HomeMy WebLinkAboutTrans 2015-10-19 Item 2A - Discussion - 42nd Avenue South Sight Distance AnalysisCity of Tukwila
Jim Haggerton, Mayor
INFORMATIONAL MEMORANDUM
TO: Mayor Haggerton
Transportation Committee
FROM: Bob Giberson, Public Works Director -/t-
By: Robin Tischmak, City Engineer
DATE: October 16, 2015
SUBJECT: 42nd Ave S - Multiple Intersections
Sight Distance Analysis
ISSUE
Investigate sight distance and visibility for the existing conditions on 42nd Ave S at the
intersections of S 140th St, S 146th St and S 148th St.
BACKGROUND
Multiple complaints /concerns from local residents and drivers have recently been received by
the City regarding limited sight distance entering 42 "d Ave S from stop controlled side streets
that include S 140th St, S 146th St and S 148th St. Public Works Engineering staff has reviewed
each location and compiled data and photographs of the existing conditions. Engineering
standards for sight distance were applied at each location to determine if any deficiencies
exist and whether or not any improvements or other action is warranted.
ANALYSIS
A sight distance analysis for each location will be presented at the meeting, including any
recommended corrective actions. Information regarding sight distance design standards and
driver responsibility is attached for reference.
RECOMMENDATION
For Transportation Committee discussion only.
Attachment: Excerpts from 2011 AASHTO Manual
RCW 46.61.190
C \Users \susan \Desktop \TC Agenda Items \Info Memo Sight Dist.docx
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5 -20
A Policy on Geometric Design of Highways and Streets
width provided, traffic volume, remaining life of the structure, pedestrian volume, snow storage, design
speed, crash history, and other pertinent factors.
Vertical Clearance
Vertical clearance at underpasses should be at least 4.3 m [14 ft] over the entire roadway width, with an
allowance for future resurfacing. Pedestrian, bicycle, and sign structures should be provided with a verti-
cal clearance of at least 4.5 m [15 ft].
5.3.4 Roadside Design
Clear Zones
Clear zones are not applicable to local urban streets.
Lateral Offset
Lateral offset is defined in Section 4.6.2. Further discussion and suggested guidance on the application of
lateral offsets is provided in the AASHTO Roadside Design Guide (9).
On all streets a minimum lateral offset of 0.5 m [1.5 ft] should be provided between the curb face and
obstructions such as utility poles, lighting poles, and fire hydrants. In areas of dense pedestrian traffic, the
construction of vertical curbing (typically 150 to 225 mm [6 to 9 in.] high) aids in delineating areas with
high- volume pedestrian traffic.
Trees are acceptable along local streets where speeds are 60 km /h [40 mph] or less, where curbs are pres-
ent, and where adequate sight distance is available from intersecting streets and driveways.
Guardrail is not used extensively on local streets except where there is a significant risk to motorists and
pedestrians, such as along sections with steep foreslopes and at approaches to overcrossing structures.
On facilities without a curb and with a shoulder width less than 1.2 m [4 ft], a minimum lateral offset of
1.2 m [4 ft] from the edge of the traveled way should be provided.
5.3.5 Intersection Design
Intersections, including median openings, should be designed with adequate intersection sight distance,
as described in Section 9.5, and the intersection area should be kept free of obstacles. To maintain the
minimum sight distance, restrictions on height of embankment, locations of buildings, on- street parking,
and screening fences may be appropriate. Any landscaping in the clear -sight triangle should be low grow-
ing and should not be higher than 1.0 m [3 ft] above the level of the intersecting street pavements.
Intersecting streets should meet at approximately a 90- degree angle. The alignment design should be ad-
justed to avoid an angle of intersection of less than 60 degrees. Closely spaced offset intersections should
be avoided, whenever practical.
The intersection and approach areas where vehicles are stored while waiting to enter the intersection
should be designed with a relatively flat grade; the maximum grade on the approach leg should not exceed
5 percent where practical. Where ice and snow may create poor driving conditions, the desirable grade on
the approach leg should be 0.5 percent with no more than 2 percent wherever practical.
2
Chapter 5 —Local Roads and Streets
At street intersections, there are two distinct radii that need to be considered —the effective turning radius
of the turning vehicle and the radius of the curb return (see Figure 5 -3). The effective turning radius is
the minimum radius appropriate for turning from the right -hand travel lane on the approach street to the
appropriate lane of the receiving street. This radius is determined by the selection of a design vehicle ap-
propriate for the streets being designed and the lane on the receiving street into which that design vehicle
will turn. Desirably this radius should be at least 7.5 m [25 ft].
