HomeMy WebLinkAboutA001 - Commission Action Form dated December 16, 2015 ITEM # 9
DATE 12-16-15
COMMISSION ACTION FORM
SUBJECT: ZONING TEXT AMENDMENTS RELATING TO VISIBILITY TRIANGLE
SECTION 29.408 (5)
BACKGROUND:
For safety reasons, the City of Ames Zoning Ordinance prohibits obstructions at the
intersecting corners of streets, defined as a vision triangle. A vision triangle is generally
concerned about the height of obstructions and their width or opacity. The City's current
standard for a visibility triangle is measured at 20 feet along two intersecting streets.
The measurement is along the right-of-way line (property line), not the edge of the curb
or pavement edge. Within this visibility triangle, no structures, fences, trees, or other
vegetation higher than 36 inches above the curb line can be placed. The current
ordinance is Section 29.408 (5) and can be found in Attachment A.
The Ames Transportation Division is interested in updating our standards. Recent traffic
engineering professional research has suggested a more refined approach to defining a
vision triangle based upon the speed of traffic on a street and whether there is a control
feature (stop sign, yield sign, or stop light) at the intersecting street. The recommended
height obstruction limitations are typically between the height of 2.5 feet and 10 feet. In
some cities there are also site vision triangles established for driveways. In addition to
travel speed, there are other factors to consider, such as the particular zoning setback
allowances which might encroach into the visibility triangle. For example, in downtowns
there is usually no requirement for a vision triangle at corners to allow for buildings to be
located up at a sidewalk.
Staff has looked at how other communities define and regulate encroachments into the
visibility triangle. Full portions of these cities' ordinances are found in Attachment B. In
summary, Mason City's definition of the visibility triangle is identical to Ames' although
the restrictions are less. For example, they allow trees and other features provided there
is no visual obstruction wider than 2 feet between 3 feet and 10 feet above grade.
Ankeny defines the visibility triangle similar to Ames except for a 30 foot distance. In
addition, the city also has a definition for a driveway visibility triangle of 20 feet on a
side. Obstructions between 30 inches above grade and ten feet above grade are
prohibited.
Iowa City has a more extensive approach. Their visibility triangle is defined by the
functional classification of a street (local, collector, or arterial) which in turn defines
where the length of the triangle is measured (whether along the curb line or along the
right-of-way line). And because the triangle includes public right-of-way (since in many
instances it is measured along the curb edge), there are exemptions that explicitly allow
public signs, street lights, and traffic lights.
Wichita, Kansas has an approach based on the number of lanes, the posted speed limit,
and whether on-street parking is allowed. Furthermore, the legs of the visibility triangle
are measured along the centerline of the street. Legs of the triangle vary based on
whether it is to the right or left of the intersection and can be as long as 580 feet to the
right and 445 feet to the left (for a 4-lane road posted at 55 mph) or as short as half the
street width plus 15 feet for the controlled street.
In developing amendments for Ames, staff kept four criteria in mind: not creating an
unreasonable amount of nonconformities, being simple to evaluate during site plan
review, being able easily describe to homeowners, and being able to easily determine
compliance in the field.
Proposed Amendments
Staff is considering two factors to determine the delineation of the visibility triangle
under different circumstances. The first factor is the posted speed limit of the street.
Speed is used as a proxy for the type of street and its function as an arterial, collector or
local street. The second factor is whether it is a controlled intersection (one or more
pairs of stop signs, yield signs, or stop lights) or an uncontrolled intersection (having the
standard rule of the road to "yield to the right-of-way").
The triangle legs would be measured from the back of the curb or the edge of the
pavement if there were no curb. This differs from the current regulation of measuring
from the right-of-way line as a lack of sidewalks or other features sometimes makes the
location of the right-of-way line problematic.
The length of the triangle legs of the intersecting street would depend on the posted
speed limit of the through street. For instance, a street having a posted speed of up to
and including 30 miles per hour would have a 50 foot leg. A street having a posted
speed above 30 mph would have a 100 foot leg. Attachment C shows the existing
standard and how the new standard would be applied to particular scenarios
based on speed limits and whether it is a controlled intersection.
While these would seem significantly greater than the current 20 feet, the length of the
legs would be reduced by 50 percent if the intersecting street was a controlled
intersection. For instance, two streets posted at 30 mph having a four-way stop would
have a visibility triangle of 50 feet on each leg measured along the edge of the
pavement (compared with the current standard of having legs of 20 feet measured
along the right-of-way line).
