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HIGHWAY DESIGN MANUAL 900-3 July 1, 1990 tance must be kept clear of the encroachment in areas protected by other means such as line shown in Figure 405.7. structures or guardrailing placed for other con- siderations or on cut slopes as discussed under "General Policies" above. Care must be taken to 902,3 Trees insure that tree growth will not encroach on (1) General Policies. For purposes of this sight distance requirements, particularly on policy, large trees are defined as plants which at loop ramps. maturity, or within 10 years, have trunks 4 Large trees should not be planted in un- inches or greater in diameter, measured 4 feet protected areas of freeway or expressway me- above the ground. Small trees are those with dians with the.possible exception of separated smaller trunks or plants usually considered roadways with very wide medians. shrubs, but trained in tree form which would not develop 4-inch diameter trunks within 10 (3) Trees Planted Along Conventional High- years. Examples of small trees are crape myrtle wars. Safety, sight distance standards, and (Lagerstroemia), bottle brush (Callistemon), ole- environmental needs are the primary concerns ander (Nerium), and mayten tree (Maytenus). when establishing the locations for tree planting The general policy is that large trees must on conventional highways. be planted at least 30 feet from the edge of the Maintenance considerations are also im- ultimate traveled way on any State highway, portant. A minimum height clearance of 14 feet but with a desirable clearance of 40 feet or 6 inches from the pavement to the lower foliage more. The 30-foot distance is measured hori- of overhanging branches is necessary to provide zontal from the trunk of the tree. Large trees for the passage of trucks. In order to ensure may be planted within the 30-foot limit where this clearance and allow for the effects of they will not constitute a fixed object, for exam- growth, wind, and snow. Trees should be ple, on cut slopes above a retaining wall or in trimmed to 17 feet above the pavement. The areas in back of guardrailing which has been size, shape, and maturity of the tree should be placed for other considerations. considered when trimming is necessary to Exceptions to the 30-foot offset may also be maintain vertical clearances. Trees which will considered on cut slopes which are 2:1 or ultimately become very wide are undesirable if steeper and where ground cover and an irriga- their maintenance will cause Interference with tion system will be placed on the slopes and the traffic flow, trees cannot be offset at least 30 feet due to Large trees may be planted within 30 feet of right of way restrictions or physical restrictions the outside edge of traveled way provided all of such as retaining walls. The minimum offset in the following requirements are met: these cases should be 25 feet. (a) There is an existing speed zone of 35 miles Offset distances greater than 30 feet should per hour or less. be provided at locations such as on the outside (b) There is a curb or other barrier between the of horizontal curves, near ramp gores, at points traveled way and the trees to be planted. of congestion, or where evasive maneuvers,may be required. In areas subject to frost or snow, (c) The trees will not lower sight distance be- trees should not be planted where they will low required standards (see Topic 201). shade the pavement area, thereby isolating sec- tions of slippery pavement. (d) All trees must be located so that the mo- torist's clear vision of any highway signs or (2) Trees Planted Along Freeways and Ex- signals will be assured at all times. pressways. The policy along freeways and ex- pressways including interchange areas should (e) Trees will be planted a minimum of 2,5 feet be to strive for 40 feet or more of clearance be- behind a curb and located a minimum distance of 10 feet from any driveway, util- tween the edge of traveled way and large trees, but with a minimum clearance of 30 feet. Spe- cial considerations should be given to providing (n Single stem trees, such as palms, cedars, additional clearance in potential recovery areas. pines, and other trees that do not lend Exceptions to the minimum may be considered