Extension of the SR-241 Toll Road
The transportation engineers at Centennial Transportation were charged with the task of
designing a two lane highway extension of the toll road SR-241 that would connect it to SR-74,
the Ortega Highway. This extension, approximately 4 miles long, would span along the Cota de
Caza mountainous terrain. In order to devise the desired design that would allow a converging
path joining the 241 and 74 corridors, it was necessary for the specific safety and design
standards of Caltrans and AASHTO to be satisfied while producing the project plan. It was also
important to take into consideration the environmental impacts of this freeway which would span
along untouched nature inhabited by many forms of wildlife. The challenge before us was to
meet standards, please nearby residents, environmentalists, and future travelers of the road all
while keeping construction cost and time at a minimum to increase efficiency. A more detailed
description of the design standards that we went by is given in Chapter One of this report.
The five skilled engineers that worked on this project started off by discussing
environmental concerns. It would have been easiest just to construct the highway along the path
with the most consistent elevation, but we wanted to take into consideration the surroundings of
the highway that residents use every day. Once we factored in the surrounding area of the
highway, we were then able to choose a path connecting the corridors. This straight path was
then connected with horizontal curves. The lengths and radii of these curves was set by safety
standards of the design speed of seventy miles per hour. The highway extension has many
horizontal curves since it goes out of its way to try and not disturb nature and wildlife habitats as
much as possible. Horizontal curves are discussed further in Chapter Two of this report.
The next task at hand for the team was designing vertical curves. Since our highway goes
through mountainous terrain and changes grade and elevation often, it was necessary to make
sure that the ride was as smooth as possible for drivers, as well as safe. In order to ensure this,
fifteen vertical curves were put in place along our highway. In placing these vertical curves, it
was necessary to ensure that they were all contained within horizontal curves for good design
practice and it was necessary to make sure that cut and fill levels of earthwork were relatively
even. More about the vertical curves in the highway extension can be found in Chapter Three of
Superelevation is an extremely important factor in ensuring the safety of drivers along the
highway. Banked roads help make horizontal curves more safe by reducing sliding friction. This
is especially important since the highway design speed is relatively high. Because of the need for
superelevated roads, much time and care was put into calculating superelevation transition
lengths of the important stations along our curves. The full list of superelevation transition
calculations can be found in Chapter Four of this report.
The last step in the design of our highway was choosing the proper pavement design.
Several different combinations of materials were available for selection, but it was important to
choose the right combination that had the least cost. All thicknesses were calculated from
AASHTO safety standards and all prices were calculated from those given in the project
description. A more detailed description of the pavement design chosen for the freeway
extension is given in Chapter Five of this report.
In the end, through the careful planning of the highway extension, construction time and
cost were minimized, while safety and environmental protection were maximized.
Chapter 1: Overview of Design
Section 1.1 Summary:
In order to produce a proper highway design that would join the CA-241 and SR-74
corridors, many factors were taken into consideration. In order to reach our goal, it was
necessary to follow all Caltrans design standards and the AASHTO pavement design method.
Here we highlight some key points of the project design including components and
characteristics of the project that are vital in many, if not all, areas of the design process. A
simple, yet critical factor was the design speed of 70 miles per hour, which determined many
design features from the early stages of the project. Also, the project’s region, residing in a rural
and mountainous area, constantly changing in elevation, also dictated certain components due to
design standards. Another factor taken into consideration was the fact that the highway only
consists of two lanes, with lane widths of 12 feet and shoulder widths of 8 feet. These basic, yet
crucial, elements of the project gave us the fundamental standard to start with and allowed us to
go forth in producing a proper design.
The production of the horizontal alignment involved the implementation of tangent paths,
with minimum elevation changes using the given topographic map. The starting and ending
points match the latitude, longitude, and elevation given as well. Once completing tangent lines,
circular curves are used to connect the tangents. These circular curves follow the Caltrans
standard radii from its design speed in the Caltrans Design Manual (Table 203).
Following the creation of the horizontal alignment, the vertical alignment was created
following Caltrans design standards, taking into consideration factors such as maximum and
minimum grades for rolling terrain (Table 204.3), the algebraic grade differences (Section
204.4), Sight Distance standards (Table 201.1), and minimum curve lengths (Figure 201.4 and
201.5) depending on tangents created. Also, the design is congruent with the initial and final
grade at the BP and EP specified by the project description while crest curves length exceeding
more than half a mile were avoided. We also made sure to practice good design form by keeping
all vertical curves within horizontal curves.
In designing the superelevation runoff of the highway, we made certain to meet
AASHTO minimum standards, selecting the appropriate superelevation for the horizontal curves
from the Caltrans design manual from Table 202.2. Once the proper superelevation rate was
chosen, the proper crown runoff and superelevation runoff lengths were chosen from Figure
202.5A on superelevation transition.
Finally, considering the pavement design, we chose an optimal design following
AASHTO flexible pavement design methods and AASHTO’s standards for minimum thickness
for each layer of pavement found in the project description. We compared the cost estimate for
each design option to find the minimal pavement cost in order to efficiently execute this project.