II
_Apo
Ri = Actual Curb Radius
R2 = Effective Radius
R2
0IJ_
Figure 5 -3. Actual Curb Radius and Effective Radius
for Right -Turn Movements at Intersections
The radius of the curb return should be no greater than that needed
radius. However, the curb return radius should be at least 1.5 m [5
sweeping equipment.
In industrial areas with no on- street parking, the radius of the curb
[30 ft]; the use of a three - centered curve with sufficiently large radius
expected with some frequency is desirable.
to accommodate the design turning
ft] to enable effective use of street-
return should not be less than 10 m
to accommodate the largest vehicles
Further information pertaining to intersection design appears in Chapter 9.
5.3.6 Railroad- Highway Grade Crossings
Appropriate grade- crossing warning devices should be installed at all railroad- highway grade crossings
on local roads and streets. Details of the devices to be used are given in the MUTCD (12). In some states,
the final approval of the devices to be used may be vested in an agency having oversight over railroads.
Sight distance is an important consideration at railroad - highway grade crossings. There should be suffi-
cient sight distance along the road and railroad tracks for an approaching driver to recognize the crossing,
perceive the warning device, determine whether a train is approaching, and stop if necessary. (For further
information on railroad - highway grade crossings, see Section 9.12.) Signalized intersections adjacent to
signalized railroad grade crossings should be designed with railroad preemption.
5 -21
3
9 -30
A Policy on Geometric Design of Highways and Streets
Clear Sight Triangle
Decision Point
2
Major Road
Major Road
at a2
b
Clear Sight Triangle
Decision Point
Approaching Sight Triangle for Viewing Traffic Approaching Sight Triangle for Viewing Traffic
Approaching the Minor Road from the Left Approaching the Minor Road from the Right
Approach Sight Triangles (Uncontrolled or Yield- Controlled)
—A—
Clear Sight Triangle
Decision Point
D
Major Road Major Road
a t
b
a2
Clear Sight Triangle
Decision Point
Departure Sight Triangle for Viewing Traffic Departure Sight Triangle for Viewing Traffic
Approaching the Minor Road from the Left Approaching the Minor Road from the Right
Departure Sight Triangles (Stop - Controlled)
—B-
Figure 9 -15. Intersection Sight Triangles
The vertex of the sight triangle on a minor -road approach (or an uncontrolled approach) represents the
decision point for the minor -road driver (see Figure 9 -15A). This decision point is the location at which the
minor -road driver should begin to brake to a stop if another vehicle is present on an intersecting approach.
The distance from the major road, along the minor road, is illustrated by the distance at to the left and
a2 to the right as shown in Figure 9 -15A. Distance a2 is equal to distance at plus the width of the lane(s)
departing from the intersection on the major road to the right. Distance a2 should also include the width of
any median present on the major road unless the median is wide enough to permit a vehicle to stop before
entering or crossing the roadway beyond the median.
The geometry of a clear sight triangle is such that when the driver of a vehicle without the right -of -way
sees a vehicle that has the right of way on an intersecting approach, the driver of that potentially conflict-
ing vehicle can also see the first vehicle. Distance b illustrates the length of this leg of the sight triangle.
Thus, the provision of a clear sight triangle for vehicles without the right -of -way also permits the drivers
of vehicles with the right -of -way to slow, stop, or avoid other vehicles, if needed.
4
Chapter 9— Intersections
Although desirable at higher volume intersections, approach sight triangles like those shown in
Figure 9 -15A are not needed for intersection approaches controlled by stop signs or traffic signals. In
that case, the need for approaching vehicles to stop at the intersection is determined by the traffic control
devices and not by the presence or absence of vehicles on the intersecting approaches.
Departure Sight Triangles
A second type of clear sight triangle provides sight distance sufficient for a stopped driver on a minor -road
approach to depart from the intersection and enter or cross the major road. Figure 9 -15B shows typical
departure sight triangles to the left and to the right of the location of a stopped vehicle on the minor road.
