1C-1: Selecting Design Criteria

Design Criteria

Project design criteria are established before beginning a project.  Although specific criteria are developed for each project, the worksheets linked from this section contain criteria reflecting the best practices for most projects.  Best practice criteria come from:

• Requirements defined in the federal law governing highway design are found in 23 CFR Sec 625.4.  These requirements are highlighted in the criteria tables.
• Requirements defined in other state or federal laws.
• Guidance provided in AASHTO documents.
• Research findings.
• Provisions from the 3R Agreement with FHWA.
• Departmental decisions to balance initial costs, maintenance costs, mobility, safety, and other concerns.

Controlling Criteria

The Code of Federal Regulations (23 CFR Sec 625.4) lists the design standards for highways.  On NHS projects, federal law requires formal design exceptions when the 10 controlling criteria listed in Federal Register Vol. 81, No. 87, page 27187 are not met.  These controlling criteria are listed in Table 1 and are highlighted (as explained in Table 1) in the design criteria tables. Refer to Section 1C-8 for guidance related to writing a formal design exception.

Table 1: Controlling Criteria for Design*

*   Yellow highlight in the design criteria worksheets means design exception required if acceptable value is not met.  Green highlight in the design criteria worksheets means design exception required if design speed ≥ 50mph and acceptable value is not met.

Selecting Project Design Criteria

In the past, designers typically began with a list of standard features that were incorporated into all projects.  These features were only omitted if unique physical or financial constraints were identified.  New research and the recent focus on substantive safety measures have demonstrated this standards-based design approach normally yields suboptimal results. In a fiscally constrained condition, designing to these standards can result in a decrease in the safety and operations of the overall system, because when funds are invested in areas that do not provide a return on the investment, funding isn’t available to correct more critical locations.

When selecting design criteria for a project within an existing corridor, designers should consider the safety and operations of the existing route for all users including bicyclists and pedestrians.  Designers should identify areas with crash histories, locations where delays are common, and infrastructure durability problems, and analyze solutions for these problems.  These solutions should be identified in the project design criteria.  Designers should avoid investing in features that don’t solve tangible problems and don’t provide a cost effective improvements.  They should instead focus on upgrading features that will solve tangible problems and provide cost effective improvements. Some features that don’t show up on standardized criteria lists, such as intersection type, can provide significant improvements and should be considered for incorporation into the project.

A benefit-cost (B/C) analysis is the most reliable method to determine the cost effectiveness of design solutions.  For a project that is not within an existing corridor, a direct safety analysis of the system is not available, but similar routes can be evaluated. The Highway Safety Manual and CMF Clearinghouse are useful tools designers should consult when determining the correct design criteria for a given project.

Design Criteria Worksheets

For each project, a project design criteria worksheet is developed by the project PMT or developed with a project concept statement.  A completed Project Design Criteria Worksheet is approved by the Office Director or ADE responsible for the design of the project.

The following design criteria worksheets contain design elements based upon various roadway types:

Secondary Roads

The design criteria worksheets apply to the primary roadway system. Secondary roads are designed to guidelines within Instructional Memorandum (I.M.) No. 3.210 or guidelines specifically requested by a county or city.

The following worksheet is for project documentation:

Secondary Roads

Rural Areas

Roadways in rural areas are usually designed for higher speeds, serve longer trips with minimal interruption to through movements, and are usually located outside of boundaries established by state and local officials.

Urban Areas

Urban roadways are usually designed for lower speeds, have a high number of access points which interrupt through movements, usually serve multiple modes of travel, and have a higher traffic density.  Designers need to consider future growth, as it could change the limits of the existing urban area.

A roadway within a corridor may change from a rural environment to an urban environment.  Designers should understand criteria from both types of roadways in order to blend the two areas together as the roadway transitions from one area to the other. 

Designers should also understand the differences between reconstruction and 3R projects.

Reconstruction

Reconstruction projects include overlays adding more than 4″ (typically) of pavement structure, inlays, new construction, and conversion of a two lane roadway to a four lane roadway, or any project on the Interstate.  The same design criteria worksheets listed above are used for reconstruction projects. 

Resurfacing, Rehabilitation, and Restoration (3R)

3R projects include overlays adding 4 inches or less (typically) of pavement structure (not on the Interstate system). The 3R Agreement outlines allowable exceptions to the reconstruction design criteria. Contact the Methods Engineer for a copy of the current 3R Agreement.

3R Project Design Criteria Worksheet

Working within Project Constraints

To meet the constraints of a project, a roadway element may need to be designed using values less than the acceptable values listed in the worksheets.  A case such as this requires judgment and serious consideration of many factors:

• The type of project (i.e., new construction, reconstruction, or 3R).
• The functional classification of the road, the amount and character of the traffic, and the crash history of the road.  A crash analysis may need to be performed.
• The degree of variance from the acceptable value.
• The effect the variance has on the safety and operation of the facility, and its compatibility with adjacent sections of the roadway.
• How the feature will affect other criteria.
• The introduction of mitigating features (e.g. pavement markings, signing, delineation, or rumble strips).
• The impact of the full variance on scenic, historic, or other environmental features.

