CE 361 Design Project

Preliminary Design Project

Corey Barrow, Wesley King, Preston Merrell, Kelsey Stuhltrager, and Carl Vermillon

Mechanical Engineering Department

Brigham Young University- Idaho

April 8, 2014

Introduction

Many intersections exist that need improvement. When it comes to fixing a current design there is a whole

planning and programming process that needs to take place. Some of the steps involve defining the needs,

generating and evaluating alternatives, public involvement, programming and implementation, and others.

It is important to generate more than one alternative to evaluate to figure out what will create the best new

intersection as far as traffic and safety. The generating and evaluating alternatives step is the focus of this

project.

Intersection Background

The intersection of Yellowstone Highway and Trejo Street in Rexburg, Idaho is the intersection chosen for

evaluation. Currently the intersection consists of a major collector (Yellowstone Highway) and a local road

(Trejo Street). The current layout can be see in Fig. 1. The major issues facing this intersection are the

illegal through traffic from northwest bound traffic on Trejo St. and the lack of a designated left turn lane

for the northeast bound traffic on Yellowstone Highway. For northwest bound traffic on Trejo St. there

is a designated left turn lane and a designated right turn lane. No cars are supposed to cross Yellowstone

Highway to continue on Trejo St; however, there are plenty of cars that do cross Yellowstone Highway and

continue straight. This is one issue that needs to be addressed in an alternative. With the current design,

Yellowstone Highway begins to narrow when traveling southwest. This creates a pocket in between the

southwest and northeast traffic that is too small for a designated left turn lane but big enough for cars to

pull into to try and get out of the way of cars continues along Yellowstone Highway. Since this space is not

actually big enough for a car to be completely clear of the through lane, traffic still gets backed up when

cars try to turn left. This design flaw is another that needs to be addressed in a design alternative. Traffic

counts were conducted and are complied in Fig 2. As seen in the last line of Fig 3, the intersection level of

service (LOS) is a ”C”.

Design Alternatives

The original traffic counts were used to analyze alternative designs, though some adjustments had to be

estimated. PTV Vistro was used to analyze the traffic counts and produce a LOS for the original data

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Figure 1: Google Maps image of the existing intersection of Yellowstone Highway (Highway 191) and TrejoStreet.

Figure 2: Original distribution of the traffic counts obtained by members of the team

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Figure 3: The intersection level of service (LOS) of the existing intersection at Yellowstone Highway andTrejo Street.

and all the alternatives. AutoCAD Civil 3D Imperial was used to produce horizontal alignments, vertical

alignments, cross sections, and superelevations to determine how much change to the existing road would

be needed to implement the new designs.

Option 1

The first alternative that was proposed what to slightly widen Yellowstone Highway to make room for a

designated left turning lane for Northeast bound traffic. Along with the addition of the designated left turn

lane, restring would be applied to Trejo Street to allow for legal through traffic. Fig 4 illustrates the proposed

design for alternative one. This option is a relatively simple design alternative that seems to correct both the

major issues with the intersection. Fig 5 and 6 show how the traffic counts were redistributed and the new

intersection LOS, which is ”C”. From the level of service, it does not appear that the changes implemented

had a significant impact; however, the widening of Yellowstone Highway and restriping of Trejo Street will

make actions already happening safer and legal.

Option 2

As a rather extreme alternative to fixing the illegal through traffic from Trejo St, a concrete “S” barrier was

placed on Yellowstone Highway to facilitate left turns off of Trejo St but to eliminate all through traffic from

Trejo St across Yellowstone Highway. This would also not allow for any left turns from traffic traveling on

Yellowstone. With a signalized intersection a few blocks southwest and a round-about a little further away

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Figure 4: Computer generated image of alternative 1 for the intersection of Yellowstone Highway and TrejoStreet

Figure 5: New distribution of original traffic counts for alternative 1 of the redesign of the intersection ofYellowstone Highway and Trejo Street.

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Figure 6: Intersection level of service (LOS) of alternative 1 is a level ”C”

northeast of the intersection, traffic would have places to turn around if need be. The layout of this alternative

can be seen in Fig 7. The original counts obtained by members of the team were redistributed to account for

no left turns off of Yellowstone Highway and no through traffic from Trejo St across Yellowstone Highway.

Fig. 8 and 9 show how the counts were redistributed along with the intersection LOS, which is “C”. Even

though this a more extreme option that probably would not hold public favor if it was truly going to go

through the planning process, the newly implemented concrete barrier did not effect the level of service for

better or worse.

Option 3

When presenting options on how to improve a design, one option is always to not change anything. This

option is usually employed if none of the alternatives provided viable options to the issues. Original layout,

traffic counts, and LOS are present in Figures 1, 2, and 3 in the Intersection Background section.

Proposed Solution

Based off the traffic analysis, no alternative provided an increased level of service, but this is not the only

indication that an intersection needs to be redesigned. It is proposed that Option 1 be implemented for

various reasons. The main reason is safety. With illegal through traffic and a section of road that isn’t large

enough to be a designated left turn lane, there is a greater potential for traffic incidences. The second reason

is the relatively low cost. The cost of restirping Trejo street is very minimal. The more expensive part would

be acquiring the land needed to make enough room for a whole left turn lane. This however would not even

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Figure 7: Computer generated image of alternative 2 for the intersection of Yellowstone Highway and TrejoStreet

Figure 8: New distribution of original traffic counts for alternative 2 of the redesign of the intersection ofYellowstone Highway and Trejo Street.

