APPENDIX M

Procedures for Estimating Weighted Design Speed

Weighted design speed (WDS) is defined as the weighted average of the
design speeds within the section, when each curve and tangent segment
within the section is considered to have an individual design speed.

The weighted design speed is determined for each paved standard sample
section. If the section has curve data coded, the curve data is used to
determine the weighted design speed. When curve data is not coded for
the sample section, the weighted design speed is a default value based
on functional system and facility type.

Section with Curve Data Coded

When the paved sample section has curve length reported for at least one
of the curve classes, the coded curve length(s) are used to determine
the weighted design speed.

+-----------------+-----------------+-----------------+-----------------+
| # Curve Classes |                 |                 | **Length of     |
|                 |                 |                 | Curves in Class |
|                 |                 |                 | (Miles)**       |
+-----------------+-----------------+-----------------+-----------------+
| **Curve Class** | **Radius Length | **Degree of     |                 |
|                 | (Metric)**      | Curvature       |                 |
|                 |                 | (English)**     |                 |
+-----------------+-----------------+-----------------+-----------------+
| A               | 506+            | 0.0-3.4         | xx.xxx          |
+-----------------+-----------------+-----------------+-----------------+
| B               | 321-5-5         | 3.5-5.4         | xx.xxx          |
+-----------------+-----------------+-----------------+-----------------+
| C               | 206-320         | 5.5-8.4         | xx.xxx          |
+-----------------+-----------------+-----------------+-----------------+
| D               | 126-205         | 8.5-13.9        | xx.xxx          |
+-----------------+-----------------+-----------------+-----------------+
| E               | 61-125          | 14.0-27.9       | xx.xxx          |
+-----------------+-----------------+-----------------+-----------------+
| F               | \< 61           | 28+             | xx.xxx          |
+-----------------+-----------------+-----------------+-----------------+

Weighted Design Speed = (Sample Section Length / Total Travel Time) \*
60 minutes/hour

Where: Total Travel Time (in minutes) = Sum of the travel time for each
curve class with a length coded for the class.

The travel time for each curve class (A-F) with a nonzero length is:

Travel Time = (60 / Design Speed for the Curve Class) \* Length of Curve
Class

Where the design speed for the curve class is defined in the following
table:

  ------------------------ ----------------------------------------------
  **Curve Class**          **Design Speed (MPH)**

  A                        70

  B                        60

  C                        50

  D                        40

  E                        30

  F                        25
  ------------------------ ----------------------------------------------

Section with Curve Data NOT Coded

When the paved sample section does not have curve data coded for the
section (all curve classes have length coded zero), the weighted design
speed is a default value based on functional system and facility type.

The default table is defined as:

<table style="width:100%;">
<colgroup>
<col style="width: 32%" />
<col style="width: 8%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 6%" />
</colgroup>
<tbody>
<tr class="odd">
<td></td>
<td colspan="9"><h1 id="functional-system">Functional System</h1></td>
</tr>
<tr class="even">
<td>Facility Type</td>
<td>1</td>
<td>2</td>
<td>6</td>
<td>7</td>
<td>11</td>
<td>12</td>
<td>14</td>
<td>16</td>
<td>17</td>
</tr>
<tr class="odd">
<td>Multilane Divided</td>
<td>70</td>
<td>70</td>
<td>70</td>
<td>65</td>
<td>70</td>
<td>70</td>
<td>70</td>
<td>60</td>
<td>55</td>
</tr>
<tr class="even">
<td>Multilane Undivided</td>
<td>70</td>
<td>70</td>
<td>70</td>
<td>60</td>
<td>70</td>
<td>70</td>
<td>70</td>
<td>55</td>
<td>45</td>
</tr>
<tr class="odd">
<td>2 or 3 Lane</td>
<td>70</td>
<td>70</td>
<td>65</td>
<td>60</td>
<td>70</td>
<td>65</td>
<td>65</td>
<td>55</td>
<td>45</td>
</tr>
</tbody>
</table>

A multilane divided roadway is a section with four or more through lanes
and a median type of curbed or positive barrier or median width greater
than or equal to four feet. A multilane undivided roadway is a section
with four or more lanes without a qualifying median.

Rounding the Weighted Design Speed

The weighted design speed is rounded to the nearest 5 MPH using the
following rules:

> WDS \< 32.5 set to 30 MPH
>
> WDS 32.5 to \< 37.5 set to 35 MPH
>
> WDS 37.5 to \< 42.5 set to 40 MPH
>
> WDS 42.5 to \< 47.5 set to 45 MPH
>
> WDS 47.5 to \< 52.5 set to 50 MPH
>
> WDS 52.5 to \< 57.5 set to 55 MPH
>
> WDS 57.5 to \< 62.5 set to 60 MPH
>
> WDS 62.5 to \< 67.5 set to 65 MPH
>
> WDS \>= 67.5 set to 70 MPH

Determining the WDS for a Section

A worksheet for WDS calculation is provided in Figure M-1. The steps to
be taken are as follows:

1.  For the section of highway being analyzed, list each nonzero length
    for the appropriate class in the column headed "Length of Curves in
    Class."

