The major considerations in alignment design are safety, grade, profile, street width, design speed, sight distance, topography, drainage and the maneuverability, braking and performance of heavy duty vehicles. Alignment should provide for safe and continuous operation at a uniform design speed. In mountainous areas, consideration should be given to locating the street so that a southern exposure will be obtained wherever possible. Street layout should bear a logical relationship to existing or platted streets in adjacent properties and to the principles of good engineering practice. All streets shall be designed such that the streets are centered in the right of way.
In general, criteria for the horizontal and vertical geometrics of street design provided within the following tables will be the minimum acceptable values. Unless otherwise specified, refer to "A Policy On Geometric Design Of Highways And Streets", latest edition by the American Association of State Highway and Transportation Officials for geometric criteria. Other factors must also be considered in a balanced design.
A. Vertical Alignment:
1. Minimum And Maximum Grades: Minimum and maximum sustained grades shall be 0.5 percent and eight percent (8%), respectively, except as provided below. Increases in grade will be permitted for sections of street where the horizontal radius of curvature exceeds one thousand five hundred feet (1,500') as follows:
One percent (1%) for five hundred feet (500') maximum to a maximum of nine percent (9%).
Two percent (2%) for two hundred feet (200') maximum to a maximum of ten percent (10%).
a. No two (2) such sections will be within five hundred feet (500') of each other or within five hundred feet (500') of a curve having a radius of less than one thousand five hundred feet (1,500').
b. All grades will flatten to three percent (3%) or less for at least one hundred feet (100') approaching intersections, measurements taken from the intersection of the rights of way lines.
c. Grades approaching the turnarounds in cul-de-sacs will be four percent (4%) or less for at least fifty feet (50'), measurements taken from the point of tangency of the road and bulb.
d. The maximum design grade should be used infrequently rather than as a value to be used in most cases.
2. Exception To Maximum Grades: A local access or low volume street may have sections with a grade of nine percent (9%) provided all of the following conditions are met:
a. The section shall be surfaced with six inches (6") of aggregate base course and two inches (2") of PMBP asphalt.
b. The section shall be no longer than five hundred feet (500').
c. The section shall have a horizontal radius of one thousand five hundred feet (1,500') or greater.
d. Grades shall not exceed six percent (6%) for five hundred feet (500') on either end of the section.
e. Curves with a horizontal radius of less than six hundred feet (600') shall not be within five hundred feet (500') on either end of the section.
f. Land on each side of the section must be designated permanent open space.
3. Vertical Curves: Vertical curves must be designed to provide adequate stopping and passing sight distance, headlight distance, driver comfort and good drainage.
a. Stopping sight distance requirements control minimum lengths of crest vertical curves. Then minimum length for sag and crest vertical curves will be determined by table 7-B of this section.
b. Vertical curves that are long and flat may develop poor drainage and should therefore be avoided.
c. Vertical curves are not required where the algebraic difference in grade is less than 0.5 percent where curb and gutter is used in the road section and four percent (4%) otherwise.
4. Site Distance: The grade line must meet sight distance requirements for the design speed. Table 7-B of this section provides the minimum passing sight distance for the crest of vertical curves.
TABLE 7-B
Design Speed | Stopping Sight Distance | Passing Sight Distance | ||
15 | 100 | 500 | ||
20 | 150 | 700 | ||
25 | 200 | 900 | ||
30 | 250 | 1,100 | ||
35 | 300 | 1,300 | ||
40 | 400 | 1,500 | ||
45 | 500 | 1,650 | ||
50 | 600 | 1,800 | ||
55 | 700 | 1,950 | ||
B. Horizontal Alignment:
1. Stopping Site Distance: Horizontal alignment must provide at least the minimum stopping distance for the design speed at all points. This includes visibility at intersections as well as around curves and roadside encroachments. The minimum stopping sight distance is the distance required by the driver of a vehicle traveling at the design speed to bring the vehicle to stop after an object on the street becomes visible. Stopping sight distance should be in accordance with table 7-B of this section.
a. Where an object off the pavement restricts site distance, the stopping sight distance determines the minimum radius of curvature, but in no case will it be less than as specified in table 7-C of this section.
b. Offset clearance to achieve stopping sight distance on horizontal curves can be obtained from table 7-C of this section. The centerline of the inside lane is used, with the offset distance measured from the centerline of the inside lane to the obstruction.
