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900-2 HIGHWAY DESIGN MANUAL July 1, 1990 performance, and quality requirements, Ade- 902.2 Sight Distance and Safety quate plant establishment and functional test Requirements periods should be provided. (1) Basic Requirements. As sight distance (4) Cost-effectiveness. All elements of a de- and safety are of primary importance, these sign should provide reasonable benefit for the considerations must not be subordinated to money spent. Materials and methods specified aesthetics. Applicable horizontal and vertical should be carefully matched to the project con- sight distance standards are set forth in Topic ditions. 201. Established vegetation should be saved Proposed planting should maintain horizon- wherever practical. Mature plants, trees in tal and vertical sight distance required by the particular, represent a substantial investment design speed of the project. In cases where, of money and resources. due to geometric restrictions, the existing fa- When developing highway planting restora- cility does not provide 70 miles per hour sight distance in urban areas and 80 miles per hour tion projects, the need and justification for in rural areas. no further reduction should be planting and irrigation facilities should be rean- alyzed and the design should reflect current caused by planting. Particular attention should be paid to planting on the inside of curves, in standards. It may be proper and cost-effective median areas, and on cut slopes so that de- to replace or eliminate plants that require ex- cessive watering or pruning. Areas of the right signed sight distances in the adjacent traffic lanes are retained. of way with low public visibility should be screened rather than fully planted. (2) Zonal Concept. To provide a sound en- Specifications for materials and methods gineering basis for retaining the designed sight should not be overly restrictive. Allow the con- distances a zonal concept should be used. This tractor as many options as may be acceptable. requires setting zonal areas wherein planting Locally available materials and innovative work will be restricted either by height or by a sight methods devised by the contractor may help distance clearance line. In the first instance, the permissive height of planting should be de- lower construction costs. termined by plotting "zones" or 'belts" of equal (5) Resource Conservation. The main- height differential between the driver's line of tenance of planting and irrigation facilities rep- sight and the ground line. In the second in- resents considerable use of water, fuel, electri- stance, the clearance line should be set by hori- cal power, fertilizer, and pesticides. Minimize zontal sight distance requirements, and plant- the use of these.resources by design. ing between the edge of the roadway and the clearance line will be restricted to low ground (6) Aesthetics. A pleasant, compatible, at- cover. The determination and setting of zonal tractive appearance is a primary objective in all pareas should be by the District and suitable planting efforts. Basic principles of design should be applied. Materials and planting drawings of these determinations furnished to compositions should be compatible with the the appropriate landscape section: Finite zonal descriptions are not included in this as these surrounding landscape. A highly urban area may warrant a more cultivated appearance than may best be determined at the time the draw- Ings are prepared. rural areas. Plantings should be designed according to (3) Interchanges. All planting must be such the perspective of the viewer. For example, that ramp and collector-distributor road sight compositions. viewed by freeway motorists distance is maintained equal to or greater than . should be simple, large scale, and linear. that required by the initial design criteria with a minimum provision of 40 miles per hour sight Contour grading, and careful preservation distance. Tree planting in interchange areas Is and enhancement of existing plants, rock out- discussed under Index 902.3. croppings, and other features should be irate- At points within an interchange area where grated into the overall composition. ramp connections or channelization are pro- vided, plantings which might restrict sight dis-