Chapter 4 - Elements of Design

4.5. Superelevation

4.5.3. Axis of Rotation

Roadway alignments are generally defined by a centerline (CL) and a profile grade line (PGL). The roadway may be rotated about various points on the typical section to achieve superelevation. Typically, the point of superelevation rotation (axis of rotation) corresponds to the PGL located on the inside edges of the travel lanes. On two-way roadways with a flush, raised or no median, the axis of rotation typically corresponds to the roadway centerline. Generally, rotation will occur about the centerline on roadways with an urban typical section. In most instances, the axis of rotation, the PGL or centerline and the pavement crown line are the same – although it is not mandatory.

The following represent GDOT guidelines when establishing the location of the superelevation rotation point:

• For almost all situations involving two-lane, three-lane and four-lane (with raised median) typical sections, the axis of rotation is located on the centerline of the proposed pavement. One exception to this is three-lane section which is widened to one side from two-lane sections. In this case, the axis of rotation typically follows the location of the former centerline of two-lane pavement.

• The actual point of rotation with a raised median is an imaginary point which is developed by projecting the left and right pavement cross slopes respectively and intersecting them with the project centerline to form a common point.

• In four-lane and six-lane typical sections involving depressed medians, the axis of rotation generally follows the inside edge of the inside travel lane (Lane 1). This approach facilitates consistent median drainage but can create drainage problems near median breaks. Particular attention should be paid to pavement drainage in the areas near median breaks and should examine the pavement profile of the median break crossover. A point of rotation at the centerline where depressed medians are in urban areas or where there is a potential for future development and the addition of future crossovers should be considered. In areas where superelevation rates would create median slopes greater than 4:1 it will be necessary to use split rotation points. When the median width is 44-ft. this typically occurs when the superelevation rate exceeds 5%.

• In a six-lane or 8-lane typical section involving a concrete median barrier, the axis of rotation will follow the lane line separating Lane 1 from Lane 2.

• In typical sections which involve more than three-lanes in each direction, the profile grade line (crown point or axis of rotation) will generally begin to move from its standard location on the inside edge of the inside travel lane towards the outside – in one-lane increments. This is due to the need to balance pavement drainage and to maintain practical superelevation transition and tangent runout lengths. There should never be more than a three-lane difference between the number of lanes on one side of the pavement “crown” vs. the other side.

Table 4.10. Superelevation Rotation Points and Rotation Widths, summarizes the location of the axis of rotation for various typical sections utilized by GDOT. For further information or more detail regarding typical sections, refer to Chapter 6 of this Manual or consult the typical section cells associated with the GDOT Electronic Data Guidelines.

Table 4.10. Superelevation Rotation Points and Rotation Widths

Number of Lanes

Median Width (ft.)

Median Type

Barrier Inside

Barrier Outside

Symmetric Widening

Asymmetric Widening

New Location

Reconstruction

Horizontal Location of Rotation Point

Rotation Width (ft.)

Urban Typical Section

2

0

N/A

X

X

CL of pavement

12

3

14

Flush

X

X

CL of new pavement

19

3

14

Flush

X

CL of old pavement

26

5

14

Flush

X

X

X

X

CL

31

4

20

Raised

X

X

X

X

CL

34

6

20

Raised

X

X

X

X

CL

46

4

24

Raised

X

X

X

X

CL

36

6

24

Raised

X

X

X

X

CL

48

Rural Typical Section

2

0

N/A

X

X

CL of pavement

12

3

14

Flush

X

X

CL of new pavement

19

3

14

Flush

X

CL of old pavement

26

5

14

Flush

X

X

X

X

CL of pavement

31

4

20

Raised

X

X

X

X

CL of pavement

34

6

20

Raised

X

X

X

X

CL of pavement

46

4

24

Raised

X

X

X

X

Inside of TL 1

36

6

24

Raised

X

X

X

X

Inside of TL 1

48

4

32

Depressed

X

X

X

X

Inside of TL 1

24

6

32

Depressed

X

X

X

X

TL 1

-

4

44

Depressed

X

X

X

X

Inside of TL 1

24

6

44

Depressed

X

X

X

X

TL 1

-

Ramp Typical Section

1*

0

N/A

X

X

X

X

Outside edge of TL

16

2*

0

N/A

X

X

X

X

Outside edge of TL 2

24

1*

0

N/A

X

X

X

X

X

X

Outside edge of TL

16

2*

0

N/A

X

X

X

X

X

X

Outside edge of TL 2

24

Limited Access Typical Section

1*

0

N/A

X

X

X

X

X

X

Inside of TL 1

12

2*

0

N/A

X

X

X

X

X

X

Inside of TL 1

24

3*

0

N/A

X

X

X

X

X

X

Varies

-

4*

0

N/A

X

X

X

X

X

X

Varies

-

Hybrid Typical Section

2

44

Depressed

X

X

X

X

X

Inside of TL 1

24

3

44

Depressed

X

X

X

X

X

Inside of TL 1

36

4

44

Depressed

X

X

X

X

X

Between TL 1 and TL 2

36

2

52

Depressed

X

X

X

X

X

Inside of TL 1

24

3

52

Depressed

X

X

X

X

X

Inside of TL 1

36

4

52

Depressed

X

X

X

X

X

Between TL 1 and TL 2

36

2

64

Depressed

X

X

X

X

X

Inside of TL 1

24

3

64

Depressed

X

X

X

X

X

Inside of TL 1

36

4

64

Depressed

X

X

X

X

X

Between TL 1 and TL 2

36

3

28

Flush w/ Barrier

X

X

X

X

X

Between TL 1 and TL 2

24

4

28

Flush w/ Barrier

X

X

X

X

X

Between TL 1 and TL 2

36

5

28

Flush w/ Barrier

X

X

X

X

X

Between TL 2 and TL 3

36

3

32

Flush w/ Barrier

X

X

X

X

X

Between TL 1 and TL 2

24

4

32

Flush w/ Barrier

X

X

X

X

X

Between TL 1 and TL 2

36

5

32

Flush w/ Barrier

X

X

X

X

X

Between TL 2 and TL 3

36

3

40

Flush w/ Barrier

X

X

X

X

X

Between TL 1 and TL 2

24

4

40

Flush w/ Barrier

X

X

X

X

X

Between TL 1 and TL 2

36

5

40

Flush w/ Barrier

X

X

X

X

X

Between TL 2 and TL 3

36

* One-Way Notes:       1. Outside Typical Sections                                                2. Assume 12-ft. Lane Widths                                            3. On raised medians, PGL is located by projecting pavement cross slope to the centerline          Symbols: CL = Centerline     TL = Travel Lane