Section 1.2 Alternate Designs:
While producing this project, two different sets of alignments were constructed with
varying design elements. By comparison, we were able to implement transportation engineering
judgment in order to choose between the more superior of the two possible plans.
The first of the two possible highway design options, Design A, had a total length of
27,262 feet. It contained 22 horizontal curves and 14 vertical curves. This particular design was
comprised mostly of earthwork area which needed to be filled that was much greater than the
amount of cut needed.
In comparison to the first possible design, the second design option, Design B, features a
total design length of 26,883 feet. It consists of 11 horizontal curves and 15 vertical curves. This
design provided more evenly distributed areas of cut and fill, which in nature is more efficient in
overall project production, as it not only is beneficial to the budget, but also time spent in
constructing the highway.
Section 1.3 Selection of Optimal Design:
The optimal design between the two choices, we felt, was Design B. The cost of
construction in Design A would be greater than that of the latter design, due to the fact that its
length is greater than that of Design B. Also, the fact that Design A has many horizontal curves
contributes to why it is not the primary choice in design, as having a minimal amount of curves
provides a more aesthetically pleasing environment for drivers. In addition, the overall grade
change of design A was more than the grade change produced in Design B, which contains
smoother transitions in the elevation change throughout the alignment.
In consideration of the earthwork involved in the project, it costs more to seek out soil
from other locations than to use the cut area of soil from the same project to fill the necessary
space. Design A requires a significant amount of soil to be filled, which exceeds the amount of
area cut out from the design. This would result in more time and money spent displacing soil in
order to achieve adequate results. In comparison, Design B allows for cut soil from this project to
be transported to areas close by needing to be filled, which in nature is more efficient in
earthwork costs and time spent on cut and fill.
The final factor in the decision to choose Design B, was that the initial and final grades
and elevation of Design A did not correspond to that of the project description. In Design A, the
end point elevation is not the specified elevation in the provided project description and the final
grade of +3.0% did not agree as well. Though Design A did not meet the required grades and
elevation, Design B executes these necessary factors in designing the projected highway.
Chapter 2: Horizontal Alignment
Section 2.1 Design Criteria:
This section outlines the design standards followed in creating the highway’s horizontal
alignment. The focus of the highway was to keep the road on the same contour line, in order to
keep the elevation difference to an absolute minimum to keep cost low. As a part of our public
outreach, the group decided to avoid building too close to a school, Tesoro High School, located
near our starting point. Due to environmental issues, the design also took into account the
location of the Thomas F. Riley Wilderness Park, located east of the planned highway. The last
restricted area was a power plant near the end point of the highway. All precautions were taken
into account and all sensitive sites will not be disturbed by this freeway design. This project
followed its own design standards, in accordance with the standards provided by AASHTO and
Caltrans. Based off of the geometric design Table 203.2 in the Caltrans Design Manual, with a
highway design speed of 70 miles per hour, the horizontal curve’s minimum radius in the
alignment is equivalent to 2,100 feet in addition to a minimum horizontal curve length of 750
feet. This planned horizontal alignment does not implement any broken back, reverse, or
compound horizontal curves.
Section 2.2 Design Methodology:
Several factors were considered in designing the highway’s horizontal alignment:
Follow the Caltrans design standards:
A study of the vertical profile of the land before finalizing the design
Minimizing cost where applicable without losing any safety factors
Avoiding any restricted areas
Following the BP and EP latitude/longitude points given by the project guidelines
Keeping all vertical curves within horizontal curves
Exceeding minimum curve radii
The maximum horizontal curve length of 2,100 feet was not exceeded; therefore the
earthwork’s cost was minimized. In general, horizontal curves with bigger curve lengths incur a
greater cost, compared to shorter horizontal curve lengths. Table 1 represents all the information
of the eleven horizontal curves present in our project. It was crucial that our team avoided steep
drops and great inclines in the route, as this would incur great costs by requiring large amounts
of cut and fill. The highway’s horizontal alignment consisted of curves that followed the design
standards for curve radius and length while providing the highest level of safety for the lowest
Chapter 3: Vertical Alignment
Section 3.1 Design Criteria:
In this project were able to designate vertical curves which coincide with standards of
both the given project as well as the corresponding rules and regulations specified in the Caltrans
Highway Design Manual. In designing these Vertical curves, grade standards were kept in mind.
Referencing to Table 204.3, which provides the maximum grades for various forms of terrain
and highways, the corresponding grade for rolling terrain is 5%. This was the maximum grade
that was allowable in the project which was not exceeded. Also, the minimum grade of 0.3% was
met, also mentioned in Table 204.3.
An important factor in geometric design standards is sight distance, consisting of both
Stopping Sight Distance and Passing Sight Distance. The Caltrans Highway Design Manual
provides Table 201.1: Sight Distance Standards which gives a list of design speeds and their
corresponding stopping sight distances and passing sight distances in accordance with
AASHTO’s standards. With a design speed of 70 mph, the corresponding stopping sight distance
and passing sight distance are 750 ft and 2,500 ft, respectively. The consideration of the passing
sight distance is particularly important in this two lane design, for section 201.2 highlights that
only in 2-lane roads would there be consideration of a passing sight distance.
