# What is (are) the Minimum Speed on the USA Interstate Highway System?
**URL:** https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683
**Category:** Factual Questions
**Created:** [August 22, 2016, 11:02pm UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683 "2016-08-22T23:02:01Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@Bullitt](https://boards.straightdope.com/u/Bullitt)
#### Post date: [August 22, 2016, 11:02pm UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/1 "2016-08-22T23:02:01Z")
Is there one minimum speed requirement that applies to all interstate highways in all states, or is the requirement a local one, or is the requirement state-specific? I would think it would be a federally mandated requirement.
I seem to remember 45 MPH as being the min speed, but can’t remember where that came from. And that may have been during the double nickle, and may no longer be fast enough.
I am interested in interstate highways only, not US or state divided highway systems, even though in some places the highway systems may coincide. If it’s an interstate, I want to know.
So, what is the minimum required speed on interstates?
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### Author:  [@dofe](https://boards.straightdope.com/u/dofe)
#### Post date: [August 22, 2016, 11:48pm UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/2 "2016-08-22T23:48:05Z")
There is no federal speed limit, either maximum or minimum – these limits are set by the States, even on interstate highways.
As far as California is concerned, there is no CA code setting a specific minimum speed limit. However, a person who is driving unreasonably slow may be cited under [CA Vehicle Code 2204(a)](http://www.leginfo.ca.gov/cgi-bin/displaycode?section=veh&group=22001-23000&file=22400-22413), which provides that:
> [@](#):
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> No person shall drive upon a highway at such a slow speed as to impede or block the normal and reasonable movement of traffic unless the reduced speed is necessary for safe operation, because of a grade, or in compliance with law.
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### Author:  [@Kent\_Clark](https://boards.straightdope.com/u/Kent_Clark)
#### Post date: [August 23, 2016, 12:03am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/3 "2016-08-23T00:03:43Z")
In Missouri [(Section 304.001.1)](http://www.moga.mo.gov/mostatutes/stathtml/30400000111.html)
> [@](#):
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> No vehicle shall be operated at a speed of less than forty miles per hour on any highway which is part of the interstate system of highways, unless:
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> (1) A slower speed is required for the safe operation of the vehicle because of weather or other special conditions; or
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> (2) Agricultural implements, self-propelled hay-hauling equipment, implements of husbandry and vehicles transporting such vehicles or equipment may be operated occasionally on interstate highways for short distances at a speed of less than forty miles per hour if such vehicle or equipment is operated pursuant to a special permit issued by the chief engineer of the state department of transportation pursuant to section 304.200 and the regulations established pursuant to such section.
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### Author:  [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)
#### Post date: [August 23, 2016, 12:08am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/4 "2016-08-23T00:08:03Z")
I have only seen posted signs for minimum speed in Florida (I-75 I believe).
As noted, California is at discretion, and I think it would be very difficult to enforce any arbitrary speed on e.g. I-80 through the Sierras, where semi truck (and sometimes passenger vehicle) speed is limited by the uphill grade.
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### Author:  [@Lord\_Feldon](https://boards.straightdope.com/u/Lord_Feldon)
#### Post date: [August 23, 2016, 12:17am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/5 "2016-08-23T00:17:15Z")
Ohio has a law _allowing_ ODOT to establish a minimum speed limit between 30 and 50 MPH on a controlled-access road, but I don’t think this is currently done. In the absence of a sign, the law is just that you can’t go “unreasonably slow.”
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### Author:  [@Bullitt](https://boards.straightdope.com/u/Bullitt)
#### Post date: [August 23, 2016, 1:07am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/6 "2016-08-23T01:07:09Z")
Thanks, guys. So, this is quite interesting - the states set the requirement. It’s not a federal requirement.
It sounds like there is some discretion allowed. Safety is, of course, first and foremost. Trucks up a steep grade, sure, I can understand not enforcing a (say) 45 MPH minimum speed. If you’re loaded and cannot climb the grade faster, then no reasonable infraction should be penalized.
And, e.g., on a California interstate such as I-5 south of Sacramento and going towards Los Angeles (but before the ‘Grapevine’ - the high mountain passes south of the Central Valley), where in the Central Valley the highway is flat and straight, with very few turns at all and even then they are quite mild, traffic often flows at 80 MPH when the speed limit is 70. The highway is only two lanes, so if a car were going very slowly in the right-most lane, say at 40 MPH, and if there’s no legitimate reason to drive so slowly, then you can definitely get a ticket for creating a hazard: the normal flow of traffic in the right lane may have to swerve suddenly to avoid hitting such a slow car.
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### Author:  [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)
#### Post date: [August 23, 2016, 1:14am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/7 "2016-08-23T01:14:12Z")
In Colorado there are sections where a minimum speed is posted.
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### Author:  [@Bullitt](https://boards.straightdope.com/u/Bullitt)
#### Post date: [August 23, 2016, 1:20am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/8 "2016-08-23T01:20:06Z")
I have seen some places where a minimum speed is posted on interstates, but I can’t remember what states those were. I’ll have to pay closer attention.