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Figure 9: Intersection level of service (LOS) of alternative 2 is a level ”C”

be too difficult or expensive because only a few feet of extra land is needed. Overall, the implementation

of this alternative would improve the rider’s experience due to safer conditions and fresh pavement that is

much needed.

AutoCAD Civil 3D Design- Option 1

This project presented the opportunity to get acquainted with the abilities of AutoCAD Civil 3D. Horizontal

alignments, vertical alignments, and cross sections were developed for each alternative. Since Option 1 is

the solution proposed to implement, the following AutoCAD renderings in Fig 10 - 14 are only for that

option. Fig 10 shows horizontal alignments of Yellowstone Highway and Trejo Street along with the layout

of the intersection. Fig 11 and 12 are the vertical alignments of Yellowstone Highway and Trejo street,

respectively. The cross sections are illustrated in the AutoCAD assemblies of Fig 13and 14. These figures

illustrate that very little change is needed to implement this new alternative as far as cut and fill and elevation

change. Renderings for the other two options can be seen in the appendix.

ESALs calculation

Creating the pavement design was based on the data collected from our traffic counts. The volume of

vehicles was calculated for a whole year using an interval of 18 hours a day with this amount of traffic.

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Figure 10: AutoCAD Civil 3D intersection rendering along with some horizontal alignments for Yellow-stone Highway and Trejo Street-Option 1.

Figure 11: AutoCAD Civil 3D vertical alignment for Yellowstone Highway-Option 1

Figure 12: AutoCAD Civil 3D vertical alignment for Trejo Street-Option 1

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Figure 13: AutoCAD Civil 3D cross section for Yellowstone Highway-Option 1

Figure 14: AutoCAD Civil 3D cross section for Trejo Street-Option 1

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An conservative average of 200 vehicles per hour per lane (VPHPL) was used from the traffic count with

an average of 2 percent of vehicles being heavy trucks (3-S2). The calculation of Equivalent Single Axial

Loads (ESALs) is shown as follows:

Passengervehicle =200veh

hr/ln∗ 18hr

1day∗ 365days

1year= 1, 314, 000 (1)

18 − wheelers =2veh

hr/ln∗ 18hr

1day∗ 365days

1year= 13, 140veh (2)

An average weight for a passenger vehicle of 2,000 lbs and an 80,000 lbs gross weight for the heavy trucks

were assumed. The following calculations were done to determine the estimated annual ESAL.

ESAL(Passengervehicles) = 2 ∗ (1

18)4 = 1.9(10)−5 (3)

ESAL(HeavyTrucks) = (12

18)4 + (

36

33.2)4 + (

32

33.2)4 = 2.44 (4)

To get the annual ESALs for each type of vehicle, these single ESALs were multiplied by the number of

each vehicle per year. The ESALs for heavy trucks and cars were 32,061 and 25 respectively, with a total

annual ESAL of 32,086 per year. The goal in roadway design is to make the roadway last for 20 years with

minimal maintenance. Therefore the ESAL also must be adjusted to account for the population growth rate

in Rexburg, ID of 4.3%. The following equation was used:

ESAL = AnnualESAL ∗ [(1 + g)N − 1]

g(5)

where g is the annual growth rate and N the number of the expected lifetime of the pavement in years. The

20 year ESAL after population growth adjustment is 9.85(10)5. This is an important value when doing the

roadway design.

To design the different layers associated with roadway design, a few assumptions must be made: a1=0.41

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and a2=0.12, and Mr =25,000psi for the subbase and an Mr for the subgrade layers is 5,000psi. A

∆PSI=2.0, R=95%, and standard deviation=0.35 was used. Using these assumptions and the 20 year ESAL

calculated previously, along with the AASHTO nomograph, the structural numbers (SN) that allow layer

thickness calculations were obtained. The SN for the layer below the hot mix asphalt (HMA), a1, was found

first, followed by the SN of the two layers. The values for SN1 and SN2 were found to be 2.4 and 3.8,

respectively. With these values, the equation below is used to determine the depth of the layers, d:

SN = a1d1 + a2d2 (6)

The a values are used to calculate the thickness of the HMA and the subbase layers. The thickness of the

HMA is found by

2.4 = (0.41)d1 (7)

and therefore, d1 = 5.85” which is rounded up to 6.0”. The thickness of the subbase was found by

3.8 = (0.41)(6.0”) + (0.12)d2 (8)

and therefore, d2 = 11.17” which is rounded up to 11.5”. The total layer thickness is 17.5”.

Conclusion

The illegal through traffic and lack of a designated left turn lane for Northeast bound traffic at the intersection

of Yellowstone Highway and Trejo Street in Rexburg, Idaho prompted a need for a new design to fix the

problem. Three options were proposed and evaluated to try and alleviate the issues. After traffic analysis

and evaluating costs, safety, and public opinion, one option was decided upon. Yellowstone Highway was

to be widen to make room for a designated left turn lane and Trejo Street was to be restriped to no longer

make through traffic illegal.

All options and designs presented in this project were preliminary designs. The results are based off of

limited data that was able to be collected during a short period of time. With more data, such as long traffic

counts, accident reports, and public opinion surveys, more detailed and accurate alternatives would be able

to be proposed and presented.

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Appendix

The following images are from Option 2 and Option 3 of the proposed alternatives.

Option 2

Figure 15: AutoCAD Civil 3D intersection rendering along with some horizontal alignments for Yellow-stone Highway and Trejo Street-Option 2


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Option 3

Figure 20: AutoCAD Civil 3D intersection rendering along with some horizontal alignments for Yellow-stone Highway and Trejo Street-Option 3


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Documents

CE 361 Design Project