Section 6: Determining Length of Need of Barrier
Anchor: #i1005497Overview
The shape of the obstacle, its location with respect to travel lanes, the volume of traffic and its corresponding clear zone width are the primary variables influencing length of barrier need. Barrier can be considered rigid, and semi rigid such as single slope and Fshape concrete barriers, and metal beam guard fence.
Anchor: #i1005508Variables
After all practical means to free the roadside of obstacles have been exhausted, certain areas may remain which constitute an obstacle to errant vehicles. These areas, as illustrated in Figure A7, will be referred to as an “area of concern.”
Figure A7. Areas of Concern
Figure A7 illustrates the variables of interest in the layout of approach barrier to shield an area of concern. Length of need is equal to the sum of the following variables:
 Anchor: #WLGNJODL
 Length of upstream barrier, L_{u}, Anchor: #KNVVKJNU
 Length of barrier parallel to the area of concern, L_{p}, and Anchor: #HJELSGVD
 The length of downstream barrier, L_{d}.
When discussing length of need as it pertains to metal beam guard fence, L_{u} is the length of guard fence needed to protect traffic adjacent to proposed guard fence. Upstream refers to the guard fence upstream of traffic adjacent to proposed guard fence. While L_{d} is the length of guard fence needed to protect the opposing traffic. For roadways serving oneway traffic operations, L_{d} = 0. L_{d} is greater than zero for twoway operations when the area of concern lies within the clear zone of opposing (northbound in Figure A8) traffic as measured from the centerline pavement markings.
Figure A8. Variables Involved in Barrier Layout.
In certain instances, judgment should be exercised to supplement design chart solutions and provide for public safety. For example, high severity fixed objects (e.g., bridge columns) may justify minimum guard fence treatment where located slightly outside the clear zone if geometric conditions (i.e., steep fill slope, outside of horizontal curvature, etc.) increase the likelihood of roadside encroachments. Also, bridge class culverts require protection inside and outside the clear zone. If a bridge class culvert is outside the clear zone, consider increasing the offset of the metal beam guard fence to decrease the length of need. Maintain a 4'0" minimum distance away from the obstacle and provide a 10:1 slope for the placement of the metal beam guard fence. If the bridge class culvert is outside the clear zone, D_{u} equals the clear zone distance. If the bridge class culvert is inside the clear zone distance, D_{u} equals the distance to the outside edge of the bridge class culvert.
Anchor: #i1005574Design Equations
To determine needed length of guard fence for a given obstacle, design equations have been formulated for low volume (ADT 750 or less) and higher volume (ADT more than 750) conditions. A clear zone width of 16 ft [4.9 m] and length of roadside travel of 200 ft [61 m] are incorporated in the low volume design equation (for use on roadways when the present ADT volume is 750 or less). Also, if the clear zone required is less than 16 ft [4.9 m] and the present ADT is 750 or less, use Equation A1 for calculating the guard fence length of need.
ADT < 750 
Equation A1. 
Equation A2. 

ADT > 750 
Equation A3. 
Equation A4. 
Where:
 Anchor: #QTBBWRPC
 L_{u} = Length of guard fence needed (upstream of area of concern), ft Anchor: #CIAFQBTN
 L_{d} = Length of guard fence needed (downstream of area of concern), ft Anchor: #FFCIWTET
 D_{u} = Distance from edge of travel lane to far side of area of concern or to outside edge of clear zone, whichever is least, ft (for upstream direction of traffic) Anchor: #AULTNOII
 D_{d} = Distance from edge of travel lane to far side of area of concern or to outside edge of clear zone, whichever is least, ft (for opposing direction of traffic) Anchor: #GAKRJQCT
 G_{u} = Guard fence offset from edge of travel lane adjacent to proposed guard fence, ft Anchor: #SLPXXCBS
 G_{d} = Guard fence offset from edge of opposing direction of travel lane (centerline)
For low volume conditions, if the clear zone width 16 ft [4.9 m] is met or exceeded, L=0.
For higher volumes, a clear zone width of 30 ft [9 m] and length of roadside travel of 250 ft [76 m] are incorporated into the design equation (for use on roadways when the present ADT volume is more than 750 or the recommended clear zone is greater than 16 ft [4.9 m]):
For high volume conditions, if the clear zone width (30 ft [9 m]) is met or exceeded, L=0.
The length of need for guard fence, as illustrated below in equation A5, is equal to the sum of the required upstream length (L_{u}), the guard fence length parallel to the area of concern L_{p}, and the required downstream length.
Equation A5.
Where:
L_{total} = Length of guard fence needed
L_{u} = Guard fence Length Upstream of Area of Concern
L_{p} = Guard fence Length Parallel to Area of Concern
L_{d} = Guard fence length Downstream from area of concern
Anchor: #i1005640Using Design Equations to Determine Length of Guard Fence
Before determining length of guard fence, the designer should assemble the following pertinent data:
 Anchor: #QTPNTPUJ
 Present ADT volume, Anchor: #BPSYUJGG
 Clear zone (horizontal clearance), Anchor: #LYUKSMBT
 Traffic operations (oneway or twoway), Anchor: #XXECBAWF
 Lateral and longitudinal dimension of the area of concern, Anchor: #VFMLXNWL
 Shoulder width, Anchor: #FDNJOTGE
 Offset distance of the area of concern from the edge of travel lane (including from the centerline markings for twoway traffic operations), Anchor: #SGNUIBUJ
 Design slope conditions, (i.e. will slopes be 1V:10H or flatter?), Anchor: #TAAYTPVY
 Placement location (alongside shoulder vs. near object, flared, etc.), and Anchor: #INEMGNJG
 Presence of other nearby areas of concern which should be considered simultaneously.
Once this design data has been assembled, the appropriate equation can be used.
Where the prescribed length of the guard fence cannot be installed at a bridge end due to an intervening access point such as an intersecting roadway or driveway, the length of guard fence may be interrupted or reduced. This change in length is acceptable only in locations where the Department must meet the obligation to provide access and this access cannot be reasonably relocated. Alternative treatments in these situations include installing an appropriate radius rail, of the access location, terminating the guard fence prior to the access location with an appropriate end treatment and continuing the guard fence beyond the access location if necessary or using an alternate bridge end treatment. The selected treatment should consider potential sight line obstructions, cost and maintenance associated with the selected treatment and any accident history at the site. Reduced guard fence length to accommodate access points will not require a design exception or a design waiver.
The Example Problems section provides example problems and solutions using the design equations. The guard fence lengths produced by the equations should be rounded up to an even length of guard fence. In circumstances where site conditions permit, the rounded up length of need should terminate at the end of guard fence; any additional length of need component available from an end attenuator should be considered an additional buffer.