Departure sight triangles should be provided in each quadrant of each intersection approach controlled
by stop or yield signs. Departure sight triangles should also be provided for some signalized intersection
approaches (see Case D in Section 9.5.3 on "Intersection Control "). Distance a2 in Figure 9 -15B is equal
to distance al plus the width of the lane(s) departing from the intersection on the major road to the right.
Distance a2 should also include the width of any median present on the major road unless the median is
wide enough to permit a vehicle to stop before entering or crossing the roadway beyond the median. The
appropriate measurement of distances al and a2 for departure sight triangles depends on the placement of
any marked stop line that may be present and, thus, may vary with site - specific conditions.
The recommended dimensions of the clear sight triangle for desirable traffic operations where stopped
vehicles enter or cross a major road are based on assumptions derived from field observations of driver
gap- acceptance behavior (12). The provision of clear sight triangles like those shown in Figure 9 -15B also
allows the drivers of vehicles on the major road to see any vehicles stopped on the minor -road approach
and to be prepared to slow or stop, if needed.
Identification of Sight Obstructions within Sight Triangles
The profiles of the intersecting roadways should be designed to provide the recommended sight distances
for drivers on the intersection approaches. Within a sight triangle, any object at a height above the eleva-
tion of the adjacent roadways that would obstruct the driver's view should be removed or lowered, if
practical. Such objects may include buildings, parked vehicles, highway structures, roadside hardware,
hedges, trees, bushes, unmowed grass, tall crops, walls, fences, and the terrain itself. Particular atten-
tion should be given to the evaluation of clear sight triangles at interchange ramp /crossroad intersections
where features such as bridge railings, piers, and abutments are potential sight obstructions.
The determination of whether an object constitutes a sight obstruction should consider both the horizontal
and vertical alignment of both intersecting roadways, as well as the height and position of the object. In
making this determination, it should be assumed that the driver's eye is 1.08 m [3.50 ft] above the roadway
surface and that the object to be seen is 1.08 m [3.50 ft] above the surface of the intersecting road.
This object height is based on a vehicle height of 1.33 m [4.35 ft], which represents the 15th percentile of
vehicle heights in the current passenger car population less an allowance of 250 mm [10 in.]. This allow-
ance represents a near - maximum value for the portion of a passenger car height that needs to be visible
for another driver to recognize it as the object. The use of an object height equal to the driver eye height
makes intersection sight distances reciprocal (i.e., if one driver can see another vehicle, then the driver of
that vehicle can also see the first vehicle).
9 -31
5
9 -32
A Policy on Geometric Design of Highways and Streets
Where the sight- distance value used in design is based on a single -unit or combination truck as the design
vehicle, it is also appropriate to use the eye height of a truck driver in checking sight obstructions. The
recommended value of a truck driver's eye height is 2.33 m [7.6 ft] above the roadway surface.
9.5.3 Intersection Control
The recommended dimensions of the sight triangles vary with the type of traffic control used at an in-
tersection because different types of control impose different legal constraints on drivers and, therefore,
result in different driver behavior. Procedures to determine sight distances at intersections are presented
below according to different types of traffic control, as follows:
• Case A— Intersections with no control
• Case B— Intersections with stop control on the minor road
– Case Bl —Left turn from the minor road
– Case B2 —Right turn from the minor road
– Case B3— Crossing maneuver from the minor road
• Case C— Intersections with yield control on the minor road
– Case CI— Crossing maneuver from the minor road
– Case C2 —Left or right turn from the minor road
• Case D Intersections with traffic signal control
• Case E— Intersections with all -way stop control
• Case F —Left turns from the major road
Case A— Intersections with No Control
For intersections not controlled by yield signs, stop signs, or traffic signals, the driver of a vehicle ap-
proaching an intersection should be able to see potentially conflicting vehicles in sufficient time to stop
before reaching the intersection. The location of the decision point (driver's eye) of the sight triangles on
each approach is determined from a model that is analogous to the stopping sight distance model, with
slightly different assumptions.
While some perceptual tasks at intersections may need substantially less time, the detection and recogni-
tion of a vehicle that is a substantial distance away on an intersecting approach, and is near the limits of
the driver's peripheral vision, may take up to 2.5 s. The distance to brake to a stop can be determined from
the same braking coefficients used to determine stopping sight distance in Table 3 -1.
