La. Admin. Code tit. 70 § I-313

Current through Register Vol. 50, No. 11, November 20, 2024

Section I-313 - Landscaping

A. General

1. Highway landscape design should begin with an analysis and the inventory of the landscape features in order to identify, evaluate and locate the features to be conserved, further developed and/or incorporated in the highway corridor.

2. Careful and proper landscaping of the right-of-way should result in the conservation, enhancement and effective display of the urban and rural countryside through which the highway passes. A properly landscaped highway will conserve the historical features and natural landscape assets while improving the aesthetic and functional quality of the highway.

3. There are two general classes of vegetation along highways: turf, such as grasses and legumes, and taller growing types consisting primarily of woody plants which are shrubs and trees. This Section is devoted to the planning and design of the taller growing plants as seen against the foundation of turf. Woody plants create a three-dimensional effect in the landscape and require special design considerations. Natural growth that exists may provide part or all of the desired planting effects in rural areas. Where possible, the retention of desirable natural existing growth is extremely important and requires consideration early in design. Planting is important along highways on new location and many times it may be more important along reconstructed highways on existing location because of restricted right-of-way and adjacent development.

4. The motorist should be able to view complete vistas and changing scenes in scale with the travel speed. Widely spaced plantings of individual trees or shrubs create a spotty and disturbing effect. Massed plantings are the form and texture of the landscape viewed at highway speeds. Tree plantings should be set back from the traveled lanes, not only for safety but also to insure spatial continuity and the strong visual effect of a wide turf area between pavement and plantings. Generous sight distance must be maintained at all times. The plants used must be capable of growing relatively well with minimum maintenance to serve their purpose under the highway conditions they may encounter.

5. Design and choice of plant materials vary considerably from region to region. Rural locations may only require supplementing existing growth with small sized new plants and planting for special functions while the urban and suburban highway may require extensive plantings with larger sized plants.

6. Planting designs should be created in accordance with the requirements of the highway and serve a justifiable purpose. They should be planned objectively on a broad scale before consideration is given to the actual selection of plants to be used. Their composition should be pleasing and coordinated with the total highway environment with safety being the most important consideration.

7. Planting plans should be clear, concise, easily understood and presented on drawings separate from the highway construction plans. The plans should indicate type of adjacent land use, topographic features, such as slope limits and utility installations in addition to the location of plants and their area of occupancy at maturity. A plant list should also be included in the plans. This will provide information concerning the species, size, condition, fertilizing requirements and other pertinent general notes which may apply. The latest and best planting techniques should be used along with top quality plants. Specifications for nursery stock, planting and other types of landscape construction should be clear, concise and describe the quality of work desired.

B. Functions of Highway Planting Design

1. In design form follows function. Some functions of highway planting design are as follows.

a. Planting for Highway Safety

i. Screening Headlight Glare. Plantings can be very effective in screening headlight glare from oncoming vehicles. Blinding vision due to headlight glare can be a cause of accidents. In addition to curved median areas, headlight glare can also be a problem between interchange loops and from frontage roads, service roads and parking areas. Shrub plantings may help prevent head-on collisions in these conditions.

ii. Delineation. Plants may be used to delineate changes in highway alignment. Headlight glare reduction plantings may serve a dual purpose in this regard. Shrubs or trees on the outside of curves may aid in directing a motorist, particularly in fog or rain storms and during night driving. Plants may also be used to aid a motorist in seeing directional signs by framing or forming a background.

iii. Psychological Design Considerations. Existing and new plantings may help to alleviate driver fatigue brought about by long stretches of riding surface that call for no change of eye focus which may even lull the driver to sleep. Emphasis may be given to directional changes by delineation plantings which aid in a driver's decision by making it easier to discern the outline of a curved roadway. These plantings may be in the median or on the outside of curves. This may be of particular importance at night when the plants are illuminated by headlights. Plantings placed beyond the junction of a "T" intersection may aid in informing a motorist of a change in direction. High headed trees may be placed within an interchange to make it conspicuous in the landscape for approaching roadways. The steepness of a cut slope may be accentuated by using vertical plant forms, or minimized by using horizontal plant forms and patterns.

