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6.  Vertical Alignment Checks

This section describes each individual PRM vertical alignment element check. Included in the discussion is a definition of the check, the input data requirements necessary to perform the check, PRM check process, boundary and rounding conditions, special conditions, and the output generated by the PRM for each check. Notes are made to illustrate the differences between the 1990, 1994, 2001 and 2004 editions of the AASHTO A Policy on Geometric Design of Highways and Streets (AASHTO policy).

There is a general consensus of state DOTs and the FHWA that there are 13 controlling geometric elements that are of sufficient importance and are basic to the operational quality of the road, and so should explicitly be considered in design exceptions. Nine of these geometric elements are reviewed in the IHSDM policy checks.

The following vertical alignment checks are included in this section (those denoted with * are controlling criteria):

Table 23., Summary of Input Data for Vertical Alignment Checks is a summary of the input data required to perform each vertical alignment check.

Table 23.:  Summary of Input Data for Vertical Alignment Checks
Vertical Alignment Check/Input Data
 Vertical Tangent Grade
 Vertical Curvature
Project Type
*
Functional Classification
 *
Highway Terrain
 *
Design Speed
 *
 *
Horizontal Alignment Data
 *
 *
Vertical Alignment Data
 *
 *
AADT
 *



6.1  Vertical Tangent Grade

Grade is defined as the rise or fall in elevation expressed as meters per 100 m [feet per 100 ft] horizontal and is expressed in percent. Grade is among the 13 controlling criteria.

Ascending grades are positive and descending grades are negative. A positive or negative gradient is determined with reference to stationing of a particular alignment. An alignment that increases in elevation in the direction of increasing stationing is said to have a positive grade.

6.1.1  Input Data Requirements

To check tangent grade, the PRM user must provide the following data

6.1.2  The PRM Process

The PRM determines the tangent grade from the IHSDM highway data for the highway being evaluated. These road values are then compared to the relevant policy values, which are determined as outlined below.

AASHTO policy recommends minimum and maximum grade values. A minimum grade of 0.30 percent is recommended with curbed highways and streets (AASHTO 1990, Page 234; AASHTO 1994, Page 234; AASHTO 2001, Page 242; AASHTO 2004, Page 236). Recommended maximum grades depend on functional classification and design speed and are provided in Tables V-4, VI-3, and VII-1 of the 1990 and 1994 AASHTO policies and Exhibits 5-4, 6-4, and 7-2 of the 2001 and 2004 AASHTO policies. Additionally, the maximum values referenced in the tables may be exceeded for short lengths of grade or on low-volume rural highways.

6.1.3  Boundary Conditions and Rounding

Road and policy values for tangent grade are rounded to the nearest 0.01 percent.

6.1.4  Special Conditions

The 1990 and 1994 AASHTO policies (on pp. 233-234), the 2001 AASHTO policy (on p. 242) and the 2004 AASHTO policy (on p. 236) include provisions that: for short grades less than 150 m [500 ft] in length and for one-way downgrades, the maximum gradient may be 1 percent steeper than the values given in the tables; and for low-volume rural highways (which the PRM interprets as an ADT less than 400), the maximum gradient may be 2 percent steeper than the values given in the table.

6.1.5  Output

Results of the check are summarized in a single table of the PRM Evaluation Report. In the table, each row represents a segment of uniform grade. The table includes the following columns:

Table 24.:  Summary of Comment Statements for Tangent Grade Check
Comment
 Description of the Situation
Road value is within controlling criteria
 The tangent grade is less than or equal to the recommended maximum grade referenced in policy and greater than 0.3 percent.
Road value varies from controlling criteria
 The tangent grade is greater than the policy value for tangent grade.
Road value may vary from recommended values, check drainage
 The tangent grade is less than 0.3 percent.
No data: (comment specific to missing data element)
 Required data are missing.
No policy: (comment specific to the variable)
 Controlling variables are out of the range of the policy look up tables



6.2  Vertical Curvature

Vertical curves are used to transition from one tangent grade to another. Equal-tangent (i.e., symmetrical) parabolic curves are typically used in highway design. The curve is defined by the first grade, length of curve, and the second grade. Vertical curves are referred to as crest vertical curves or sag vertical curves. Design policy on vertical curves is based upon stopping-sight distance requirements. Vertical alignment is among the 13 controlling criteria.

6.2.1  Input Data Requirements

To check a vertical curve, the PRM user must provide the following data:

6.2.2  The PRM Process

The PRM determines the length and algebraic difference in grades for each vertical curve from the IHSDM highway data for the highway being evaluated. The PRM then computes the rate of vertical curvature, K, i.e., the length of curve divided by the algebraic difference in grades or the horizontal distance to effect a 1 percent change in gradient along the vertical curve. These computed K values are then compared to the relevant policy values, which are determined as outlined below.

AASHTO policy values for vertical curve length depend on the design speed, type of vertical curve (sag or crest) and the algebraic difference in grades between the first and second grade. Recommended minimum K values are provided in Tables III-40 and III-42 in the 1990 AASHTO policy, Tables III-35 and III-37 in the 1994 AASHTO policy, Exhibits 3-76 and 3-79 in the 2001 AASHTO policy, and Exhibits 3-72 and 3-75 in the 2004 AASHTO policy for crest and sag vertical curves, respectively. The 1990 and 1994 AASHTO policies provide a range of K values for each design speed. The PRM uses the upper value as the policy value for new construction and the lower value as the policy criterion for reconstruction. The 2001 and 2004 polices reference a single K value for each design speed. This value is used as the policy value in PRM for new construction and reconstruction.

The PRM also reports an effective design speed for each curve. The effective design speed is the speed for which the K value for the curve would be equal to the recommended minimum K value for that speed. The PRM computes effective design speed by interpolation of values in the AASHTO policy tables for recommended minimum K values.

6.2.3  Boundary Conditions and Rounding

The road and policy values for K in the vertical curve output tables are rounded to the nearest 0.01 m [0.1 ft].

6.2.4  Special Conditions

No special conditions are identified for vertical curve. Where no vertical curve exists, the PRM review for vertical curve does not apply.

6.2.5  Output

Results of the check are summarized in one table of the PRM Evaluation Report. Each row in the table represents a vertical curve. The table includes the following columns:

Table 25.:  Summary of Comment Statements for Vertical Curve K Value Check
Comment
 Description of the Situation
Road value is within controlling criteria
 The K value for the vertical curve is greater than or equal to the recommended minimum K value referenced in policy.
Road value varies from controlling criteria
 The K value for the vertical curve is less than the recommended minimum K value referenced in policy.
No data: (comment specific to missing data element)
 Required data are missing.
No policy: (comment specific to the variable)
 Controlling variables are out of the range of the policy look up tables.




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