Within that triangle, no obstruction would be allowed that would impede or obstruct
vision between a height of 3 feet and 10 feet above the grade at the curb line. It would
generally allow deciduous trees provided the trunk is trimmed between 3 and 10 feet. It
would prohibit fences and shrubs above three feet, and evergreen trees. Exceptions
would be written to allow power poles, street lights, signs, and buildings allowed by
building setbacks (such as in downtown or campustown).
The proposed language can be found in Attachment D.
ALTERNATIVES:
1. The Planning & Zoning Commission can recommend that the City Council adopt the
amendment regarding the visibility triangle.
2. The Planning & Zoning Commission can recommend the City Council decline to
adopt the proposed amendment regarding the visibility triangle.
3. The Planning & Zoning Commission can recommend alternative language for the
proposed amendments regarding the visibility triangle.
RECOMMENDED ACTION:
Staff believes the proposed amendments will improve in some areas the clear visibility
at intersections. At controlled intersections there will only be a minimal change in the
visibility triangle areas. However, administration and enforcement of the rules will be
easier for staff with the updated language. Staff believes the posted speed limit option
for defining the triangle is the preferred method. There are more involved methods of
addressing speed and sight distance at intersections, but in staff's opinion due not result
in significantly better outcomes. Alternatively to speed limits, staff could use a street
classification map to define the lengths of vision triangle legs. Although, there is a wide
range of methods and details for defining triangles at intersections, staff believes the
proposed option addresses the interests of Ames.
Therefore, it is the recommendation of the Department of Planning and Housing
that the Planning and Zoning Commission act in accordance with Alternative #1.
ATTACHMENT A: EXISTING LANGUAGE
Current language is shown here:
29.408 (5) Visibility at Intersections.
(a) Definition. "Visibility Triangle" means the area created by the intersection of
property lines at the corner of 2 abutting streets and a line connecting 2 points on
these property lines 20 feet from the point of intersection.
(b) Within the Visibility Triangle on any corner lot located in Agricultural,
Residential, or Hospital/Medical districts, no fence, wall, or other structure shall
be erected and no foliage plant permitted to grow to a height of more than 3 feet
above the elevation of the established street grade measured at the curb line at
the intersection of the streets abutting the corner lot.
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ATTACHMENT B: OTHER COMMUNITIES' APPROACHES
Mason City: (Zoning Ordinance)
Corner Lots: On corner lots in all zoning districts, except for buildings in the Z5 district,
nothing shall be erected, placed, planted or allowed to grow in such a manner as to
materially impede vision between a height of three feet (3') and ten feet (10') within
twenty feet (20') from the intersecting property lines. Said obstruction shall not be wider
than two feet (2').
Ankeny: (Zoning Ordinance)
No fence, wall, shrubbery, earthen berm, sign, billboard, or other obstruction to vision
shall be permitted which serves to obstruct vision between a height of 30 inches and ten
feet on any corner lot within a triangle of 30 feet formed by intersecting street right-of-
way lines or, in the case of interior lots, within a triangle of 20 feet formed by
intersecting driveway edge and street right-of-way lines.
As an alternative to the 30-foot and 20-foot visibility triangles defined in the paragraph
above, an unobstructed area between a height of 30 inches and ten feet at intersections
formed by intersecting street right-of-way lines or driveway edge and street right-of-way
lines may comply with the sight distance specifications established by Iowa SUDAS
(State Urban Design Standards Manual) in Chapter 5, Roadway Design, Urban
Geometric Design Criteria if approved by the City.
Iowa City: (Zoning Ordinance)
The intersection visibility standards establish triangular areas on corner lots, referred to
herein as "vision triangles", within which the placement of buildings, fences, hedges,
walls, and other structures is restricted in order to maintain clear lines of sight at street
intersections for the purposes of traffic and pedestrian safety.
14-51D-2: APPLICABILITY: Lots located at the corner of any street intersection must
comply with the requirements of this article.
14-5D-3: VISION TRIANGLES: The dimensions of vision triangles are determined as
follows: (See also figure 5D.1, located at the end of this section.)
A. At the intersection of two (2) local streets, a local street with a collector street,
or two (2) collector streets, the legs of the vision triangle will be thirty feet (30') in
length, as measured along the curb line from the point where the curbs of the two
(2) streets intersect. In the absence of a curb, the legs of the triangle will be thirty
feet (30') in length, as measured along the edge of the street pavement from the
point where the pavement of the two (2) streets intersect.
