HomeMy WebLinkAbout2018-05-07; Traffic Safety Commission; ; AMBROSIA LANEItem #3
Meeting Date: May 7, 2018
To: Traffic Safety Commission
Staff Contact: Doug Bilse, Senior Engineer
doug.bilse@carlsbadca.gov or 760-602-7504
Subject: Recommend that City Council establish a 30 mile per hour (mph) prima facie speed limit for Ambrosia Lane between Conosa Way and Aviara Parkway.
Recommended Action
Recommend that City Council establish a 30 mile per hour (mph) prima facie speed limit for Ambrosia Lane between Conosa Way and Aviara Parkway.
Executive Summary
The current posted speed limit on Ambrosia Lane between Conosa Way and Aviara Parkway (see
attached Exhibit 1) is 35 mph and is based on an engineering and traffic survey (E&TS) that
reached the regulatory expiration date. The new E&TS (Exhibit 2) supports a lowered speed limit
of 30 mph.
Background
The setting of speed limits requires a rational and defensible procedure to maintain the
confidence of the public and legal systems. By following a uniform procedure, agencies can
establish speed limits that are uniform throughout the state and avoid influence from political
pressure or emotional perceptions. Setting the speed limit arbitrarily low often makes violators
of a disproportionate number of drivers, does not facilitate the orderly movement of traffic,
and requires constant enforcement to maintain compliance. The Federal Highway
Administration conducted studies demonstrating that setting the speed limit too high or too
low can increase collisions.
The California State Legislature sets in place the regulations for California speed limits, and the
California Vehicle Code (CVC) places these regulations into language used primarily for
enforcement purposes. The California Manual for Setting Speed Limits is published by Caltrans
to establish standard procedure for setting legally defensible speed limits in compliance with
the CVC, preparing an adequate E&TS, and installing appropriate signage according to the
California Manual for Uniform Traffic Control Devices (CAMUTCD).
Speed Limits for Special Classes of Roadway Locations
CVC Section 22352 sets predetermined speed limits in California for specific classes of locations that do not require a specific speed limit sign to be posted. An E&TS is not required for these locations. There are six classes of locations where these predetermined speed limits apply:
• uncontrolled railroad crossings (15 mph)
• blind uncontrolled intersections (15 mph)
• alleyways (15 mph)
• business and residential areas that have met specific requirements (25 mph)
• school zones (mph)
• areas immediately around senior centers (25 mph)
Speed Limits Requiring An E&TS
CVC Section 22358 allows local jurisdictions to define speed limits in locations that do not meet
the six classes listed above. The speed limit determination for these locations relies on the
premise that a reasonable speed limit is one that conforms to the actual behavior of the
majority of drivers. Carlsbad follows the standard procedures included in the California Manual
for Setting Speed Limits to set these speed limits using an E&TS to document the supporting
data including: the 85th percentile speed, accident history, daily traffic volumes, roadway
conditions and any special conditions.
85th Percentile speed: Posted speed limits are normally set near the 85th percentile speed defined
as the speed at or below which 85 percent of the free flowing traffic (i.e., a 5 second minimum
gap between vehicles) is moving. The 85th percentile speed is also known as the “critical speed”
and statistically represents one standard deviation above the average speed. According to the
California Manual for Setting Speed Limits:
speed limits that are set near the 85th percentile speed of free flowing traffic are safer
and produce less variance in vehicle speeds. Because of this, the 85th percentile is used to
establish the upper limit of operating speeds that are considered reasonable and prudent.
Traffic Factors: The most pertinent traffic factor is the average daily traffic volume. Other factors include
adjacent land uses and pedestrian/bicycle use.
Accident History: A comprehensive review of the collision history on a roadway is another
important element in the process to establish a speed limit. The review of collisions reveals if
there is a high incidence of speed-related collisions on the road at specific locations. Absent a
significant collision history, it may be concluded that most drivers are operating their vehicle in a
prudent manner; this supports using the 85th percentile speed to establish the speed limit. A
significant collision history could justify a lower speed limit.
Roadway Conditions: Drivers adjust their vehicle speed to the readily apparent roadway
conditions present including: number of lanes, lane width, on-street parking, traffic control,
horizontal and vertical curves. The 85th percentile speed is a way to determine how drivers are
reacting to roadway conditions readily apparent to them. If readily apparent conditions are of
great concern, advisory speed signs may be posted for specific roadway conditions as noted
below.
Special Conditions: Drivers adjust their vehicle speed to the readily apparent roadway conditions
discussed above. Special events, construction, or congestion are also apparent when present.