Field observations indicate that vehicles approaching uncontrolled intersections typically slow to ap-
proximately 50 percent of their midblock running speed. This occurs even when no potentially conflicting
vehicles are present (12). This initial slowing typically occurs at deceleration rates up to 1.5 m /s2 [5 ft /s2].
Deceleration at this gradual rate has been observed to begin even before a potentially conflicting vehicle
comes into view. Braking at greater deceleration rates, which can approach those assumed in stopping
6
9 -38
A Policy on Geometric Design of Highways and Streets
intersection is located on a 4 percent upgrade, then the time gap selected for intersection sight distance
design for left turns should be increased from 8.0 to 8.8 s, equivalent to an increase of 0.2 s for each per-
cent grade.
The design values for intersection sight distance for passenger cars are shown in Table 9 -6. Figure 9 -17
includes design values, based on the time gaps for the design vehicles included in Table 9 -5.
No adjustment of the recommended sight distance values for the major -road grade is generally needed be-
cause both the major- and minor -road vehicle will be on the same grade when departing from the intersec-
tion. However, if the minor -road design vehicle is a heavy truck and the intersection is located near a sag
vertical curve with grades over 3 percent, then an adjustment to extend the recommended sight distance
based on the major -road grade should be considered.
Table 9 -6. Design Intersection Sight Distance -Case Bl, Left Turn from Stop
.{is
U.S. Cts'omary
Design
Speed
(km /h)
Stopping Sight
Distance (m)
Intersection Sight
Distance for
Passenger Cars
Design
Speed
(mph)
Stopping
Sight
Distance (ft)
Intersection Sight
Distance for
Passenger Cars
Calculated
(m)
Design
(m)
Calculated
(ft)
Design
(ft)
20
20
41.7
45
15
80
165.4
170
30
35
62.6
65
20
115
220.5
225
40
50
83.4
85
25
155
275.6
280
50
65
104.3
105
30
200
330.8
335
60
85
125.1
130
35
250
385.9
390
70
105
146.0
150
40
305
441.0
445
80
130
166.8
170
45
360
496.1
500
90
160
187.7
190
50
425
551.3
555
100
185
208.5
210
55
495
606.4
610
110
220
229.4
230
60
570
661.5
665
120
250
250.2
255
65
645
716.6
720
130
285
271.1
275
70
730
771.8
775
-
-
-
-
75
820
826.9
830
-
-
-
-
80
910
882.0
885
Note: Intersection sight distance shown is for a stopped passenger car to turn left onto a two -lane highway with
no median and grades 3 percent or less. For other conditions, the time gap should be adjusted and the
sight distance recalculated.
Sight distance design for left turns at divided - highway intersections should consider multiple design ve-
hicles and median width. If the design vehicle used to determine sight distance for a divided - highway
intersection is larger than a passenger car, then sight distance for left turns will need to be checked for
that selected design vehicle and for smaller design vehicles as well. If the divided - highway median is wide
enough to store the design vehicle with a clearance to the through lanes of approximately 1 m [3 ft] at
both ends of the vehicle, no separate analysis for the departure sight triangle for left turns is needed on the
minor -road approach for the near roadway to the left. In most cases, the departure sight triangle for right
7
Chapter 9— Intersections
Table 9 -8. Design Intersection Sight Distance —Case B2, Right Turn from Stop, and Case B3, Crossing
Maneuver
Metric
U.S. Customary
Design
Speed
(km /h)
Stopping
Sight
Distance
(m)
Intersection Sight
Distance for
Passenger Cars
Design
Speed
(mph)
Stopping
Sight
Distance
(ft)
Intersection Sight
Distance for
Passenger Cars
Calculated
(m)
Design
(m)
Calculated
(ft)
Design
(ft)
20
20
36.1
40
15
80
143.3
145
30
35
54.2
55
20
115
191.1
195
40
50
72.3
75
25
155
238.9
240
50
65
90.4
95
30
200
286.7
290
60
85
108.4
110
35
250
334.4
335
70
105
126.5
130
40
305
382.2
385
80
130
144.6
145
45
360
430.0
430
90
160
162.6
165
50
425
477.8
480
100
185
180.7
185
55
495
525.5
530
110
220
198.8
200
60
570
573.3
575
120
250
216.8
220
65
645
621.1
625
130
285
234.9
235
70
730
668.9
670
—
—
—
—
75
820
716.6
720
—
—
—
—
80
910
764.4
765
Note: Intersection sight distance shown is for a stopped passenger car to turn right onto or to cross a two -
lane highway with no median and with grades of 3 percent or less. For other conditions, the time gap
should be adjusted and the sight distance recalculated.
