Camille from Henderson writes, “What's driving you crazy? What, if anything, is the minimum speed limit on a highway with a speed limit of 60 mph? Is there an actual number? A suggested number? Anything I read just says so as not to impede the normal flow. I understand that conditions affect speed, but I'm asking about normal road and traffic conditions.”
There is no set number, Camille, and most roads in Colorado don't have an official minimum speed limit. When you do see a posted minimum speed limit, it will be on a heavily traveled section of that particular interstate. One of those sections I’ve seen these minimum speed signs is along Interstate 25 in downtown Denver. I counted 23 of them between 58th Ave. and Broadway, 10 northbound and 13 southbound. The posted minimum speed is 40 mph with a maximum of 55 mph. I looked for speed limit signs along Highway 85 and Interstate 76 around Henderson and didn’t see any minimum limits attached to the maximum speed limit posted on either highway.
You referred to the Colorado law stating drivers are not to impede the normal flow of traffic. That comes from Colorado Revised Statutes 42-4-1103 where in part 1 it states, “No person shall drive a motor vehicle on any highway at such a slow speed as to impede or block the normal and reasonable forward movement of traffic, except when a reduced speed is necessary for safe operation of such vehicle or in compliance with law.”
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There are caveats within the law, including one that refers to what you are asking. In the second section of part 2 of CRS 42-2-1103 it reads, “Local authority may determine and declare a minimum speed limit below which no person shall drive a vehicle, except when necessary for safe operation or in compliance with law.”
Again, the law never states exactly what that minimum speed is or should be leaving that decision to the local authority and to local law enforcement who need to determine if a driver is going too slowly.
There are times when driving too slowly can be dangerous for other drivers on the road, especially at highway speeds. One CDOT traffic and safety engineer told me, a driver should not be going so slow to the point of impeding the normal and reasonable movement of traffic.
“The question becomes, ‘what is normal and reasonable?’ If you are driving on a freeway with a posted 60 mph speed limit, and you are driving at 50 mph on the right side with multiple lanes and other drivers can easily pass you, you are not impeding the normal flow of traffic, thus you are not creating a hazard," the CDOT engineer said. "The primary goal of setting the right speed limit is to reduce differentials in speed between vehicles in the traffic stream as much as practical because large differences in speeds often correlate to higher crash frequency to a far greater degree than higher speeds overall.”
The CDOT engineer said all drivers should be driving at or below the speed limit. However, speed differential can be a problem as going slow on a higher speed roadway can increase the risk of a crash.
“The expectation is 85% of drivers are traveling at speeds somewhere near the speed limit, so driving at a significantly slower speed would surprise other drivers and can create an unexpected reaction. If there is a hazard in the road or adverse weather conditions, slower speeds are encouraged for safety,” the CDOT engineer said.
Another part of the minimum speed law says, "When driving on a highway at a speed less than the normal and reasonable speed of traffic so that traffic is impeded immediately behind, then such driver shall either drive in the right-hand lane or on the extreme right side of the roadway or pull off the roadway at the first available place where such movement can safely and lawfully be made until such impeded traffic has passed by."
That is often the case on mountain roads where an additional climbing lane has been added or a roadside turnouts are provided for drivers of all vehicles proceeding at less than the normal and reasonable speed of traffic.
Additionally, to the west of Denver, there is a sign posted at the Morrison exit stating the left lane minimum speed limit of 55 mph. It stems from Colorado law 42-4-1106 that sets a minimum speed for the left lane on I-70 in the mountains.
The law states, “Where the average grade is six percent or more uphill for at least one mile, no person shall operate a motor vehicle in the far left lane of traffic of interstate 70 at a speed of less than the lower of ten miles per hour below the speed limit or the minimum speed set by the department of transportation.”
There are a few exceptions like due to weather conditions or lane blockages.
Depending on what exactly a driver is cited for, the fines for driving too slowly vary. Driving at a speed that is not reasonable and prudent given road conditions carries a $100 fine with a $10 surcharge. Driving at such a slow speed that the normal and reasonable forward movement of traffic is impeded, carries a $50 fine with a $6 surcharge.
If you would like to really get into this topic, Victor Muchuruza and Renatus Mussa from the joint college of engineering of Florida A&M University and Florida State University published a research paper titled, “Nationwide survey on the posting of minimum speed limit signs on Interstate freeways,” dated November 2006. In that paper, the authors found that the most common minimum speed limit posting is 40 mph. There are few states that post 45 mph and 55 mph in some sections on interstate freeways. The survey results have also discovered that many states raised the maximum speed limits on interstate freeways as the consequence of the National Highway System (NHS) Designation Act of 1995 without revising or studying the effect of the existing minimum speed limits on traffic operation.
This is a good time to remind drivers that the speed limit — high and low — are limits, not the required speed you have to drive. I reminded my teenage daughter of that often when she was learning to drive saying, “The speed limit is the speed you aren’t supposed to drive more than, not the speed you have to drive.”
As CDOT advises, you should be going at about the same speed as all other traffic, whether that is above or below the speed limit because if you don’t, you are disrupting the traffic flow and could be a risk for other drivers.
Denver7 Traffic Expert Jayson Luber says he has been covering Denver-metro traffic since Ben-Hur was driving a chariot. (We believe the actual number is over 25 years.) He's obsessed with letting viewers know what's happening on their drive and the best way to avoid the problems that spring up. Follow him on Facebook, Twitter or Instagram or listen to his award winning Driving You Crazy podcast on any podcast app including iTunes, iHeartRadio, Spotify, Podbean, or YouTube.
