9A-4: Regulatory Speed Limit Changes

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.

In general, attempts to reduce speeds will increase the likelihood of crashes. 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.

Inattentiveness, not speed, is the most dangerous factor in a temporary traffic control zone. By making work areas more visible, drivers will be more alert to changing conditions and have time to respond appropriately. Signs, flaggers, and other traffic control devices alert motorists and guide them safely through the temporary traffic control zone. The safest traffic control zone is where everyone pays attention and travels at the same speed.

Accommodating traffic at the normal operating speed is not always possible. The following guidelines provide uniformity when reduced speeds are necessary in temporary traffic control zones.

Two Lane Highways

• Maintain existing regulatory speed limit if at all possible.
• On-site detours, shoulder runarounds, and temporary connections should have advisory speeds posted in accordance with the geometric design.
• An advisory speed of 35 mph should be posted for one-lane bridges when traffic is controlled by temporary signals.

Multi Lane Divided Highways

Four Lane

• Existing regulatory speed limit (65 mph, 60 mph, or 55 mph) maintained if all existing lanes are open to traffic and the width between barriers (other than spot locations) is 30’ or greater.
• Regulatory 55 mph speed limit where the roadway width between barriers is less than 30’ other than spot locations.
• Regulatory 55 mph speed limit with single lane closure.
• Regulatory 55 mph speed limit where construction vehicles must frequently merge into high volume traffic lanes.
• Regulatory 55 mph speed limit with single lane closure using temporary barrier rail due to potential side friction from use of barrier rail.
• Regulatory 55 mph speed limit for two-lane, two-way operation (TLTWO).

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.
• Regulatory 55 mph speed limit when cross section configuration is less than 30’.

• Regulatory 55 mph speed limit with only single lane available to traffic.

 

Attention Required

	Any other situation which may justify a reduction in the regulatory speed limit should be discussed with the Work Zone Traffic Control Engineer for the Office of Traffic and Safety and the Traffic Safety Field Engineer for the Office of Construction. Concurrence with the regulatory speed limit reduction must be received from both offices before adding it to the project plans.

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Chronology of Changes