Various factors are taken into consideration when designing vertical curves, including
curve length as well as sight distance which may be considered further. In order to consider the
proper curve lengths and stopping sight distance on crest vertical curves, Figure 201.4 was
utilized to take the correct precautions. It provides two equations which factor in sight distance,
curve length, and algebraic grade difference which provides us with proper curve length. Crest
curve lengths exceeding more than half a mile were avoided, following standards. Also, the
Figure 201.5 provides proper stopping sight distance on sag vertical curves and also provides a
reference for proper curve length based on specified design speeds. There are two unique
equations to sag vertical curves relating the curve length, sight distance, and algebraic grade
difference for each curve. Sample calculations for both crest and sag curves are provided;
however, one must also consider the minimum curve lengths specified in the manual.
Due to standards, specified in Section 204.4 of the manual, an algebraic grade difference
between initial and final grades of less than 2% has a corresponding minimum vertical curve
length of 200 feet. Also, in this section is a reference to minimum curve lengths in situations
where the grade difference is greater than 2 percent and with a design speed exceeding 40 miles
per hour, stating that in such situations the minimum length of the vertical curve would be 10
times the velocity. With a design speed of 70 miles per hour, the minimum vertical curve length
is 700 ft, which would override the sample calculations of curve lengths below this number. This
condition was applied to several curves as the grade difference and design speed satisfies the
requirements. Overall, the vertical alignment design was created accurately, following standards,
and agrees with the beginning and end points, as specified in the project description.
Section 3.2 Design Methodology:
Using the proper minimum grades and minimum vertical curve lengths, we were able to
produce a design that coincides with the Caltrans safety standards. However, this project enabled
us to practice our personal opinions as transportation engineers on design as well, particularly in
terms of earthwork. As previously mentioned, we chose a design which resulted in more evenly
distributed volume between cut and fill, which would reduce both time and monetary needs to
conclude the project. Though we used more Vertical Curves than initially intended with the
previous design, we were able to reach our goal of a more efficient project result in earthwork.
Also, we hoped to produce a corridor path that would be both aesthetically pleasing and
safe for drivers. The given project, in nature, was a challenge in producing a design in such a
terrain as the Coto de Caza mountainous area, with large elevation differences. However, we
were able to design this project under such conditions while following proper Caltrans standards.
Overall, as section 204.4 in Caltrans manual states, correctly designed vertical curves
should provide safety, comfortable driving, proper drainage, adequate sight distance, and be
aesthetically pleasing. We believe that we have complied with all of the above.
Section 3.3: Summary Table
Table 2: Vertical Alignment Summary
Final Grade [%]
Min. Length of
Curve Length [ft]
Initial Grade [%]
Final Grade [%]
Min. Length of
26+81.74 34+91.00 44+76.55 60+00.00 70+10.75 80+85.46 121+50.00 146+64.20
28+81.74 36+91.00 46+76.55 62+00.00 72+10.75 82+85.46 128+50.00 148+64.20
Chapter 4: Superelevation Runoff
Section 4.1 Selection and required lengths:
For this highway design there are 11 horizontal curves, all with equal radii of 2100 feet.
Referencing to the Table 202.2 in the Caltrans Design Manual, for a 2-lane conventional
highway with a curve radii between 1900 and 2199 feet, will have a superelevation of 0.06 feet
per foot when the curve is in full superelevation. Therefore all of the horizontal curves’
superelevation runoff lengths are the same and have a value of 150 feet. The curves also all have
the same crown runoff lengths of 50 feet, as well as a normal crown of 2% and a shoulder cross
slope of 5%. Curves were designed with minimum lengths to ensure that the requirements for
driver safety and comfort are achieved.
Section 4.2 Summary Table
Table 3: Superelevation Transition Summary
Chapter 5: Pavement Design
Section 5.1 Optimal Design:
The pavement of the highway extension was designed using the AASHTO Flexible
Pavement Design Method. The cross section of the pavement design chosen, design #2, is shown
in the picture below. The surface course is made up of 5.7 inches of plant mix asphalt, with a 6
inch, cement treated base course and a 6 inch sandy clay subbase course. The objective was to
keep the cost for pavement construction as low as possible. This particular design was chosen as
the optimal design because it meets all the standards for safety and is the most affordable of all
three alternative designs. About 9,000 vehicles utilize the highway each day per lane, but only
about 720 of these vehicles are trucks that cause significant damage to the pavement over time.
The thicknesses of each section of the pavement are based on the daily axle loading of the trucks
assuming that the structural number was 4 and checking this assumption with the AASHTO
Nomograph. Because of the small percentage of trucks on the highway, it was okay to stick with
the minimum allowable thickness for the subbase layer when designing the pavement. The cost
to construct each layer of the pavement was based on the cost per volume of its individual
material. Volumes were computed from the thickness of each layer, the width of the highway
(12.2m), and the length of the entire stretch of highway (8,193.6m).
Figure 1: Cross Section of Optimal Pavement Design