During the concept field review, record existing features such as foreslopes, provided clear zone distance, shoulder cross-slopes, and shoulder widths to aid in the selection of an appropriate design concept.  Having a record of the features is important since meeting the initial design criteria for one feature may require significant improvements to other features.  Understanding how all the features affect each other will help to avoid designing projects that can escalate beyond their initial scope, both in cost and in type of work.  Consider all design options and select the best overall design that will balance safety and funding.

Safety Repairs

Safety repairs should be addressed whenever possible.  Expecting safety repairs to be included in future resurfacing projects can lead to a steady degrading of the highway system as repairs are continually delayed.  Because funds are limited, targeting funds towards projects that will provide the best value is very important.  Funding spent completing repairs that provide marginal safety benefits in one area may be better spent repairing more pressing safety issues in other areas.

Mitigation Strategies

If a project element cannot be designed to meet the acceptable values recommended in the criteria worksheets, designers should consider how mitigation strategies may help to reduce operational or safety problems.  A list of possible mitigation strategies is included in Chapter 4 of Mitigation Strategies for Design Exceptions at http://safety.fhwa.dot.gov/geometric/pubs/mitigationstrategies/index.htm

Examples: Adjusting for Overlays

Typically, repair projects involve an overlay which consequently adds height to the roadway.  This can result in a vertical drop at the edge of a paved shoulder.  Erosion and settlement tend to increase this drop.  To avoid vertical drops at the edge of a paved shoulder, the foreslope and the shoulder should be adjusted to meet at the same elevation.  To achieve this balance, three options are available: steepening shoulders, steepening slopes, and narrowing shoulders.  The first step for determining the appropriate design for the project is to evaluate which features can be adjusted and still maintain, at a minimum, the acceptable values in the design criteria tables listed above.

• Steepening shoulders to 6%.

Table 2 shows the thickness of overlay that can be accommodated by building 6% shoulders rather than 4%.

• Steepening foreslopes.

   4:1 foreslopes are generally considered acceptable; however, many types of guardrail, including some cable guardrail, are not designed for slopes steeper than 6:1.  Existing guardrail should be analyzed if slopes are steepened.  In some situations, 3:1 slopes are acceptable, but clear zone and many other factors need to be considered prior to designing a 3:1 slope.  Slopes steeper than 4:1 will not typically be approved on Interstate routes.  Table 3 shows the width of foreslope reconstruction required to tie into an existing slope for various overlay depths.  To avoid erosion issues and the expense of erosion control features, rock fillets are desirable if the width of the fillet does not exceed 4 feet.  When the width is greater than 4 feet, a rock fillet can appear to be a shoulder, so earth fill is more desirable.  Whenever earth fill is required, consider upgrading the foreslopes to the preferred values since the equipment and erosion control measures will already be in place.

• Narrowing shoulders. 

  In some situations, e.g. interstate inside shoulders, Iowa DOT builds shoulders wider than AASHTO minimums.  In that case, the inside shoulder can be narrowed; however, the outside shoulder needs to be reviewed as well.

  Note: Do not narrow shoulders where bicyclists currently are, or are expected to be, accommodated.

Example 1C-1_1: Overlay Project

Example 1C-1_2: Interstate Overlay Project

Acceptable Values for 3R Roadway Features
(requires approval according to Section 1C-8)

(a) Any length of stopping sight distance within the range of values established
in the Current AASHTO "Greenbook" is acceptable for a specified speed. 

(b) Based on e maximum = 0.04 (urban) and e maximum 0.08 (rural). Nonsuperelevated curves should be according to Table 111-12 in AASHTO "Greenbook" for high speed and Figure 111-17 for low speed.

(c) Use 12' lanes if:

• (1) Route is on National Truck Network, or
• (2) Design year ADT is greater than 2,000 and truck volume is greater than 10%, or
• (3) Design year ADT is greater than 750 and truck volume is greater than 10% and regulatory speed is greater than 50 m.p.h.

(d) If transverse slopes are modified with project, use 8:1 with drainage structures and 10:1 without drainage structures.

(e) Current AASHTO Roadside Design Guide.

(f ) Fixed objects (utility poles, fire hydrants, traffic signals, etc.) which are installed or relocated due to the project should be placed at the right-of-way line or at 10' from the traveled lane.

Design loading - legal load
Bridge rail and approach guardrail to be upgraded.

(h) Railroad clearances are higher, generally 23.5' over a railroad track. Need to
verify each Railroad's requirement.

Example Problem 1C-1_1: Overlay Project

A project has been initiated to repair the pavement on a section of roadway. A pavement analysis concludes that an 8″ overlay is required. A 3.5″ overlay was previously placed.
Step 1 – Identify the design criteria that may be affected.
In this case, adding 8″ of elevation to the roadway and tying into the existing ground will impact one or more of the following:

• Lane width.
• Roadway cross-slope.
• Shoulder cross-slope.
• Shoulder width.
• Foreslopes.
• Clear zone (because it is directly related to the foreslopes).
• Ditch width.

