MEMORANDUM
DEPARTMENT OF TRANSPORTATION
ITS Branch
4201 East Arkansas Avenue, EP 770
Denver, Colorado 80222
(303) 512-5805
FAX (303) 757-9219
DATE:
February 18, 2004
TO:
Robert Torres, Regional Transportation Director
FROM:
Saeed Sobhi, ITS Planning and Special Projects
SUBJECT:
The use of variable speed limit systems on Interstate 25 north of Colorado Springs
This letter summarizes the findings of the research performed by the CDOT ITS Branch into the
application of variable speed limit technology to Colorado highways. It also offers ITS solutions for
the safety issues occurring on I-25 between Baptist Road and Woodman Road.
The subject started with a regional visit from the ITS Branch. Mr. Saeed Sobhi, Mr. Frank Kinder, and
Mr. John Nelson met with Mr. Robert Torres to discuss the application of ITS technology for solutions
to regional traffic problems. During that meeting, Mr. Torres suggested using a variable speed limit
system on the I-25 corridor north of Colorado Springs. The system would decrease the speed limit in
the southbound lanes prior to entering a freeway segment that was temporarily congested due to
regularly scheduled sporting events. The ITS Branch decided to examine this concept to see what can
be done.
In order to get started, the boundaries and conditions of the targeted freeway segment need to be
examined. The boundaries for the freeway segment are the Baptist Rd. interchange on the north and
extend southward to the Woodman Rd interchange. Within these boundaries are two distinctly
different freeway segments with two different speed limits. The Baptist Rd. interchange lies within a
75 mph speed limit zone and is semi rural. The Woodman Rd. interchange is inside the Colorado
Springs city limits and has a 55 mph speed limit. From a discussion with Mr. Torres, the problem
appears to be as follows. The southbound vehicles are traveling at high speeds and enter the outer
metro area of Colorado Springs and encounter congestion and thus, reduced vehicle speeds. On some
occasions, whether it is due to non-recurring congestion caused by Air Force Academy football games
or recurring congestion caused by the evening rush hour, the traffic backs up into the Woodman Rd.
area and beyond. Southbound vehicles on I-25 enter the congested area and begin to maneuver around
slower moving traffic. Thus, unsafe driving conditions are created resulting in a high potential for
vehicular crashes.
The Safety and Traffic Engineering Branch was contacted in order to discuss the legal aspects of this
traffic problem and to produce a detailed traffic summary report. Mr. Dwayne Wilkinson forwarded
the C.R.S. 42-4-1101 thru CRS 42-4-1105 and Mr. Randall Reyes was called upon to prepare a
detailed accident summary report. A discussion of each will follow.
The limits of the detailed accident summary report extend from MP 148 to MP 159. The accident data
was collected from January 1, 2000 to December 31, 2002. The report identifies a total of 1734
accidents in that freeway segment and 1019 are rear-end collisions. The largest number of accidents
occur at the Woodman Road interchange and the next largest occurs at the Academy Boulevard
interchange. The rear-end collisions in the southbound lanes from MP 148 to MP 150 out number the
rear-end collisions in the northbound lanes by nearly 4 to 1.
The Woodman Road interchange is located at MP 148.85 and lies within the Colorado Springs city
limits. The speed reduces from 60 mph to 55 mph at MP 149.32 just north of the interchange. The
end of the 75 mph speed zone is just north of the Briargate Blvd. interchange at MP 151.74.
Therefore, there is a 2.42-mile deceleration distance before the city limits are reached. Between MP
149.90 and MP 148 there were 500 rear end collisions. There were 390 in the southbound lanes alone.
This unusually high number of rear end collisions typically indicates a congestion problem. In order to
examine the problem further, the conditions for the Academy Blvd. interchange were examined.
This interchange is located in a 60 mph speed zone at MP 150.36 and has a 1.38-mile deceleration
distance from the 75 mph speed limit zone. The rear end collision data shows a sharp decline. The
number falls from 390 to 35 in the southbound lanes. This sudden drop suggests that the congestion
problem in the southbound lanes has ended and free flow conditions are prevalent.
The table below shows the rear end accidents for freeway segments under study.
Rear end accident data from January 1, 2000 to December 31, 2002
Rear End Collisions
Interchange and MP
MP
MP
Mainline SB
Mainline NB
Speed
Limit
Woodman Rd.
148.85
148.00
149.00
148.99
149.90
193
197
67
43
55
60
Academy Blvd.
150.36
150.00
150.99
35
7
60
151.64
151.00
151.72
32
22
60
457
139
South Gate Blvd. to N. Academy Blvd.
Briargate Rd.
