FSH 7709.56 - ROAD PRECONSTRUCTION HANDBOOK
10/85 R-1 SUPPLEMENT 1
CHAPTER 4 - DESIGN
4.2 - DESIGN ELEMENTS AND STANDARDS
4.24 - Road Bed.
1. Traveled Way. The minimum single-lane road widths, including shoulders and traveled
way, shall be:
a. 12 feet for road with a defined ditch.
See example in directive.
b. 14 feet for roads without a defined ditch.
SEE EXAMPLE IN HARD COPY
Add additional width for curve widening, slough or fill widening, turnouts, etc., to these widths
when required to accommodate the traffic expected to use the road
On local roads where the traveled way surface and fill slopes are highly resistant to erosion and
in areas to be tractor logged only (no skyline yarding now or in the foreseeable future), the
minimum width may be reduced to 12 feet without a ditch. Roads shall not be designed or
constructed narrower than 12 feet. Not only are construction costs likely to be higher for
narrower roads, but also the operational, safety, and maintenance problems and costs associated
with substandard, narrow roads are unacceptable.
3. Turnouts. Provide turnouts on single-lane roads which carry two-way traffic unless other
traffic control measure are established to provide effective control of opposing traffic without
inducing a traffic hazard or unusual delay time. Effective control could be provided by a
flagperson at a gate for a short road. The gate would be closed when the flagperson was not
present. The fact that many using vehicles are equipped with CB radios does not constitute
effective traffic control or hazard reduction. Consider the desired travel speed (efficiency),
density and type(s) of traffic as well as the function and service-life of the facility, in the design,
location, and spacing of turnouts. Coordinate turnouts with horizontal and vertical alignment and
locate to take advantage of natural "daylight" sections to minimize excavation.
4. Turnarounds. Provide turnarounds at 2.5 mile minimum intervals, near road ends, or
near road closures. Turnarounds should be designed to allow the design vehicle to safely turn
around in one backing and turning manuver.
5. Curve Widening. Design of curves should not result in a more restrictive basic traveled
way width than is provided on tangents. Therefore, determine the minimum lane width on curves
using the formula shown below or Exhibits 1 and 2 in lieu of the formula and exhibits (8 and 9)
FSH 7709.56 - ROAD PRECONSTRUCTION HANDBOOK
10/85 R-1 SUPPLEMENT 1
in the parent text. Since the minimum lane width designated in this Supplement (4.24--1) is 12
feet, the National formula is revised to reflect this change in the basic width of the traveled way.
Curve widening based on a formula of 10 feet + curve widening would, in effect, reduce the
basic traveled way width on a curve without the knowledge of the driver(s), thus creating a more
hazardous or restrictive driving conditions on a curve than on tangents.
Determine minimum land (MLW) from the formula:
MLW = 12 ft + Curve Widening
SEE FORMULA IN HARD COPY
Use the following value for L:
Log truck: L = 20 feet L = 10 feet L = 20 feet
1
2
3
(Tractor wheelbase) (Stinger length) 5th wheel to trailer
duals length
minus stinger length
SEE FORMUAL IN HARD COPY
SEE EXHIBITS 1 AND 2 IN HARD COPY
To provide necessary curve widening in road designs, basic curve data (deflection angle and
radius) should be taken when:
Deflection angle greater than 15 degrees
Radius less than 175 feet on roads designed for log trucks
Radius less than 400 feet on roads designed for lowboys
This data can be recorded during location, survey, or the designer's field classification.
It is usually desirable to have at least a rough horizontal plot for use during design. It is not
necessary to plot horizontal alignment on the construction drawings. Curve widening must be
included on the drawings or in staking notes.
When designing roads with reverse or compound curves or curves separated by short tangents
(tangents shorter than the curve widening transition length), particular attention should be given
to ensure that adequate curve widening is provided.
Based on limited initial studies on reverse curves, minimum lane width at the point of reverse
curvature should be:
FSH 7709.56 - ROAD PRECONSTRUCTION HANDBOOK
10/85 R-1 SUPPLEMENT 1
12ft + 2/3(curve widening for the first curve) + 2/3(curve widening for the second curve)
Example: Lowboy design vehicle
Deflection angle #1 = 60 degrees
Deflection angle #2 = 90 degrees
Radius #1 = 100 feet
Radius #2 _ 125 feet
Minimum lane width at point of reverse curvature =
12 ft + 2/3(8 ft + 7 ft) = 22 ft
For compound curves, curve widening should be calculated using the total of the deflection
angels of both curves. The curve widening at the point of compound curvature (PCC) should be
the greater of the indicated curve widening for the two adjacent curves.
Example: Low boy design vehicle
Deflection angle #1 = 30 degrees
Deflection angel #2 = 40 degrees
Radius #1 = 125 feet
Radius #2 = 100 feet
Normal minimum lane width for curve #1 = 17 ft
Normal minimum lane width for curve #2 = 18 ft
Total deflection angle = 70 degrees
Revised minimum land width for curve #1 with deflection angle of 70 degrees = 19 ft
Revised minimum land width for curve #2 with deflection angle of 70 degrees = 20 ft
Minimum land width at PCC = 20 ft
Curve widening for reverse or compound curves separated by tangents shorter than the curve
widening transition length should be computed using the above procedures.
On Traffic Service Level C and D roads, it may be desirable to provide curve widening for
occasional lowboy traffic by designing turnouts where significant curve widening would
otherwise be required, such as on short radius curves. Where this is done, the minimum land
width should be the greater of:
1. Single land width + log truck curve widening + turnout width
or
FSH 7709.56 - ROAD PRECONSTRUCTION HANDBOOK
10/85 R-1 SUPPLEMENT 1
2. Single lane width + lowboy curve widening
The turnout should extend from PC to PT.
7. Slough Widening. Provide slough (fill) widening to ensure the minimum traveled way
width will be stable after sloughing of the cut bank and after settling and consolidation of the
outside edge of the roadbed. Inclusion of slough (fill) widening does not indicate that the entire
cut slope or the entire outside of a fill will slough or erode away. The narrow construction limits
and steep terrain often encountered preclude compaction of fills or the flattening of cut slopes
that could reduce the need for slough (fill) widening.
SEE EXAMPLE IN HARD COPY
8. Clearing Width. Clearing widths less than 30 feet may reduce the efficiency of
construction equipment and increase construction costs and time. Use more restrictive clearing
widths only when necessary to reduce impacts on critical resource values.
4.25 - Speed and Sight Distance
3. Stopping Sight Distance. Turnouts may be designed and constructed to provided the
required stopping sight distance (SSD) for crest vertical curves and horizontal curves in lieu of
double-laning that segment of road if the turnout is of sufficient length; i.e., it starts where the
SSD constraint begins and continues till the SSD constraint ends (usually PC to PT).