iv. No vegetation shall be planted that will hide or obscure visibility of any official highway sign.

b. Planting for Environmental Mitigation

i. Traffic Noise. Traffic noise is a serious environmental problem to people living adjacent to major highways carrying large volumes of traffic. Plants absorb and scatter sound waves to a small degree. The effectiveness of plants as noise barriers is very limited because of the considerable width, height and density required. The principle noise reduction effect of plantings is psychological. When it is possible or feasible to use barriers or other actual means of attenuation, plantings may reduce human annoyance and awareness of the problem by screening the noise source from view. Evergreens are best suited for this purpose; however, they mat be used in combination with dense deciduous plants. Planting should be an integral part of noise barrier design due to their length and height. Plants can visually soften their effect and reduce the perceived massiveness of the barriers. In addition to trees and shrubs, vines are very effective for this purpose.

ii. Wildlife Habitat. Roadside plantings can provide food in the form of berries, browse and forage. Nesting cover is also provided for birds and other mammals. Preservation of existing trees and shrubs is important and the regeneration of native growth can be hastened by the establishment of mowing limits.

iii. Revegetation. Where climatic and soil conditions permit, all exposed soil surfaces should be revegetated. This may be in the form of turf, herbaceous or woody vegetation. Through the establishment of mowing limits, regeneration of native growth from adjacent seed sources will be encouraged and a natural blending with surrounding areas will occur. This form of naturalization may be hastened and supplemented by the planting of young trees and shrubs and proper maintenance activities. When reconstruction of a highway occurs, tree and shrub restoration should be included in the landscape plans to serve plant functions wherever this is feasible. This is important where existing roadside buffers must be destroyed for roadway construction.

c. Planting for Aesthetics

i. Visual Quality. Planting is one of the several methods used to improve visual quality in transportation facilities. Through the application of landscape design principles, the functional and aesthetic can blend to produce safe and pleasant highways. The highway should reflect the character of attractive communities. Trees and shrubs can provide a green buffer between the traveled way and adjacent development. Plants of larger size may be necessary in urban areas to give an immediate effect. The selection of suitable species is important in urban areas and should be based on experience in similar areas. Street tree plantings can significantly improve the visual quality of communities. Flowering trees and shrubs and wildflowers enhance the highway environment and offer pleasant and changing scenes for the motorist and adjacent property owners.

ii. Screening Undesirable Views and Objects. Screening undesirable views seen from and toward the highway can be performed with plants, earth berms, fences and combinations thereof. Space permitting, plantings offer a variety of forms and combinations which can be arranged to obtain the desired results. Although effective screening with plants may take several years to achieve, this should not deter or discourage the use of this method. Sight lines from and toward the highway, of the object to be screened, should be studied and a determination of the type of screening to be used should be made. Where a year-round effect is desired, evergreen plants should dominate and deciduous should be added for seasonal and textural interest. Whenever possible, consideration should be given to the removal of the objectionable object.

d. Setback Distances for Trees

i. These guidelines may be applied to new plantings of trees whose trunk diameter at maturity will be 4 inches or greater. Setback distances or vehicle recovery areas are related to type of slope, slope ratio, traffic volumes and design speed of the highway. The setback is from the traveled way, which is the portion of the roadway for the movement of vehicles, exclusive of shoulders and auxiliary lanes. Minimum horizontal and vertical clearance setbacks for all roads will be governed by the Louisiana Department of Transportation Design Standards.

ii. Given distances will not always be practical. Variations in site-specific conditions need to be considered and may warrant special treatment. Existing historic, aesthetic or environmentally important trees may be retained within the recovery area if they are protected or are not in a target position, such as the outside of horizontal curves. Shrubs and ground cover may be planted within the recovery area for safety and aesthetic purposes.