Total - MP 148 to MP 151.72
Interquest Pkwy.
153.22
151.74
153.99
35
68
75
Gleneagle Rd.
155.91
154
156.99
32
27
75
157
159
*48
11
75
151.74
159
115
106
75
572
245
North Gate Blvd. (By MSN Maps & Directions)
Baptist Rd.
Total - 75 MPH Zone
158.25
Total number of rear end collisions, excluding ramps
Note: * Baptist Rd. SB - 14 rear ends occurred on either rain/snow/sleet/hail conditions
Note: The names of the cross streets over the interchanges were derived from the CDOT Strip Maps
Does the freeway segment warrant a variable speed limit system? Based on the existing speed limits
and the location of the rear-end accidents, it does not appear to be warranted. However, due to the
abnormally high accident rate, the ITS Branch is recommending other ITS solutions.
As previously noted, the Woodman Rd. interchange has a severe rear end collision problem.
Therefore, the ITS Branch would like to suggest two ITS solutions to help reduce the accident
numbers. One is an Advanced Traveler Information System consisting of variable message signs that
warns drivers of congestion ahead and, the other is to install ramp meters at the freeway entrance
ramps to help maintain the traffic flow during the peak periods. Combining both would be optimal to
increase safety and reduce travel time through the corridor. However, the installation of the ramp
meters requires that the traffic volumes exceed specified warrants.
CDOT is currently using a Ramp Metering Feasibility Study that was conducted for Region 6. It
recommends using a three-tiered approach for determining viable ramp meter locations. Two of the
tiers are derived from warrants established by Arizona Department of Transportation (ADOT) and
California Department of Transportation (CALTRANS). The third tier is based on the Region’s field
observations and experience with the current ramp metering system. The study performed by the
ADOT provides volume thresholds for installing ramp meters. “The warrant criterion states if a ramp
plus mainline volume upstream of the gore exceeds the following volume thresholds, a meter may be
warranted”.
ADOT
 2 mainline lanes: 2,650-vph
 3 mainline lanes: 4,250-vph
 4 mainline lanes: 5,850-vph
In addition to the ADOT warrants, CALTRANS provides guidelines for installing single-lane metered
entrance ramps versus two-lane metered entrance ramps. The suggested volume thresholds are listed
below.
CALTRANS
 Single-lane metered entrance ramp for volumes up to 900-vph
 Two-lane metered entrance ramps for volumes above 900-vph
In order to check the volume in the corridor, traffic counts were obtained from the CDOT - Division of
Transportation Development website. The most recent traffic count was performed in December 2003
at MP 142.10 near the Bijou St. interchange. The count in the southbound lanes reveals that the
average volumes between the hours of 11:00 AM and 5:00 PM range from 2,794-vph to 3,052-vph.
The freeway segment between MP 142.10 and the Woodman Rd. interchange is 2-lanes wide in each
direction. The data indicates that the traffic volume downstream from the Woodman Rd. interchange
exceeds the 2,650-vph thresholds and is 2-lanes wide. Therefore, based on the ADOT warrants, a
ramp metering system appears to be necessary to help vehicles move downstream through the I-25
corridor.
Ramp metering systems have proven their benefits by reducing travel time through a congested area.
The United States Department of Transportation’s website for Intelligent Transportation Systems lists
the benefits as follows: “Freeway management systems, primarily through ramp metering, have
reduced crashes by 24 to 50 percent while handling 8 to 22 percent more traffic at speeds 13 to 48
percent faster than pre-existing congested conditions”.
The ITS Branch strongly recommends completing a three tiered study in order to verify the need to
install a ramp metering system and, to help in designing it.
As previously mentioned above, the ITS Branch recommends extending the Advanced Traveler
Information System into the Colorado Springs metro area. It could be comprised of a traffic counting
system that operates variable message signs and could work in conjunction with the ramp meters. For
example: when the traffic volume at the Woodman Rd. interchange (or downstream) exceeds the
specified limit, a controller signals the VMS located upstream at the Briargate Rd. interchange (or
other) to display “Congestion Ahead” and “Prepare to Slow Down”. It could utilize the same vehicle
detector and controller system that activates the ramp signals or, operate as a separate system.
Currently, the Region 6 ramp metering systems work with vehicle detectors and controllers that have
preset volume thresholds. The ramp signals begin to operate and control the entrance ramps when the
upstream volume exceeds the pre-set threshold limit.
Please review the table showing the suggested ITS solutions and their estimated installation costs.
