Oregon Department of Transportation, Highway Division

Rule Rule 734-020-0019
Locations and Criteria of Variable Interstate Speed Limits


This rule is applicable only to regulatory systems and not to advisory systems.

(1)

All locations of interstate highways have maximum speed limits set in section (2) of OAR 734-020-0011 (Locations of Interstate Speed Limits Other Than Set in Statute) or statutory maximum speed limits per ORS 811.111 (Violating a speed limit).

(2)

Variable speed limits on the following sections of interstate highways are established as follows:

(a)

I-84 Eastbound, MP 277.49 – MP 305.00: The following sections each may have different speed limits based on the criteria in section (3) of this rule:

(A)

1.15 mile west of Clover Creek Interchange Structure (MP 277.49) to 0.30 mile east of North Powder River (MP 286.50).

(B)

0.30 mile east of North Powder River (MP 286.50) to 100 feet west of Culley Lane Structure (MP 295.65).

(C)

100 feet west of Culley Lane Structure (MP 295.65) to 0.86 mile east of Campbell Street Interchange Structure (MP 305.00).

(b)

I-84 Westbound, MP 277.88 – MP 306.40: The following sections each may have different speed limits based on the criteria in section (3) of this rule:

(A)

0.76 mile west of Clover Creek Interchange Structure (MP 277.88) to 0.67 mile west of North Powder Interchange Structure (MP 285.01).

(B)

0.67 mile west of North Powder Interchange Structure (MP 285.01) to 0.24 mile west of Culley Lane Structure (MP 295.43).

(C)

0.24 mile west of Culley Lane Structure (MP 295.43) to 0.13 mile west of S. Baker Interchange Structure (MP 306.40).

(3)

Criteria for Changing Speeds. The Variable Speed Limit system has two automated subsystems, 1) a congestion subsystem and 2) a weather subsystem, each determining a recommended speed based on criteria set forth below in (a) and (b). The system also includes a manual control subsystem with criteria for use as described below in (c). The system automatically displays the lowest recommended speed from the automated subsystems with the ability for limited manual intervention when appropriate.

(a)

Automated variable speed limits for congestion:

(A)

During periods of free flow or near free flow when there is little or no congestion and drivers are not impeded by other vehicles, the subsystem will be programmed to select the maximum speed limit.

(B)

During periods of congestion characterized by slower speeds on the roadway the subsystem will be programmed to select recommended reduced speeds corresponding to the congested conditions.

(C)

The 85th percentile speed will be calculated from traffic sensor data. The variable posted speed limit shall be the 85th percentile speed rounded to within 5 mph. If there are multiple traffic sensors within the segment, the 85th percentile speed will be taken from the speed sensor with the lowest speed.

(D)

If the subsystem does not have sufficient data to calculate the 85th percentile speeds, occupancy data (percent of time a vehicle was on the sensor) shall be used to determine if vehicles are stopped at the sensors. If the presence of vehicles is indicated, then the subsystem will assume traffic is stopped and display the minimum speed limit.

(E)

If the subsystem is unable to determine a recommended speed due to the lack of data, the subsystem will use the last valid speed until a new speed is calculated.

(b)

Automated variable speed limits for adverse weather conditions:

(A)

The weather responsive subsystem will select recommended reduced speeds during periods where speeds are required to be at least 10 mph below the maximum speed limit due to either adverse weather conditions or other safety hazards.

(B)

Weather sensors must be installed within the corridor. These sensors may measure such elements as the friction of the roadway (grip factor), classify roadway surface conditions (ice or snow present), measure visibility, or other factors related to weather depending on the corridor and reoccurring conditions.

(C)

The variable speed determined by weather sensor data shall generally be 10 to 20 mph lower than maximum speed limit in the segment depending on severity and number of conditions present. If the maximum speed limits on the highways are above 65 mph (i.e., speeds on interstate are 70 or 75 mph) then the variable speeds shall be lowered to 55 mph and 45 mph depending on severity and number of conditions present. The minimum speed limit of 30 mph will be reserved for the most severe weather conditions in combination with low visibility or where significant traction problems are present.

(D)

The TOC has authority to disable the weather responsive subsystem when necessary or override the speed if other conditions are present and not being detected by weather sensors.

(E)

The subsystem shall also consider the snow zone chain condition in place during inclement weather conditions. ODOT establishes the chain conditions based on OAR 734-017-0005 (Definitions) thru 0025.

(i)

When chains are required on some classes of vehicles, but not all, the subsystem will recommend a 45 mph speed.

(ii)

When chains are required on all vehicles the subsystem will recommend a 35 mph speed.

(F)

When a chain condition is present and a weather event is being detected the slowest recommended speed shall be used.

(c)

When sensors are not detecting properly or conditions are such that the automated system does not adequately address the conditions present on the roadway the TOC may establish variable speed limits other than those established by either the weather subsystem or the congestion subsystem in accordance with the following:

(A)

The TOC shall have the ability to temporarily override the system when in the judgement of the Department it is necessary to protect the safety of the public or workers, or avoid damage to any portion of the highway.

(B)

Key information such as weather and displayed speeds from the automated systems, if available, should be used to provide information for decision making.

(C)

The minimum period for changing speeds may be overridden by the TOC and the posted speed changed immediately.

(D)

Unforeseen conditions not covered by the automated system may necessitate overriding the automated system such as a major natural disaster or evacuation.

(E)

Other conditions include setting the variable speed system to a lower speed for properly documented reduced speeds. Examples include construction work zones or emergency conditions such as landslides.

(F)

When manual control is requested, the TOC shall record who made the request and the reason for the request.

(d)

General conditions for variable speed limits:

(A)

Speed signs shall not display a speed greater than the designated speed limit for the segment as set in OAR 734-020-0011 (Locations of Interstate Speed Limits Other Than Set in Statute) and if none, then the statutory maximum speed limit in ORS 811.111 (Violating a speed limit).

(B)

Speed Limits displayed shall be the lowest of the two automated subsystems, congestion or weather, unless overridden by the TOC.

(C)

Speed limits between subsequent highway speed change segments typically shall not be reduced by more than 10 to 15 MPH between adjacent segments. These may be urban situations where speed signs are separated by no more than a few miles. In some cases (such as rural locations) where there are relatively long distances between speed signs, the speed change between subsequent sections may be much greater since there may be free flow speeds in adjacent segments and thus no reason for reduced speeds.

(D)

The speed limit shall be displayed in 5 MPH increments.

(E)

The speed limit shall not be decreased more than once within a 2 minute period, unless overridden by the TOC.

(F)

The speed limit shall not be increased more than once within a 3 minute period, unless overridden by the TOC.

(G)

The minimum variable speed limit shall not be less than 30 MPH.

(H)

Variable speed signs should be posted near, and downstream of interstate entrances, typically within about 1500 to 2000 feet.

(I)

Variable speed signs for urban areas should be placed at frequent intervals. For rural areas the sign interval should be at least every five miles but no more than every ten miles.

(J)

The TOC shall log the speed limit being displayed on the variable speed signs and keep the log for a minimum of five years.

(K)

Static signs giving warning of entering the variable speed corridor shall be placed at the beginning of the corridor.

(L)

Static signs giving notice of the end of the variable speed limit may be placed at the exit points. A static speed sign shall be placed at the end of the corridor to establish the end of the variable speed and the beginning of the fixed speed limit.
Source

Last accessed
Jun. 8, 2021