B. At the intersection of two (2) arterial streets, the legs of the triangle will be
thirty feet (30') in length, as measured along the right of way line from the point
the right of way lines of the two (2) streets intersect.
C. At the intersection of a local street with an arterial or at the intersection of a
collector street with an arterial, the local or collector leg of the triangle will be
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thirty feet (30') in length, as measured along the curb line from the point where
the curbs of the streets intersect. The arterial leg of the triangle will be thirty feet
(30') in length, as measured along the right of way line from the point the right of
way lines of the two (2) streets intersect.
Figure 5D.1 - Vision Triangles At The Intersections of Various Street Types
LOCAL.or COLLECTOR
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14-5D-4: STANDARDS WITHIN VISION TRIANGLES:
A. Prohibited Obstructions: The following obstructions are prohibited within vision
triangles:
1. Hedges and walls higher than two feet (2') above the curb level.
2. Fences higher than two feet (2') above the curb level. However, fences that
are of a type that is less than twenty percent (20%) solid, such as split rail, open
weave, or wrought iron, are permitted within the vision triangle; provided, that
such fences are kept free from plantings and other materials that are more than
two feet (2') in height. Solidity is the percent of the fence over a random area that
is made up of solid, opaque material that does not allow light or air to pass
through.
3. Signs, except as specifically exempted in subsection B of this section.
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4. Structures of any type, including principal and accessory buildings, except as
exempted in subsection B of this section.
5. Items of outdoor display or storage, including ornamental features, such as
fountains, statues, garden structures and similar features.
6. Parking and vehicular display areas.
B. Exemptions: The following structures are exempt from the provisions of this article:
1. Structures, including signs, in the CB-2, CB-5 and CB-10 zones.
2. After review by the city, buildings on lots at intersections where both streets
are signalized. The city will approve or deny a request to construct a building
within the vision triangle based on such factors as street right of way width,
speed and volume of traffic through the intersection, and the number of turning
movements.
3. Utility and street light poles.
4. Traffic control equipment, including control boxes, traffic signs, and structures
that support traffic signals.
5. Building signs located on buildings that are exempt from the vision triangle
requirements as specified in subsections B1 and B2 of this section.
6. A sign established according to Chapter 5, Article B, "Sign Regulations", of this
title, provided the bottom edge of the sign and any supporting structure is at least
eight feet (8') above the adjacent curb level, so that visual clearance is
maintained within the vision triangle. Poles and supporting structures for signs
are prohibited within the vision triangle.
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ATTACHMENT C: IMPACTS OF PROPOSED APPROACH
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ATTACHMENT D: PROPOSED LANGUAGE
Section 29.408 (5) Visibility at Intersections
(a) In order to promote a safe pedestrian and vehicular environment, a "visibility
triangle" is established at each street intersection.
(i) The visibility triangle is defined by a triangle, two legs of which are
measured from the intersection of the back of the curb line or pavement
edge (if there is no curb) extended.
(ii) The legs of the triangle are based on the posted speed limit of that
street. A street with a posted speed limit of greater than 30 miles per hour
shall have a 100 foot leg. A street with a posted speed limit of up to and
including 30 miles per hour shall have a 50 foot leg.
(iii) The length of any leg shall be halved if the street has a control feature,
such as a stop sign, yield sign, or traffic signal.
(iv) Illustrative examples are provided in Figure 1, below.
(b) Within the visibility triangle, no obstruction is allowed that would visually
obstruct vision through the visibility triangle between a height of 3 feet and 10
feet above the grade of the curb or pavement edge (if there is no curb).
Examples of obstructions include:
(i) any fence, wall, shrub, berm, or sign taller than 3 feet.
(ii) any evergreen tree.
(iii) any deciduous tree with branches extending from the trunk between 3
feet and 10 feet.
(iv) any parking of vehicles.
(v) any other permanent or temporary structure or item determined by the
traffic engineer to be a visual obstruction.
(c) The following shall not be considered an obstruction:
(i) a building conforming to the setback requirements of this Zoning
Ordinance shall not be considered an obstruction.
(ii) utility poles and street light poles.
(iii) traffic control equipment including control boxes, traffic signs, and
traffic signal poles.
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(d) For unique siutations due to topogrpahy, street alignment, or other physical
condition, the city's traffic engineer may require as part of site development plan
approval, alternative dimensions to the triangle to ensure ensure a safe clear
area at street intersections
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