Intelligent Transportation System – Solution Packages
Variable message sign controlled by vehicle detectors
Estimated Cost - ITS Package
$180,000.00
This system would calculate the volume of traffic at a specified point such as the Woodman
Rd. Interchange. When specified limits are attained, the system would command the variable
message signs to display “Congestion Ahead” “Prepare to Slow Down”
Recommended based on large number of
rear end collisions at the Woodman Rd.
interchange.
Ramp meters at entrance ramps activated during congestion
A proven solution for Freeway Management Systems. This system would use ramp signals in
conjunction with vehicle detectors and controllers to meter the vehicles leaving the entrance
ramps and improve freeway congestion.
Electronic speed limit signs activated by vehicle detectors
This system would calculate the volume of traffic at a specified point such as the Woodman
Rd. Interchange. When specified limits are attained, the system would command electronic
speed limit signs to reduce the upstream speed limit. Hours of operation posted by static
signs or VMS - Note: * When using a VMS, add $160,000.00 to estimated cost.
$100,000.00 per signal
Three Tiered Study recommended, keeps
traffic flowing in the area of congestion,
reduces accidents, traffic data suggests
that it is recommendable for the corridor
$50,000.00 *
Arguments exist on how to determine the
reduced speed limits and does CDOT
want to defend them in court.
* Note: The legal research that was conducted on variable speed limits with regards to CDOT’s ability
to use such a system is discussed on the next page. The statutes and the viewpoints regarding the
statutes are included in the discussion.
The discussion of the variable speed limit concept leads to the legal aspects of establishing speed
limits. In researching the Colorado Revised Statutes pertaining to the variable speed limit system, it
was found that C.R.S. 42-4-1101 Speed Limits and C.R.S. 42-4-1102 Altering of Speed Limits.
C.R.S. 42-4-1101. Speed limits (1) No person shall drive a vehicle on a highway at a speed greater
than is reasonable and prudent under the conditions then existing.
C.R.S. 42-4-1101. Speed Limits (3) No driver of a vehicle shall fail to decrease the speed of such
vehicle from an otherwise lawful speed to a reasonable and prudent speed when a special hazard exists
with respect to pedestrians or other traffic or by reason of weather or highway conditions.
Arguments against a variable speed limit system include the view that the statutes above already
require the driver to change his speed according to the conditions. However, the statutes below allow
CDOT to alter the speed limits as necessary, based on the findings of a traffic investigation or survey.
Currently, the traffic investigation and speed limit criteria that CDOT has adopted are within the
Millennium Edition of the MUTCD.
C.R.S. 42-4-1102. Altering of Speed Limits (1) (a) Whenever the department of transportation
determines upon the basis of a traffic investigation or survey or upon the basis of appropriate design
standards and projected volumes in the case of newly constructed highways or segments thereof that
any speed specified or established as authorized under sections of 42-4-1101 to 42-4-1104 is greater or
less than is reasonable or safe under the road and traffic conditions at any intersection or other place or
upon any part of the state highway under its jurisdiction, said department shall determine and declare a
reasonable and safe speed limit thereat which shall be effective when appropriate signs giving notice
thereof are erected as such intersection or other place or upon the approaches thereto; except that no
speed limit in excess of seventy-five miles per hour shall be authorized by said department.
C.R.S 42-4-1102 Altering of Speed Limits (5) Whenever the department of transportation or local
authorities, within their respective jurisdictions, determine upon the basis of a traffic investigation or
survey that a reduced speed limit is warranted in a school or construction area or other place during
certain hours or periods of the day when special or temporary hazards exist, the department or the
concerned local authority may erect or display official signs of a type prescribed in the state traffic
control manual giving notice of the appropriate speed limit for such conditions and stating the time or
period the regulation is effective. When such signs are erected or displayed, the lawful speed limit at
the particular time and place shall be that which is then indicated upon such signs; except that no such
speed limit shall be less than twenty miles per hour on a state highway or other arterial street as defined
in subsection (3) of this section nor less than fifteen miles per hour on any other road or street, nor
shall any such reduced speed limit be made applicable at times when the special conditions for which it
is imposed cease to exist. Such reduced speed limits on streets which are state highways shall be
subject to the written approval of the department of transportation before becoming effective.
It appears from this statute that CDOT is legally able to reduce speeds during specified hours when
special or temporary hazards exist. The statute makes reference to traveling in a school or construction
area or other place. Specifically, the wording “other place” is important here. The statute also requires
posting the hours of operation. ITS solutions such as variable message signs could be used to post the
hours, if a variable speed limit system was implemented.
Here are some examples of how other states are posting variable speed limits. These photos come
from the website of the vendor “ADDCO”. They show current installations of their variable speed
limit signs on the roadway.