CVC Section 22358.5 specifically prohibits downward speed zoning for these conditions and any
other conditions readily apparent to the driver. Conditions not readily apparent to the motorist
is a condition which, if the motorist were aware, they would adjust their speed accordingly. Non-
apparent conditions include high volume traffic generators not visible (vehicular, bicycle or
pedestrian) and access points that are not visible to the motorist. Every attempt should be made
to make the motorist aware of non-apparent conditions (e.g., warning and advisory speed signs)
before a decision is made to lower the speed limit. Non-apparent conditions may be used to
lower the speed limit an additional 5 mph, except if the speed limit is being lowered for a higher
than average collision rate. Speed limits that have been lowered by 5 mph due to conditions not
readily apparent to the driver are sometimes successfully challenged in court.
Advisory Speeds
An advisory speed sign (typically black lettering on a yellow background) advises drivers of
speeds recommended for certain roadway conditions, such as horizontal curves. Advisory
speeds are determined primarily by the physical and design characteristics of the roadway, not
the 85th percentile speed. They advise the driver of a roadway condition more comfortably
traversed at the recommended speed that is lower than the posted speed limit. It is not
necessary to lower the regulatory speed to conform to the advisory speed. A driver may not be
cited solely for exceeding the speed posted on an advisory sign, but under the basic speed law
(CVC Section 22350) they can may be cited if they encounter, or create, a problem when
exceeding the advisory speed.
Length of Speed Zone
The length of any section of roadway set for a particular speed zone should be as long as possible
and consistent with changes in roadway conditions, roadside development, and land use. Speed
zone or speed limit changes need to be coordinated with changes in roadway conditions,
roadside development, and land use. Speed zones of less than 0.5 miles should be avoided.
Separate speed zones should be created at City, County or other jurisdictional boundaries with a
consistent speed limit across the boundary.
Discussion
Ambrosia Lane between Aviara Parkway and Conosa Way (Exhibit 2) does not meet the
requirement to be classified as a resident district that calls for a 25 mph speed limit. The adjacent
school meets the criteria for a school zone where the speed limit is 25 mph when children are
present. The E&TS for Ambrosia Lane between Aviara Parkway and Conosa Way (Exhibit 2)
includes the following data supporting the proposed 30 mph speed limit:
1. 85th Percentile Speed
Table 1 lists the pertinent speed information used to support the proposed speed limit.
A sample of 100 vehicles (50 vehicles in each direction of travel) was measured to
determine the critical speed. The pace speed is the 10 mph range of speeds that contain the
highest number of observed vehicles. The proposed speed limit is the 5 mph increment at
or directly below the 85th percentile speed.
Table 1: SPEED SURVEY DATA
Survey
Date
Location
Critical Speed
(MPH)
Pace Speed
(MPH)
Percent vehicles
in Pace
Proposed Speed
Limit (mph)
1/17/18 200 feet south of Cormorant Ave 33 25-35 85% 30
2. Accident History
Staff reviewed the Police Department traffic collision reports for the two-year period from
January 1, 2016 through December 31, 2017. During that time there was one reported traffic
collision; it was not speed related and did not involve a pedestrian or bicyclist. This is not
considered a significant collision history.
3. Traffic Factors
Adjacent land uses include single-family residential, multi-family residential, open space, and
an elementary/middle school. Nearby land uses include a shopping center, library, and post office. The most pertinent traffic factor is the average daily traffic volume. On August 22, 2017, traffic counts were obtained on Ambrosia Lane to measure the two-way, 24-hour
average daily traffic volume. The count data is indicated on Table 2.
Table 2: 24-HOUR TRAFFIC VOLUMES:
24-Hour
Date of Count Location Average Daily Traffic Count
8/22/2017 North of Aviara Parkway 2,430
4. Roadway Factors
This portion of Ambrosia Lane has one travel lane in each direction. Ambrosia Lane is striped with on-street parallel parking allowed on both sides along most of this section. The road follows a curvilinear alignment with grades below 12% and has a curb-to-curb width of 40 feet. Intersections exist at Cormorant Ave, Conosa Way, Aviara Parkway and the entrance and exit to Aviara Oaks Elementary and Middle School. All intersections are STOP controlled for the side streets only, except for the signalized intersection at Aviara Parkway.
5. Conditions Not Readily Apparent to the Driver
There are no conditions reported for this roadway segment that are not readily apparent to
the driver. A 5 mph reduction in the recommended speed limit is not justified.
Necessary Council Action
To establish the new speed limit on Ambrosia Lane between Conosa Way and Aviara Parkway
the City Council must adopt an ordinance at a public meeting.
Next Steps
Once an ordinance is adopted, appropriate regulatory speed limit signs are posted in
compliance with the CAMUTCD. Once the signs are installed, Police can cite violators that
exceed the posted speed limit that is legally defensible.
Exhibits
1. Location Map
2. Engineering and Traffic Survey