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The operating speed of vehicles traveling through the work area directly affects the operation and safety of the work zone. A common misbelief is that the best way to increase safety is by reducing the speed limit in the work zone. However, attempting to reduce speed increases the likelihood of crashes. The MUTCD, Part 6 specifically states, "reduced speed zoning should be avoided as much as practical.”
Signing alone does not compel most motorists to drive at the posted speed limit. The few drivers who do slow down create a speed differential in the traffic stream, increasing the likelihood of rear-end crashes. Drivers must perceive a reason to slow down. Reasons include enforcement of the posted speed limit; work activity near the open lane; adverse weather conditions; and roadway geometry.
To the extent possible, the Iowa Department of Transportation designs a temporary traffic control zone to accommodate the normal operating speed of traffic traveling on the roadway segment. This means that lane closure tapers, shift tapers and roadway geometry should be designed at the pre-construction regulatory speed limit, This section provides guidance for managing speeds in work zones.
Back to topAdvisory Speed
While the overall goal is to design work zones for normal operating speeds, there are instances that require a slower speed due to geometry and unusual operating conditions for distances of 1000’ or less. In such cases, advisory speeds should be used. An Advisory Speed is a recommended speed on a section of highway and is based on the highway design, operating characteristics, and conditions. Setting the value of the advisory speed should follow established engineering practices.
The advisory speed should be determined based on free-flowing traffic conditions. Research has shown that drivers often exceed existing posted advisory curve speeds by 7 to 10 mph.
Because work zone design can initiate changes in conditions, such as roadway geometrics, surface characteristics, or sight distance, that might affect the advisory speed, each location should be evaluated to determine if an advisory speed limit is appropriate or should be established. On-site detours, shoulder runarounds, and temporary connections have advisory speeds posted in accordance with the geometric design. Advisory speeds are only posted when the safe operating speed is 10 mph or more below the regulatory speed limit.
Advisory speeds are not enforceable. Advisory speed plaques are not a substitute for black-on-white regulatory speed limit signs. The Advisory Speed plaque is only used to supplement a warning sign and shall not be installed as a separate sign installation.
Refer to MUTCD CHAPTER 2C. WARNING SIGNS AND OBJECT MARKERS and Section 2C.59 Advisory Speed Plaque (W13-1P) for additional information.
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Regulatory Speed Limits
Regulatory speed limits are established by code, or some other ordinance established by the agency or public body having jurisdiction over a particular section of roadway. Simply erecting speed limit signing is not sufficient for enforcement of the speed limit. Regulatory work zone speed limits shown on the referenced Standard Road Plans or contained in the project plans, are considered authorized and therefor enforceable. Section 5.42 of the Construction Manual contains additional information regarding the Department’s work zone speed limit policy and procedures. It also explains the approval process to reduce a regulatory speed limit after the project is under contract.
Reduced speed zoning (lowering the regulatory speed limit) should be avoided as much as practical because drivers will reduce their speeds only if they clearly perceive a need to do so. If reduced speed limits are used, they should be used only in the specific portion of the TTC zone where conditions or restrictive features are present and only after an engineering study. However, frequent changes in the speed limit should be avoided.
A TTC plan should be designed so that vehicles can travel through the TTC zone with a speed limit reduction of no more than 10 mph. A reduction of more than 10 mph in the speed limit should be used only when required by restrictive features in the TTC zone. Where restrictive features justify a speed reduction of more than 10 mph, additional driver notification should be provided. The speed limit should be stepped down in advance of the location requiring the lowest speed, and additional TTC warning devices should be used.
The Department’s policy regarding speed limits in temporary traffic control zones is based on Part 6 of the MUTCD. Refer to MUTCD sections 6B.01 and 6C.01 for additional information.
Back to topExtra Enforcement in Work Zones
A number of conditions may indicate a need for or benefit of extra enforcement in work zones. The presence of a uniformed law enforcement officer and marked law enforcement vehicle in view of motorized traffic on a highway project can affect driver behavior which helps to maintain appropriate speeds and improve driver alertness through the work zone. Refer to the Guidelines for Use of Extra-Enforcement in Iowa Department of Transportation Work Zones at Work-Zone-Law-Enforcement for additional information.
Back to topTwo Lane Highways
- Maintain existing regulatory speed limit if possible.
- On-site detours, shoulder runarounds, and temporary connections should have advisory speeds posted in accordance with roadway geometry.
- An advisory speed should be used for one-lane bridges when traffic is controlled by temporary signals. Standard Road Plans TC-216 and TC-217 show a default advisory speed of 35 mph. This value may be adjusted in the field based on operating conditions.
Multi Lane Divided Highways
Four Lane
- Existing regulatory speed limit should be maintained if the existing number of lanes are open to traffic and the width between barriers (other than spot locations) is 30’ or greater.
- Regulatory speed reduction may be considered where the roadway width between barriers is less than 30’ for tangent lengths 1000’ or longer. Consider using an advisory speed for shorter sections.
- Consider a reduced regulatory speed limit for single lane closure where workers are present without positive protection. The normal regulatory speed should be restored when workers are not present, and the lane closure remains in place during non-working hours Speed Feedback Signs should be placed in advance of worker activity.
- Consider appropriate advisory speeds where construction vehicles must frequently merge into high volume traffic lanes.
- Post an appropriate advisory speed for single lane closure next to temporary barrier rail due to potential side friction.
- Regulatory 60mph speed limit for two-lane, two-way operation (TLTWO). Refer to Standard Road Plan TC-61.
Six Lane or More
- Existing regulatory speed limit maintained if all existing lanes are open to traffic.
- Existing regulatory speed limit maintained if at least two existing lanes are open to traffic per direction and the cross-section configuration is 30’ or greater.
Electronic Speed Displays
The 11th Edition of the Manual on Uniform Traffic Control Devices (MUTCD) defines these signs as Electronic Displays to avoid confusion with changeable or dynamic messages.
Summary of additional portions of the MUTCD allow use of changeable message signs for Regulatory, Warning and Temporary Traffic Control signs as detailed below:
Changeable Electronic Displays are defined as manually controlled electronic displays where the message is changed to reflect specific conditions such as active lane closures, workers present, night, etc.
Back to topSpeed Feedback Signs
Speed Feedback Signs are a Dynamic Electronic Display sign that automatically displays the actual vehicle speed to an approaching driver. The variable speed limit legend is displayed in white LEDs on an opaque black background with a YOUR SPEED IS sign mounted directly above this display.
Speed Feedback Signs are intended for use when workers are adjacent to open lanes without positive protection. Other use of these signs should be limited on a case-by-case basis.
Caution:
Any other situation which may justify a reduction in the regulatory speed limit should be discussed with the Work Zone Traffic Control Engineer and the Work Zone Operations Engineer. Concurrence with the regulatory speed limit reduction must be received from both offices before adding it to the project plans.
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