4.10.7 Clear Zone

A clear recovery area, or clear zone, should be provided along highways. Clear zone requirements for 4R projects are shown in
Table 4-20
. A clear zone is the unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles.
The clear zone includes shoulders, bicycle lanes, and auxiliary lanes, except those auxiliary lanes that function like through lanes. Such a recovery area should be clear of unyielding objects where practical or shielded by crash cushions or barrier.
The clear zone values shown in
Table 4-20
are measured from the edge of travel lane.
 These are appropriate design values for all cut sections (see
Section 4.12
), for cross-sectional design of ditches within the clear zone area) and for all fill sections with side slopes 1V:4H or flatter.
It should be noted that, while a 1V:4H slope is acceptable, a 1V:6H or flatter slope is preferred for both errant vehicle performance and slope maintainability. For slopes steeper than 1V:4H, errant vehicles have a reduced chance of recovery, therefore
it is preferable to provide an obstacle-free area of 10-ft beyond the toe of steep side slopes
 even when this area is outside the clear zone.
For Freeway mainlanes, or rural arterials with greater than or equal to 750 ADT, the clear zone may be limited to 30 ft. for practical purposes; however, where design speeds are 60 mph and above and side slopes are 1V:5H to 1V:4H, a clear zone of 40 ft is desirable.
Discretionary items which are items that are not necessary for the safety, maintenance, and operation of the roadway (e.g., Gateway Monuments), their foundations and any of their associated appurtenances proposed on the roadside of a facility must not be positioned within the clear recovery zone, and it is desirable that any discretionary items, their foundations and associated appurtenances be placed at least 10-ft beyond the respective minimum clear zone. If this cannot be achieved, if a more desirable alternative location is not available, and if the discretionary item must be placed within the clear zone, it must be built with appropriate breakaway supports or shielded with an appropriate crashworthy device consistent with TxDOT Roadway Standards and applications.
Table 4-20: 4R Clear Zone
Context Classification
Functional Classification
Design Speed (mph)
Avg. Daily Traffic
2
Clear Zone Width (ft)
1,3,4,5
Minimum
Preferred
Rural
Freeways
All
All
30 (16 ramp)
See 4.10.7
Rural
Arterial
All
≤ 750
> 750
16
30
30
See 4.10.7
Rural
Collector
≥ 50
All
Use above rural arterial criteria.
Rural
Collector
≤ 45
All
10
16
6
Rural
Local
All
All
10
16
6
Suburban
All
All
< 8,000
10
6
16
6
Suburban
All
All
8,000 - 12,000
10
6
20
6
Suburban
All
All
12,000 -
16,000
10
6
25
6
Suburban
All
All
>16,000
20
6
30
6
Urban
Freeways
All
All
30 (16 for ramps and collector-distributor)
Urban
All (Curbed)
≥ 50
All
Use above suburban criteria insofar as available border width permits.
Urban, Urban Core, Rural Town
7
All (Curbed)
≤ 45
All
4 from FOC
6 from FOC
Urban
All (Uncurbed)
≥ 50
All
Use above suburban criteria.
Urban, Urban Core, Rural Town
7
All (Uncurbed)
≤ 45
All
10
6
10
6
Notes:
  1. Devices such as traffic signal supports, railroad signal/warning device supports, and controller cabinets must be located as far from travel lanes as feasible. If not feasible to place outside of the clear zone, these devices may be excluded from clear zone requirements. Other non-breakaway devices must be located outside the prescribed clear zone, or these devices must be protected with barrier.
  2. Average ADT over project life (i.e., 0.5 x (present ADT plus future ADT)). Use total ADT on two-way roadways and directional ADT on one-way roadways.
  3. Without barrier or other safety treatment of appurtenances.
  4. Measured from edge of travel lane for all cut sections and for all fill sections where side slopes are 1V:4H or flatter. Where fill slopes are steeper than 1V:4H it is preferred to provide a 10-ft area free of obstacles beyond the toe of slope.
  5. Preferred, rather than minimum, values should be used where feasible.
  6. Purchase of 5-ft or less of additional ROW strictly for satisfying clear zone provisions is not required.
  7. For curbed facilities with a shoulder, bike lane or any buffer in addition to the curb offset, the minimum measurement begins at the edge of the through travel lane. The clear zone criteria is met if either 10-ft from the through travel lane or the distance measured from the FOC is met.
The designer may choose to modify clear-zone widths with adjustment factors to account for horizontal curvature, as shown in
Table 4-21
. These modifications are mainly considered when there is a high number of crashes in the area that could be cost-effectively decreased by increasing the clear zone width. Horizontal curves, particularly for high-speed facilities, are usually superelevated to increase safety and provide a more comfortable ride. Increased superelevation on curves where the superelevation is inadequate is an alternate method of increasing roadway safety within a horizontal curve. Snow and ice should be considered in this scenario when increasing the superelevation rate.
CZ
C
 = (L
C
)*(K
CZ
)
Where:
CZC =
 Clear zone on outside of curvature, ft
LC =
 Clear zone width, ft
KCZ =
 Curve adjustment factor
Table 4-21: Horizontal Curve Adjustment Factor
1
Radius, ft
Design Speed, mph
40
45
50
55
65
70
2,950
1.1
1.1
1.1
1.2
1.2
1.2
2,300
1.1
1.1
1.2
1.2
1.2
1.3
1,970
1.1
1.2
1.2
1.2
1.3
1.4
1,640
1.1
1.2
1.2
1.3
1.3
1.4
1,475
1.2
1.2
1.3
1.3
1.4
1.5
1,315
1.2
1.2
1.3
1.3
1.4
-
1,150
1.2
1.2
1.3
1.4
1.5
-
985
1.2
1.3
1.4
1.5
1.5
-
820
1.3
1.3
1.4
1.5
-
-
660
1.3
1.4
1.5
-
-
-
495
1.4
1.5
-
-
-
-
330
1.5
-
-
-
-
-
Notes:
  1. The clear-zone correction factor is applied to the outside of curves only. Corrections are typically made only to curves less than a 2,950-ft radius.