iii. The above guidelines should be used unless one of the following reasons will allow for a lesser distance or require a greater distance: For central business districts and local streets with barrier curbs, a minimum distance of 1.5 feet should be provided beyond the face of the curb to the anticipated outside diameter of the tree trunk when mature. On urban arterials and collectors with similar curbs and usually higher speeds, the offset distances should be increased.

iv. Where limited right-of-way or the necessity for planting would result in less clearance, all factors in the area should be weighted to decide if a special exception is warranted. Special exceptions or conditions may include:

(a). where exceptional or unique trees because of size, species or historic value exist;

(b). on designated scenic roads or low-speed roads, as well as low-speed urban roads;

(c). where the absence or removal of trees would adversely affect rare/endangered/threatened species (plant or animal), wetlands, water quality or result in serious erosion/sedimentation effects;

(d). locations where the cumulative loss of trees would result in a significant adverse change in character of the roadside landscape;

(e). landscape, park, recreation, horticultural, residential or similar areas where trees and other forms of vegetation provide significant functional and/or aesthetic value.

v. Trees should not be placed or remain where they are particularly vulnerable to vehicle contact or where significant incidences of run-off road accidents occur.

C. Criteria for Landscaping Interstate and Major Primary Routes

1. The clear distance from the edge of the traveled way to the face of the tree line shall be a minimum of 50 feet on the mainline and 30 feet for ramps. The setback is measured from the traveled way, which is the portion of the roadway for the movement of vehicles, exclusive of shoulders and auxiliary lanes. These distances apply to trees with trunk diameter of 4 inches or greater at maturity.

2. Trees may be planted or remain within the 50-foot clear distance or the 30-foot clear ramp area when they are protected by guardrail on nontraversable backslopes or other protected areas. Setback distances behind guardrails are as follows.

a. The minimum distance behind guardrail depends on the deflection of the guardrail as described in the AASHTO reference cited. Examples of this setback distance are 11 feet for cable guardrail, 3 feet for W-Beam guardrail and no distance for concrete barriers.

b. Although there is no minimum distance behind rigid barriers, consideration should be given to tree branching and maintenance in determining setbacks.

3. The clear distance from the edge of the travel lane to the shrubbery line shall be determined by Sheet 1 of 4 of the design standards.

4. Exit gore areas shall be free for a distance of 350 feet of trees and shrubbery which will attain a height greater than 2.5 feet. Shrubbery which will not attain a height of 2.5 feet will be permitted in the gore area. In rural areas, exit gores shall be free of trees for a distance of 600 feet along the mainline and 500 feet along the ramp.

5. Entrance ramps shall require a minimum of 350 feet along the ramp (sight triangle to the main roadway) free of trees and shrubbery. In the case of loop ramps, a point 350 feet from the gore area, intersecting the main travel lane and extended to the nose of the off ramp preceding the loop or 600 feet (whichever is longer) shall be clear of trees and shrubbery, with the exception of shrubs which will not attain a height greater than 2.5 feet. In the case of large rural diamond interchanges, there will be no planting over 2.5 feet in height along a line of sight from a point 500 feet from the gore area intersecting the mainline and a point 600 feet along the mainline from the gore area.

6. The clear distance from the edge of ramps to the shrubbery line shall be a minimum distance of 15 feet.

7. The clear distance from the edge of entrance and exit ramps to the tree line shall be a minimum of 50 feet. No trees will be permitted within 50 feet of the inside and outside edge of a loop ramp. Shrubbery may be planted in front of any group of trees planted outside the 50-foot line. Shrubbery planted within the 30-foot line on the inside of loop ramps shall not attain a height greater than 2.5 feet in order to provide adequate stopping sight distance around the loop.

8. Ramp terminals at the cross roads shall have an unobstructed view of the crossroad for a minimum of 150 feet in all directions. The view back beyond the structure from the exit ramp intersection with the cross road shall be kept unobstructed within the limits set by the columns or embankment.

9. No trees shall be planted within utility rights-of-way or in areas which may interfere with power lines once the trees mature.