9 -41
8
46.61.185 « 46.6 90 » 46.61.195
RCW 46.61.190
Vehicle entering stop or yield intersection.
(1) Preferential right -of -way may be indicated by stop signs or yield signs as authorized in
RCW
(2) Except when directed to proceed by a duly authorized flagger, or a police officer, or a
firefighter vested by law with authority to direct, control, or regulate traffic, every driver of a
vehicle approaching a stop sign shall stop at a clearly marked stop line, but if none, before
entering a marked crosswalk on the near side of the intersection or, if none, then at the point
nearest the intersecting roadway where the driver has a view of approaching traffic on the
intersecting roadway before entering the roadway, and after having stopped shall yield the
right -of -way to any vehicle in the intersection or approaching on another roadway so closely
as to constitute an immediate hazard during the time when such driver is moving across or
within the intersection or junction of roadways.
(3) The driver of a vehicle approaching a yield sign shall in obedience to such sign slow
down to a speed reasonable for the existing conditions and if required for safety to stop, shall
stop at a clearly marked stop line, but if none, before entering a marked crosswalk on the
near side of the intersection or if none, then at the point nearest the intersecting roadway
where the driver has a view of approaching traffic on the intersecting roadway before
entering the roadway, and then after slowing or stopping, the driver shall yield the right -of-
way to any vehicle in the intersection or approaching on another roadway so closely as to
constitute an immediate hazard during the time such driver is moving across or within the
intersection or junction of roadways: PROVIDED, That if such a driver is involved in a collision
with a vehicle in the intersection or junction of roadways, after driving past a yield sign
without stopping, such collision shall be deemed prima facie evidence of the driver's failure to
yield right -of -way.
[2000 c 239 § 5; 1975 c 62 § 27; 1965 ex.s. c 155 § 30.]
Notes:
Rules of court: Monetary penalty schedule -- IRLJ 6.2.
Captions not law -- 2000 c 239: See note following RCW 49.17.350.
Severability -- 1975 c 62: See note following RCW 36.75.010.
Stop signs, "Yield" signs -- Duties of persons using highway: RCW ?7.36.1 10.
Page 1 of 1
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8/27/2015
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WTSEA
CONTROVERSIES IN TSE
By Skeet Gaul (TSE Instructor & WTSEA Treasurer)
Issue: Should We Teach Students to Make `Double Stops'?
Nowhere in the RCW's does it say that double stops are required at intersections with stop
signs. RCW 46.61.190 says that after coming to a complete stop at the correct stopping
position we must "yield the right of way to any vehicle in the intersection or approaching on
another road way so closely as to constitute an immediate hazard..." The word 'yield' is clarified
in the Washington State Driver's Guide as meaning you "must do everything you can to prevent
striking a pedestrian, on foot or in a wheelchair, or another vehicle, regardless of the
circumstances "(p.40). Furthermore, RCW 47.36.110 says that when a person approaches an
intersection that has a stop sign, that the person is required to stop. Then, "A person stopping at
such a sign shall proceed through that portion of the highway in a careful manner and at a
reasonable rate of speed not to exceed twenty miles per hour.
We can certainly see if an intersection we are about to enter is clear or not. But if we have a
closed Line of sight (LOS), it's what we can't see that might hurt us. The question becomes,
"How will we know if the intersection is about to become a dosing path of travel (POT) by an
approaching vehicle we can't see, due to a closed LOS ?" We don't know the vehicle is coming,
but if we just trust what we see in front of us and enter the intersection, a collision will occur and
it will be our fault because we "failed to yield ". The law doesn't care that we couldn't see the
vehicle coming. So, after we have executed a complete stop at the correct stop location, for any
initial pedestrians or vehicles, the law puts the responsibility on each driver to determine if the
intersection will be clear when we enter it.
So, it makes sense that after making our correct and complete stop, we should proceed with
caution to determine if our Path of travel is open or closing. Proceeding with caution doesn't
mean that we have to stop again, completely. If we can make the determination that our POT is
open, while still rolling, then we may proceed safely into the intersection, without making a
second stop.
The problem becomes, what do you teach young students, whose scanning skills and gap
judgment and are not equal to more experienced drivers?
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