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Section 3: Final Alignments/Profiles

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Overview

Finalizing alignments is necessary before proceeding into detailed roadway design and plans development. Setting final alignments should be performed by an experienced roadway design engineer because it involves “fine tuning” the horizontal curve radii, superelevation, grades, and stopping sight distance to optimize the design. Environmental impact minimization, safety enhancement, operational improvement, and constructability are elements that should be taken into account during this process. Drainage headwater elevation and hydraulic grade lines are an important determinant in setting roadway grades.

This section includes the following task.

50300. Design final controlling conditions

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50300: Design final controlling conditions

Description. Finalizing controlling conditions is a necessary first step in completing roadway design. Preliminary design development does not take into account as detailed analysis of conditions as is required for final design. Issues that may warrant modifications include final design of superelevation rates, stopping sight distances (SSD), intersection geometry, grades, access connections, traffic management during construction, major utility adjustments, or drainage facilities. Changes may also be required for minimizing impacts to Section 4(f) properties, wetlands, rights of way, and threatened or endangered species.

Finalizing controlling conditions is not complete without a thorough review of preliminary hydraulic assumptions. Final design considerations for drainage facilities may require curve or grade changes.

Pertinent Project Types. Projects where controlling conditions are being established or will be revised

Responsible Party. Roadway design engineer

Subtasks.

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  • Gather previous schematic designs and engineering reports.
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  • Prepare cross sections for preliminary alignment, including sections at access connections.
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  • Reevaluate alignments for opportunities to minimize environmental impacts.
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  • Reevaluate alignments while considering constructability and final design of detailed geometrics.
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  • Ensure compliance with basic design criteria. See the Roadway Design Manual, Chapter 2.
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  • Request the additional survey data required to finalize detailed design.
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  • Prepare cross sections for modified alignments.
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  • Prepare final alignment.
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  • Review stream crossing hydraulics and hydraulic grade lines.
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  • Ensure drainage considerations are addressed by adjusting roadway grades and curves with regard to hydraulic grade lines.
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  • Verify stream crossing elevations and hydraulics.
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  • Consider a lateral offset for roadway objects in urban and suburban areas to ensure that mirrors or other appurtenances of heavy vehicles do not strike roadway objects and passengers in parked vehicles are able to open their doors.

Helpful Suggestions.

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  • Areas requiring modification become apparent after developing cross sections and layouts, which show existing right of way, stopping sight distance, intersection and connection geometry, superelevation, proposed ditches, wetland limits, headwater elevations, major utilities, etc.
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  • Side roads and driveways should be checked closely to ensure that the intersecting road can be tied in without degrading stopping sight distance.
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  • Detailed consideration of construction staging often leads to minor alignment revisions for safe maintenance of traffic.
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  • Consider modeling of areas with complex features.

For hydraulic review and design:

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  • Detailed consideration of construction staging for major drainage structures can result in alignment revisions.
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  • An area of special concern occurs at drainageway intersections with the roadway. Grade changes often occur at these intersections. Drainage structure headwater elevations need to be predicted at these locations.
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  • The interrelated elements of ditch capacity, roadside slope safety, and roadway grade need to be properly designed to provide a safe facility. Ensure that proposed ditch capacity is adequate.
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  • If the job is in a tidal area, consider storm surge (tide) analysis.

Critical Sequencing.

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  • Final, detailed alignment setting should occur after the roadway design engineer thoroughly evaluates data prepared during preliminary design and coordinates the project with stakeholders.
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  • The roadway design engineer should feel comfortable that all project issues relevant to alignment setting are understood and coordinated with others.
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  • Final, detailed hydraulic review should occur after the roadway project engineer determines final controlling criteria, and after stream crossing hydraulics are completed.

Resource Material.
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