10. Refer to the figures below for graphic illustrations of these criteria.

11. Design guidelines may be loosened so as to permit maintenance-intensive designs which might not otherwise be permitted on the state highway system. Examples might include fountains, statuary and/or planting schemes which require a high level of consistent maintenance in order to assure success.

D. Criteria for Landscaping Arterial Roads, Collector Roads, Local Roads and Streets

1. The normal set-back distance for trees (See Design Standards for Urban UA-1, UA-2) whose trunk diameter at maturity will exceed 4 inches shall not be closer than 40 feet from the travel edge of the roadway except under special conditions:

a. on the high or cut side of the roadway not in the likely path of an uncontrolled vehicle;

b. on the low or fill side if protected by a guardrail or not likely to be hazardous to an out-of-control vehicle;

c. if important historically or aesthetically and, protected by a guardrail;

d. on routes in cities and towns with speed limits 35 mph or less, a minimum of 10 feet behind a barrier curb to the face of the tree. Trees of this size will only be allowed in medians which are 30 feet or greater in width and protected by a barrier curb.

2. Small trees, with trunks normally less than 4 inches, such as crape myrtle, wax myrtle, etc., will be allowed in medians, on routes with speed limits up to 45 mph, under the following conditions:

a. minimum setback determined by design standards behind a barrier curb. Not more than 4 feet of the tree spread will be allowed to overhang the roadway;

b. a minimum setback of 30 feet beyond the edge of the travel way, for uncurbed roadways and medians, providing they do not interfere with the drainage pattern.

3. The clear distance from the right edge of the travel way to the shrubbery line shall be a minimum of 25 feet. When protected by a barrier curb, the minimum will be determined by design standards.

4. On curves, adequate sight distance for the design speed of the highway must be maintained, in accordance with design standards.

5. For safety reasons, control of landscaping at intersections is critical. Sight triangles at intersections are determined by the design speeds of the intersecting roadways. Any object within the sight triangle high enough above the elevation of the adjacent roadways to constitute a sight obstruction will not be allowed. No trees shall be planted in sight triangles. Shrubbery and ground cover will be allowed in the sight triangle providing their height does not exceed 2.5 feet above the roadway surface. Minimum sight distance requirements for intersections are illustrated in "Sight Distance Requirements at Typical Intersection."

6. Refer to the figures below for illustrations of these criteria.

7. Design guidelines may be loosened so as to permit maintenance-intensive designs which might not otherwise be permitted on system. Examples might include fountains, statuary, art and/or planting schemes which would require a high level of consistent maintenance in order to assure success.

8. A plant list outlining various species that have been used for highway planting is included in this manual (See §313. E) This should not be the only plant material considered for highway landscaping.

9. The landscape unit of DOTD will provide technical assistance, standard plans and suggestions for construction methods along highway rights-of-way to local governing bodies. The local governing body should address the request to the Secretary of the Department of Transportation and Development in order to obtain assistance. Upon completion of the planning and design phase of a project, the governing body which requested the project will obtain a permit from the department's permit unit. This permit will stipulate that the governing body will construct and maintain the project at no cost to the Department of Transportation and Development.

E. Planting List

1. The following is a listing of plants which have been used on landscaping projects with success. This list is only intended as a guide and is not considered to be all inclusive:

a. shrubs and ground covers (30" maximum height):

i. Liriope (Liriope Muscari or Liriope Spicata);

ii. Monkey Grass (Ophiopogon Japonicum);

iii. Asian Jasmine (Trachelosperum Asiaticum;

iv. Daylilly (Hemerocalis Spp.);

v. Indian Hawthorne "Clara" (Raphiolepis Indica);

vi. "Blue Rug" Juniper (Juniperus Horizontalis "Wiltonii");

vii. "Blue Pacific" Juniper (Juniperus Conferta "Blue Pacific");

viii. Parson's Juniper (Juniperus Parsoni);

b. small shrubs (4' maximum height):

i. Dwarf Yaupon (Ilex Vomitoria Nana);

ii. Dwarf Chinese Holly (Ilex Cornuta Rotunda);

iii. Indian Hawthorne "Peggy" and "Clara" (Raphiolepis Indica);

iv. Compacta Juniper (Juniperus Chinensis Pfitzeriana Compacta);

v. Dwarf Oleander "Petite Pink" or "Petite Salmon" (Nerium Oleander)*;

vi. Fountain Grass (Pennisetum Setaceum);

vii. Maiden Grass (Miscanthus Sinensis);