2.  For each curve class with length entered, determine the Travel Time
    by dividing 60 by the Design Speed for the curve class and multiply
    by the "length of curve in class". Enter this value in the column
    labeled "Travel Time."

3.  Total the Travel Time for each curve class.

4.  Divide the sample section length by the Total Travel Time and then
    multiply by 60 minutes/hour to obtain the WDS in miles per hour.

5.  Round to the nearest 5 MPH.

A sample calculation is shown on the worksheet, Figure M-1. For a rural
section, curve class D has a length of 1.20 miles for a travel time of
1.8 minutes, curve class B a length of 1.3 miles for a travel time of
1.3 minutes and curve class A a length of 3.0 miles for a travel time of
2.57 minutes. The total travel time for the curves is 5.67 minutes. The
section length (5.50 miles) divided by the total travel time (5.67
minutes) and multiplied by 60 (minutes/hour) yields a WDS of 58.2 MPH.
This is then rounded to 60 MPH.

+---------+-------------+-----------+-----------+---------------------+
| **Curve | **Degree of | **Ap      | **Length  | **Travel Time       |
| Categor | Curv        | proximate | of Curves | (Minutes)           |
| y**[^1] | ature**[^2] | Design    | in        | ((60/Design Speed)  |
|         |             | Speed**   | Class**   | \***                |
|         |             |           |           |                     |
|         |             |           |           | **Length of         |
|         |             |           |           | Curves)**           |
+---------+-------------+-----------+-----------+---------------------+
| F       | 28.0 +      | 25        |           |                     |
+---------+-------------+-----------+-----------+---------------------+
| E       | 14.0 - 27.9 | 30        |           |                     |
+---------+-------------+-----------+-----------+---------------------+
| D       | 8.5 - 13.9  | 40        | 1.20      | 1.8                 |
+---------+-------------+-----------+-----------+---------------------+
| C       | 5.5 - 8.4   | 50        |           |                     |
+---------+-------------+-----------+-----------+---------------------+
| B       | 3.5 - 5.4   | 60        | 1.3       | 1.3                 |
+---------+-------------+-----------+-----------+---------------------+
| A       | 0.0 - 3.4   | 70        | 3.0       | 2.57                |
+---------+-------------+-----------+-----------+---------------------+
|         |             | Total     |           | 5.67                |
|         |             | Travel    |           |                     |
|         |             | Time (in  |           |                     |
|         |             | minutes)  |           |                     |
+---------+-------------+-----------+-----------+---------------------+
| Section |             |           |           |                     |
| Length: |             |           |           |                     |
| *       |             |           |           |                     |
| *5.50** |             |           |           |                     |
| miles   |             |           |           |                     |
|         |             |           |           |                     |
| W       |             |           |           |                     |
| eighted |             |           |           |                     |
| Design  |             |           |           |                     |
| Speed = |             |           |           |                     |
|         |             |           |           |                     |
| (       |             |           |           |                     |
| Section |             |           |           |                     |
| Length  |             |           |           |                     |
| (*      |             |           |           |                     |
| *5.50** |             |           |           |                     |
| miles)  |             |           |           |                     |
| divided |             |           |           |                     |
| by      |             |           |           |                     |
| Total   |             |           |           |                     |
| Travel  |             |           |           |                     |
| Time    |             |           |           |                     |
| (*      |             |           |           |                     |
| *5.67** |             |           |           |                     |
| mi      |             |           |           |                     |
| nutes)) |             |           |           |                     |
| \* 60   |             |           |           |                     |
| minut   |             |           |           |                     |
| es/hour |             |           |           |                     |
|         |             |           |           |                     |
| =       |             |           |           |                     |
| *       |             |           |           |                     |
| *58.2** |             |           |           |                     |
| MPH;    |             |           |           |                     |
| Rounded |             |           |           |                     |
| WDS =   |             |           |           |                     |
| **60**  |             |           |           |                     |
| MPH     |             |           |           |                     |
+---------+-------------+-----------+-----------+---------------------+

Figure M-1. Worksheet for Calculating Weighted Design Speed (WDS)

[^1]: Curve categories are from Curves by Class (Items 63-68 in Chapter
    IV).

[^2]: For maximum super elevation rate of 0.08 foot/foot.