granite block | Good | 40.3 | 45.75
Brick block | Slightly worn | 25.1 | 28.0
Granite block with | | |
cement joints | Good | 25.5 | 30.2
Macadam, water bonded | Dry and hard | 23.3 | 25.8
Macadam, water bonded | Fair, heavily oiled | 35.9 | 38.7
Macadam, water bonded | Poor, damp, some holes| 36.3 | 41.6
Tar macadam | Good | 25.7 | 28.0
Tar macadam | Very soft | 36.8 | 38.7
Tar macadam | Many holes, soft, | |
| extremely poor | 52.4 | 60.6
Cinder | Fair, hard | 27.5 | 30.6
Gravel | Fair, dusty | 30.4 | 33.0
----------------------+-----------------------+-----------+----------
[Illustration: Fig. 8]
=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. In Fig. 8 _W_ represents the
weight of the vehicle, acting vertically downward, _w_ is the
component of the weight perpendicular to the road surface and _W_{2}_
is the component parallel to the road surface.
_W_{2}_ = _W_ tan _THETA_.
tan _THETA_ = 0.01 x per cent of grade.
_W_{2}_ = 0.01 _W_ x per cent grade.
_W_{2}_ = 0.01 x 2000 x per cent of grade, for each ton
of weight of vehicle.
Hence _W_{2}_ = 20 lbs. per ton of load for each one per cent of
grade.
The gravity force acting upon a vehicle parallel to the surface on a
grade is therefore 20 lbs. per ton for each one per cent of grade and
this force tends either to retard or to accelerate the movement of the
vehicle.
Let _F_ = the sum of all forces opposing the translation of a vehicle.
_F = f_{r} + f_{i} + f_{p} + f_{a} + f_{g}_ (1)
where
_f_{r}_ = rolling resistance of road surface.
_f_{i}_ = resistance due to internal friction in the vehicle.
_f_{p}_ = resistance due to impact of the road surface.
_f_{a}_ = resistance due to air.
_f_{g}_ = resistance due to grade, which is positive when
ascending and negative when descending.
All of the above in pounds per ton of 2000 lbs.
Let _T_ = the tractive effort applied to the vehicle by any means.
_T_ >= must be greater than _F_ in order to move the vehicle.
By an inspection of (1), it will be seen that for a given vehicle and
any type of road surface, all terms are constant except _f_{a}_ and
_f_{g}_. _f_{a}_ varies as the speed of the vehicle and the driver can
materially decrease _f_{a}_ by reducing speed. _f_{g}_ varies with the
rate of grade. For any vehicle loaded for satisfactory operation on a
level road with the power available, the limiting condition is the
factor _f_{g}_. If the load is such as barely to permit motion on a
level road, any hill will stall the vehicle. Therefore, in practice
the load is always so adjusted that there is an excess of power on a
level road. If draft animals are employed the load is usually about
one fourth of that which the animals could actually move by their
maximum effort for a short period. With motor vehicles, the excess
power is provided for by gearing.
If it be assured a load of convenient size is being moved on a level
road by draft animals, there is a limit to the rate of grade up which
the load can be drawn by the maximum effort of the animals.
Tests indicate that the horse can pull at a speed of 2-1/2 miles per
hour, an amount equal to 1/8 to 1/10 of its weight, and for short
intervals can pull 3/4 of its weight. The maximum effort possible is
therefore six times the average pull, but this is possible for only
short intervals. A very short steep hill would afford a condition
where such effort would be utilized. But for hills of any length, that
is, one hundred feet or more but not to exceed five hundred feet, it
is safe to count on the draft animal pulling three times his normal
pulling power for sustained effort.
The limiting grade for the horse drawn vehicle is therefore one
requiring, to overcome the effect of grade, or _f_{g}_, a pull in
excess of three times that exerted on the level.
A team of draft animals weighing 1800 lbs. each could exert a
continuous pull of about 1/10 of their weight or 360 lbs. If it be
assumed that the character of the vehicle and the road surface is such
that _f_{r}_ + _f_{i}_ + _f_{p}_ + _f_{a}_ = 100 lbs. per gross ton on
a level section of road, then the gross load for the team would be 3.6
tons. The same team could for a short time exert an additional pull of
three times 360 lbs. or 1080 lbs. For each 1 per cent of grade a pull
of 20 lbs. per ton would be required or _f_{g}_ for the 3.6 tons load
would be 72 lbs. for each per cent of grade. At that rate, the
limiting grade for the team would be fifteen per cent.
If, however, the character of the vehicle and the road surface were
such that _f_{r}_ + _f_{i}_ + _f_{p}_ + _f_{a}_ = 60 lbs. per gross
ton on a level section of road, the gross load for the team on the
level would be 6 tons, and the limiting grade 9 per cent.
The above discussion serves to illustrate the desirability of adopting
a low ruling or limiting grade for roads to be surfaced with a
material having low tractive resistance and the poor economy of
adopting a low ruling grade for earth roads or roads to be surfaced
with material of high tractive resistance.
It may be questioned whether horse drawn traffic should be the
limiting consideration for main trunk line highways, but it is
certain that for a number of years horse drawn traffic will be a
factor on secondary roads.
In the case of motor vehicles, excess power is provided by means of
gears and no difficulty is encountered in moving vehicles over grades
up to 12 or 15 per cent, so that any grade that would ordinarily be
tolerated on a main highway will present no obstacle to motor
vehicles, but the economy of such design is yet to be investigated.
=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.
[Illustration: Fig. 9.--Types of Guard Rails]
=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 shown in Fig. 9, 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 as shown in Fig. 10.
[Illustration: Fig. 10]
=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.