*These plants should be used in only the southern most areas of the state due to their susceptibility to freezing.

c. large shrubs:

i. Pampas Grass (Cortaderia Argentea);

ii. Eleagnus (Eleagnus Angustafolia);

iii. *Pittosporum (Pittosporum Tobira);

iv. *Oleander (Nerium Oleander);

v. Spiraea (Spiraea Reevesiana);

vi. *Primrose Jasmine (Jasminum Primulinum);

vii. Ligustrum (Ligustrum Japonica);

viii. *Viburnum (Viburnum Odoratissimum);

ix. Photinia (Photinia Fraseri);

x. Pineapple Guava (Feijoa Sellowiana);

xi. *Sago Palm (Cycas Revoluta);

*These plants should be used only in the southern half of the state due to their susceptibility to freezing.

d. small trees (25' maximum height.):

i. Crape Myrtle (Lagerstroemia Indica or Lagerstroemia Indica x Fauriel);

ii. Wax Myrtle (Myrica Cerifera);

iii. Leggy Yaupon (Ilex Vomitoria);

iv. Tree Hollies (many varieties) (Ilex);

v. Leggy Ligustrum (Ligustrum Japonica);

vi. Leggy Photinia (Photinia Fraseri);

vii. Leggy Pineapple Guava (Feijoa Sellowiana);

viii. *Leggy Viburnum (Viburnum Odoratissimum);

ix. **Crab Apple (Malus Spp.);

x. *Vitex (Vitex Agnus Castus);

xi. Japanese Magnolia (Magnolia Soulangeana);

xii. Purple Plum (Prunus Cerasifera);

xiii. *Windmill Palm (Trachycarpus Fortunei);

xiv. *Palms (Many Varieties);

*These plants should be used only in the southern portions of the state due to their susceptibility to freezing.

**These plants should be used only in the northern portions of the state.

e. medium trees:

i. Drake's Elm (Ulmus Parvifolia Sempervirens "Drake");

ii. Pistachio (Pistachia Chinensis);

iii. Bradford Pear (Pyrus Calleryana "Bradford");

iv. *Golden Rain Tree (Koelreuteria Bipinnata);

v. *Cabbage Palm (Sabal Palmetto);

vi. *Palms (Many Varieties);

*These plants should be used only in the southern portions of the state due to their susceptibility to freezing.

f. large trees:

i. Live Oak (Quercus Virginiana);

ii. Sawtooth Oak (Quercus Acutissima);

iii. Water Oak (Quercus Nigra);

iv. Shumard Oak (Quercus Shumardii);

v. Red Maple (Acer Rubrum Drummondii);

vi. Silver Maple (Acer Saccharinum);

vii. Tulip Poplar (Liriodendron Tulipifera);

viii. American Elm (Ulmus American);

ix. Cedar Elm (Ulmus Crassifolia);

x. Winged Elm (Ulmus Alata);

xi. Sweet Gum (Liquidambar Styraciflua);

xii. Cypress (Taxodium Distichum);

xiii. Southern Magnolia (Magnolia Grandiflora);

xiv. Weeping Willow (Salix Babylonica);

xv. Pines (most varieties) (Pinus).

F. Typical Urban Cloverleaf

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G. Typical Urban Diamond

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H. Typical Rural Interchange

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I. Median Planting for Barrier Curbed Roadways

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J. Minimum Setbacks for Highway Plantings without Barrier Curbs

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K. Sight Distance Requirements

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La. Admin. Code tit. 70, § I-313

Promulgated by the Department of Transportation and Development LR 17:204 (February 1991), amended LR 26:1670 (August 2000).

AUTHORITY NOTE: Promulgated in accordance with R.S. 47:820.6.