Step 2 – Identify the current values and acceptable ranges from the Initial Design Criteria Tables
For this example, let’s assume that this is a rural Expressway on an NHS route. The values from the tables are:

Step 3 – Identify all reasonable options and determine how they would affect the design criteria
This project adds 8″ of overlay, but must also account for 4″ of height due to previous rolled shoulder and edge rut. This is roughly 12″ of height to transition into the existing ground. If the previous overlay can be milled off, then only the proposed 8″ must be accommodated. For thick overlays, or overlays on pavement that was previously overlaid, projects will likely require:

• Adding fill to the foreslope for the width of the designed clear zone.
• Installing a barrier, such as high tension cable guardrail, to shield the slope.
• Reconstruction or inlay (3:1 slopes that were previously covered with a design exception will not require a new design exception to be written if the slopes are not changed).

With any of these solutions, it will be difficult to add future overlays without significant upgrades. Therefore, with a thick overlay or inlay, the designer should consider shifting the centerline of the pavement in 1′ or 2′. This will allow extra shoulder width to accommodate future overlays on the outside shoulder where foreslope corrections are more costly. Always consult the Pavement Design Engineer to ensure an overlay will be thick enough to prevent reflective cracking before shifting the centerline.
Step 4 – Select the best option
This determination will depend on the design of pavement repair and replacement and associated costs.
Step 5 – Submit the design for approval and document the process (Section 1C-8)

Example Problem 1C-1_2: Interstate Overlay Project

An Interstate project has been initiated to repair the pavement on a section of roadway. A pavement analysis determines a 6″ overlay is required.

Step 1 – Identify the design criteria that may be affected.
In this case, adding 6″ of elevation to the roadway and tying into the existing ground will impact one or more of the following:

• Lane width.
• Roadway cross-slope.
• Shoulder cross-slope.
• Shoulder width.
• Foreslopes.
• Clear zone (because it is directly related to the foreslopes).
• Ditch width.

Step 2 – Identify the current values and acceptable ranges from the Initial Design Criteria Tables
For this example, assume the DDHV truck volume is less than 250. The initial design criteria values from the tables are:

Step 3 – Identify all reasonable options and determine how they would affect the design criteria

Shoulder cross slope, inside shoulder width, and foreslopes are greater than the acceptable value thresholds, so these areas should be analyzed to meet the project requirements.

Outside shoulders

• By steepening the shoulders to 6%, 2.4″ of the 6″ is gained (per Table 1), leaving 3.6″ yet to be accounted for.
• By constructing a 4:1 foreslope adjacent to the shoulder, the width of construction required to meet the existing foreslope to compensate for the 3.6″ of depth that is still unaccounted is about 3.6′ (per Table 2). A granular fillet at a 4:1 slope will meet the acceptable criteria. A 4:1 slope is considered recoverable, so this will not affect the clear zone. Existing guardrail should be analyzed to ensure it will function properly if slopes are steepened.

Inside shoulders

• By steepening the inside shoulders to 6%, only 1.44″ of height would be gained (per Table1), leaving 4.56″ yet to be accounted.
• In addition to the first option, by constructing a 4:1 foreslope, the width of construction to meet the existing foreslope at a 4.56″ of depth is 4.56′ (per Table 2). This exceeds the desirable width for granular fillets. Additionally, bringing in this amount of earth fill would be relatively expensive because of the erosion control features required for such a small quantity.
• Another option in addition to the first option is to construct a 3:1 foreslope. The width of construction to meet the existing foreslope would be about 2.5′ (per Table 2). This is a reasonable width for a rock fillet. However, if this option is selected, consider that the 3:1 slope is not recoverable and will impact the width of clear zone provided. The 2.5′ of 3:1 slope is deducted from the provided clear zone, leaving two median pipes within a 29′ clear zone. Because this is an Interstate project (FHWA is generally reluctant to approve 3:1 slopes on Interstate projects), this option would require significant justification.
• Another option is to reduce the inside shoulder width to 4′ and adjust the cross-slope to 6%, which creates nearly an inch of fall (per Table 1). Then, the remaining 2′ of the existing shoulder can be sloped at 4:1. This will absorb about 5″ of the added height (remember the 4% slope would have provided nearly 1″ of fall in the outer 2′). By combining the shoulder cross-slope steepening and the shoulder width narrowing, the full 6″ overlay can be accommodated.
• The previous options could be partially combined for other solutions.

Step 4 – Select the best option
For this example, we’ve selected a combination of reducing the shoulder width, steepening the cross-slope of the shoulder, and adding a 4:1 rock fillet. This option is shown below in the Project Design Criteria Worksheet.

Step 5 – Submit the design for approval and document the process (Section 1C-8)

Chronology of Changes