West Davis Corridor
NSLP Engineering
Tanner K. Harris, J. Wyatt Nelson, Daxen G. Smith, Jeffrey R. Huffman
Introduction
The West Davis Corridor (WDC) is a corridor planned by the Utah Department of Transportation (UDOT) extending from approximately the Legacy/I-15 junction in Davis County and S.R. 193 in West Point. The 16-mile divided highway will
- Alleviate congestion from commuter traffic,
- Provide a route for truck traffic to communities in the western portion of Davis County, and
- Accommodate the additional 52,000 commutes that are anticipated to be generated by 2040 from 2015 levels.
NSLP engineering focused on the southern terminus of this project. The interchange needed to connect northbound Interstate 15 and Legacy Parkway to northbound West Davis Corridor, and southbound West Davis Corridor to southbound Legacy Parkway and Interstate 15.
Alternatives
WDC to Legacy, Legacy to I-15 Alternative
The WDC to Legacy, Legacy to I-15 Alternative connects the WDC directly to Legacy but not I-15. A separate ramp structure then connects Legacy to I-15, further south. The advantage of this alternative is that it has a much smaller footprint than any of the other alternatives.
Figure 1: WDC to Legacy, Legacy to I 15 Alternative
Braided Ramp Alternative
The Braided Ramp Alternative combines all ramps that cross over a highway into one single overpass. Southbound Legacy to WDC does not cross over any existing highways and would connect on the ground.This alternative would result in a small footprint and potentially fewermaterials as three of the ramps would combine into overpass.This alternative would have complex geometry as southbound I-15 andnorthbound Legacy would have to cross.
Figure 3: Braided Ramps Alternative
Separate Ramps Alternative
The Separate Ramps Alternative separates each direction of traffic into its own overpass. Once grounded, the exits would combine to make up the West Davis Corridor. No overpass is required for the southbound Legacy ramp. This alternative is simple, however requires a large footprint.
Figure 2: Separate Ramps Alternative
Figure 4: Location of interchange looking north
Decision Criteria
The alternative decision was made with a Pugh matrix with five categories: anticipated level of service (LOS), driver understanding (i.e., alignment simplicity), cost, required right of way, and impact to neighboring developments and a nearby park.
Right of way and house/park impact were considered separately as not all right of way acquisition is equally challenging or disruptive. While the separate ramps alternative required approximately the same amount of land acquisition as the braided ramps alternative, it required the acquisition and demolition of significantly more homes, and necessitated removal of a portion of a neighborhood park.
The final design was a combination of the braided ramp alternative and the separate ramp alterative. The simplicity of the separate ramp alternative provided the main framework for the final design, but it was combined with some elements of the braided ramp to reduce the footprint of the project. The final design provides an economic and relatively simple solution for the purpose and need of the project.
| Item | Quantity | Cost per unit | Cost |
|---|---|---|---|
| Area of MSE Wall | 55975 ft2 | 50 | $2,798,750.00 |
| Fill for MSE Wall section | 1119500 ft3 | 25 | $27,987,500.00 |
| Fill for non MSE Wall Section fill factor 1.2 | 7005.84 ft 3 | 25 | $175,146.00 |
| Roadway costs | 4670 ft 2 | 100 | $467,000.00 |
| Bridge pier | 7 | 250000 | $1,750,000.00 |
| Bridge deck | 1200 ft 2 | 10000 | $12,000,000.00 |
| 18 in Storm Drain | 3150 ft | 50 | $157,500.00 |
| Drainage Structure CB-9 | 13 ea | 5000 | $65,000.00 |
| Total: | $45,400,896.00 |
Table 2: Northbound I 15 to WDC Cost Estimate
Design
A complete design of this project would have entailed a full-employment contract for a team of environmental, transportation, traffic, geotechnical, drainage, and structural engineers. To that end, the scope here was significantly limited. Past the initial alignments for all four ramps, design work was only completed for the ramp connecting northbound I-15 to northbound West Davis Corridor. Design elements include
- drainage
- a cross-section of the highway
- signage
- barriers
- side slopes
- horizontal and vertical curves
- bridge pier placements, and
- bridge clearances.
The guiding design standards for this project were the UDOT Roadway Design and Structures Design Detailing Manuals and the Utah Manual on Uniform Traffic Control Devices.
| Weight | Separate | Ramps Score | WDC to Legacy, | Legacy to I 15 Score | Braided | Ramps Score | |
|---|---|---|---|---|---|---|---|
| Cost | 3 | 3 | 9 | 2 | 6 | 1 | 3 |
| Right of Way | 2 | 1 | 2 | 3 | 6 | 2 | 4 |
| Driver Understanding | 4 | 3 | 12 | 1 | 4 | 2 | 8 |
| LOS | 5 | 2 | 10 | 1 | 5 | 2 | 10 |
| Houses/Park Impact | 1 | 1 | 1 | 2 | 2 | 3 | 3 |
| Total | 34 | 23 | 28 |
Table 1: Pugh Matrix. A higher score shows a more desirable option
Conclusion
Figure 5. Final Alignment
The cost estimate for the northbound I-15 to northbound WDC ramp is $45.5 million. A full estimate is included in Table 2. Cost estimates were obtained from UDOT standards and from unit material costs for similar projects.
To complete design work for the southern interchange, additional work would include
- A full geotechnical analysis and design
- A full design of bridge structures
- Re-alignment of the Legacy Bike Path
- Hydraulic culvert design for a small stream passing west of the project
- Completed design work for all other ramps
Upon completion, the WDC is anticipated to shorten commutes for residents of western Davis County, provide a route for commercial traffic, and accommodate growth in the area.
References
Utah Department of Transportation. (2017). Chapter 1: Purpose and Need for Action. In West Davis Corridor: Final Environmental Impact Statement. Retrieved from https://westdavis.udot.utah.gov/wpcontent/uploads/2019/10/WDC_FEIS_01_Purpose.pdf
Special thanks to Dr. Michelle Mekker, Rex Harris, P.E., Professor Austin Ball, P.E., and the Utah Department of Transportation