TABLE 7-C
Design Speed | Minimum Curvature Of Radius | Minimum Tangent Lengths | ||
15 | 75 | 50 | ||
20 | 125 | 75 | ||
25 | 175 | 100 | ||
30 | 250 | 150 | ||
35 | 375 | 200 | ||
40 | 550 | 250 | ||
45 | 700 | 250 | ||
50 | 850 | 250 | ||
55 | 1,200 | 250 | ||
2. Passing Sight Distance: Passing sight distance is the minimum sight distance that must be available to enable the driver of one vehicle to pass another safely and comfortably without interfering with oncoming traffic traveling at the design speed. Two (2) lane streets should provide adequate passing zones. Required passing sight distance for given design speeds are stated in table 7-B of this section.
3. Curvature: Table 7-C of this section specifies the minimum centerline radius of curvature for specific design speeds. This table is based on speed alone and does not take into consideration sight distance factors. Every effort should be made to exceed the minimum values.
a. Consistency in design speed and curve radii should be used to avoid surprising the driver. Where changes in the design speed are necessary, the design speed between approach tangents and curves will not change by more than ten (10) miles per hour. Under no condition will a low speed curve be introduced at the end of a long tangent, where high approach speeds are anticipated. Compound curves should be avoided. Reversing curves without an intervening tangent will not be permitted where design speeds exceed twenty five (25) miles per hour. The minimum lengths of such tangents are specified in table 7-C of this section. Broken back curves are not allowed.
4. Curb Returns: Curb returns or pavement rounding radii at intersection corners are as follows:
TABLE 7-D
Street Classification | Curb Return Radius | |||
Local | 25 feet | |||
Collector | 30 feet | |||
Arterial | 50 feet | |||
a. Additional right of way will be required to provide a minimum clear distance for fifteen feet (15') between the curb or edge of pavement and the right of way limit.
b. The given criteria are intended as requirements in new developments and as desirable where feasible in redeveloping areas. All radii are measured to the back of the curb section in urban street sections and to the edge of the street surface in rural street sections.
c. The selection of appropriate curb return radii at intersections depends largely upon the governing design vehicle expected to negotiate turning movements about the return and its effect on traffic flow. Streets commonly expected to experience large commercial vehicles or bus traffic will require larger radii at intersections than local streets.
d. The designer should consult the village public works director prior to beginning design of any intersection involving principal or minor arterials and collector streets of like classifications. Radii requirements for intersections in commercial or industrial areas should also be reviewed with the village public works director prior to design. These classifications of streets frequently experience special traffic circumstances for which the village public works director will require the use of larger radii.
5. Intersections: The minimum distance between intersections for various street classifications is as follows:
TABLE 7-E
Street Classification | Distance | |||
Local | 200 feet | |||
Collector | 500 feet | |||
Arterial | 1,000 feet | |||
a. Distance is measured from the inside edge of each right of way.
C. Balanced Design: An abrupt, inconsistent change in either horizontal or vertical alignment is not acceptable. Thus, long, straight reaches of residential street, conducive to excessive speed, followed by sharp changes in alignment which can be negotiated safely only at low speed are to be avoided. Balanced design will result in appropriately consistent low speed alignment and street arrangement to preclude such hazardous situations. The designer is particularly cautioned to maintain distances and stopping distances. Production vehicles, particularly passenger cars, have rapidly changing characteristics related to driver eye height above street surface and to braking effectiveness.
When connecting street lines deflect from each other at any one point by more than ten degrees (10°), they shall be connected by a curve with a radius adequate to ensure a sight distance of not less than two hundred feet (200') for local or collector streets and for special cases of a greater radius as the village public works director shall determine. (Ord. 2007-06, 8-14-2007)