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Clear Zone and Roadside Drainage Features

1. Overview 

In addition to drainage channels (see Section 3.4 in the Clear Zone Overview article), other drainage features that may impact the roadside environment include curbs, pipes and culverts, headwalls, and drop box inlets. If feasible, locate these features outside the clear zone. If this is not practical, consider designing a feature that is traversable or shielded.


KYTC Standard Curb has a nearly vertical face and is 6 in. in height. Except at very low speeds, curbs are not able to redirect errant vehicles. Nor are they typically regarded as fixed objects requiring clear zone mitigation. Sloping curbs, especially those with heights of 4 in or less, can be readily traversed. The presence of curbs does not reduce the needed clear zone.

Curbs are undesirable on high-speed roadways. Vehicles traveling at high speeds that contact a curb could trip, overturn, or become airborne. If necessary, on high-speed roadways, a mountable curb may be used at the edge of the shoulder.  If guardrail is needed, construct it so the rail’s face is flush with the curb’s face.

Figure 2-1: Guardrail Aligned with Curb

If it is not practical to install the guardrail flush with the curb’s face, it can be located with a minimum lateral offset of 6 ft to minimize the likelihood that a vehicle could vault the barrier.

Figure 2-2: Guardrail Offset from Curb

Photo Source:  Google Earth

For more information on curbs in urban areas see the article Enhanced Lateral Offsets in Urban Areas.

3. Drainage Structures

According to KYTC’s Drainage Manual (DR 608.3) the primary drainage features of concern in roadside safety are culvert end treatments. From the standpoint of safety, these end treatments are designed to accommodate cross-drainage situations and parallel drainage situations.

Cross-drainage structures are likely to be traversed by an errant vehicle in a direction perpendicular to the headwall and pipe (across the width). They are used in the classic culvert application when a pipe conveys water from one side of the road to the other. For small culverts, no other treatment is required. For cross-drainage structures, the RDG defines a small pipe culvert as (1) a single round pipe with a diameter 36 inches or (2) multiple round pipes each with a diameter 30 inches. However, KYTC requires safety bars on metal end sections on all round pipes 30 inches or greater and on pipes that are 24-inch elliptical equivalent or greater (see KYTC Standard Drawing RDB-155). To minimize obstacles presented by larger cross-drainage structures, consider the following options:

  • Use a traversable design.
  • Extend the structure as far as practical so it is less likely to be hit (In some instances, extending the structure costs less than shielding it.)
  • Shield the structure.
  • Delineate the structure if the above first three treatments are not appropriate (Figure 3-1).

Figure 3-1: Delineation of the Structure

Photo Source:  Google Earth

Parallel drainage structures should be designed to be traversed by an errant vehicle in a direction parallel to the headwall and pipe (along the length). Parallel structures may be located under driveways, entrances, side roads, and median crossovers. Generally, single parallel pipes with diameters 24 inches do not require a grate (AASHTO 2011). When an installation involves multiple pipes, a grate for smaller pipes may be appropriate. To minimize obstacles presented by larger parallel drainage structures, consider the following options:

  • Eliminate the structure.
  • Move the structure laterally, out of the clear zone or as far as practical, to a less vulnerable location.
  • Use a traversable design (Figure 3-2).
  • Shield the structure.
  • Delineate the structure if the above options are not appropriate.


Figure 3-2: Parallel Structure with Safety Grate

Photo Source: Google Earth

For culvert headwall and end treatments in the clear zone KYTC’s Drainage Manual (DR-606) makes the following recommendations: 

  • Projected ends are generally low-cost alternatives but are not recommended in a clear zone.
  • All safety headwalls can and should be used within the clear zone. Grates used on safety headwalls are designed to make the headwalls traversable. KYTC’s Standard Drawing RDB Series includes safety grate applications for cross- and parallel drainage structures. KYTC’s Drainage Manual (DR-606.5) discusses the following safety headwalls and their applications:
    • Sloped Box Outlet Type 1
    • Sloped and Flared Box Inlet — Outlet
    • Sloped Box Inlet or Outlet Type 1 & 2
    • Metal End Sections Types 1-4
    • Sloped & Parallel Headwalls
  • Since non-safety headwalls are not considered traversable by errant vehicles, they should only be used outside of the clear zone limits. They may also be used behind guardrail or any other type of barrier to prevent an errant vehicle from reaching them.

Drop box inlets can be used in lieu of headwalls where traffic safety is a consideration (Figure 3-3). 

Figure 3-3: Drop Box Inlets within Clear Zone

Photo Source: Google Earth

KYTC’s Standard Drawing RDB Series includes safety grate applications for inlets. RDH-030 contains sloped and parallel headwall drawings.

4. See Also

5. KYTC Documentation

KYTC Highway Design Manual

  • HD-800 Roadside Design

KYTC Drainage (DR) Manual

  • DR 606-2 Culverts: Headwalls & End Treatments
  • DR 608-3 SAFETY

KYTC Standard Drawings

6. References 

AASHTO. Roadside Design Guide, Fourth Edition, RSDG-4. American Association of State Highway and Transportation Officials, Washington, DC, 2011.

AASHTO. Guidelines for Geometric Design of Low-Volume Local Roads, Second Edition. American Association of State Highway and Transportation Officials, Washington, DC, 2019.

Roadside Safety Clear Zone Concepts Knowledge Book:

Access the complete Knowledge Book here:  Roadside Safety Knowledge Book and Clear Zone Concepts Knowledge Book

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