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### Author:  [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)
#### Post date: [August 23, 2016, 1:54am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/9 "2016-08-23T01:54:06Z")
I specifically remembered Florida because of old people stereotypes.
The maximium speed limit also [varies by state](https://en.wikipedia.org/wiki/Speed_limit#/media/File:World_Speed_Limits.svg), to a high of 85 in Texas. This is after more than two decades of strict Federal restrictions on speed limits that states now set their own limits. Montana is also very “nudge, nudge, wink, wink” about not enforcing speed limits in some areas.
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### Author:  [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)
#### Post date: [August 23, 2016, 2:28am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/10 "2016-08-23T02:28:42Z")
[QUOTE=thelurkinghorror]
Montana is also very “nudge, nudge, wink, wink” about not enforcing speed limits in some areas.
[/QUOTE]
They just don’t want to let go of “reasonable and prudent.”
The idea of a minimum limit is interesting as it could conflict with the basic speed law that says you may not drive faster than what’s safe for the current conditions. Driving 45 MPH may not be safe or prudent and thereby unlawful in a heavy rainstorm but if there’s a minimum posted speed of 45, what do you do? Violate that minimum or endanger yourself and others?
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### Author:  [@Bullitt](https://boards.straightdope.com/u/Bullitt)
#### Post date: [August 23, 2016, 3:17am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/11 "2016-08-23T03:17:33Z")
> [@gotpasswords](#):
>
> … in a heavy rainstorm but if there’s a minimum posted speed of 45, what do you do? Violate that minimum or endanger yourself and others?
Good point. But I think the Basic Speed Law, to never drive faster than the conditions dictate to be safe, always prevails.
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### Author:  [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)
#### Post date: [August 23, 2016, 3:43am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/12 "2016-08-23T03:43:42Z")
> [@gotpasswords](#):
>
> They just don’t want to let go of “reasonable and prudent.”
>
> The idea of a minimum limit is interesting as it could conflict with the basic speed law that says you may not drive faster than what’s safe for the current conditions. Driving 45 MPH may not be safe or prudent and thereby unlawful in a heavy rainstorm but if there’s a minimum posted speed of 45, what do you do? Violate that minimum or endanger yourself and others?
E.g. California: [Wet road–go 5 to 10 mph slower.](https://www.dmv.ca.gov/portal/dmv/detail/pubs/hdbk/idt_hand_emerg)
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### Author:  [@Lord\_Feldon](https://boards.straightdope.com/u/Lord_Feldon)
#### Post date: [August 23, 2016, 3:49am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/13 "2016-08-23T03:49:27Z")
> [@gotpasswords](#):
>
> The idea of a minimum limit is interesting as it could conflict with the basic speed law that says you may not drive faster than what’s safe for the current conditions.
I’d expect any minimum speed law to be written like the Missouri law posted earlier, which has an exception for when conditions require you to go slower.
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### Author:  [@Mr\_Downtown](https://boards.straightdope.com/u/Mr_Downtown)
#### Post date: [August 23, 2016, 4:05am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/14 "2016-08-23T04:05:49Z")
It’s worth noting that nothing about the Interstate System is actually under federal control. Interstates, like “US highways,” are owned and operated by the individual states. A nongovernmental body, the American Association of State Highway & Transportation Officials (AASHTO) is the arbiter of whether a highway meets the standards to display an Interstate System sign, and whether the proposed route number is acceptable as part of the [interstate numbering scheme.](https://en.wikipedia.org/wiki/Interstate_Highway_System#Numbering_system)
AASHTO also developed the minimum criteria to be considered part of the Interstate network, though that was formally adopted by the Federal Highway Administration. The 1956 Highway Act offered 90 percent federal funding for interstates, but they were all built by the states.
As for the original question, both minimum and maximum speeds are set by individual states, though the feds did set a nationwide maximum from 1974 until 1995.
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### Author:  [@BigT](https://boards.straightdope.com/u/BigT)
#### Post date: [August 23, 2016, 4:47am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/15 "2016-08-23T04:47:06Z")
I was taught that it wasn’t a fixed speed, just that you can’t go more than 25MPH under the speed limit (except due to bad conditions). That type of thing makes more sense to me than a fixed speed. Don’t know how true it is, though, even in my state.
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### Author:  [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)
#### Post date: [August 23, 2016, 10:35am UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/16 "2016-08-23T10:35:22Z")
I live in NY state. The maximum on our interstates is 65 mph, and although I’ve never seen any ‘minimum speed’ signs I have seen ones on interstates that say ‘UNDER 30 MPH MUST USE FLASHERS’. I think it was 30 mph, maybe it was 40.
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### Author:  [@joe.russell](https://boards.straightdope.com/u/joe.russell)
#### Post date: [August 23, 2016, 1:06pm UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/17 "2016-08-23T13:06:43Z")
Iowa has a law of a min of 45 mph on all interstates. Mostly because of farm equipment I believe. There is also a grade limit on all(?) Interstates of x° where a heavy vehicle should have no problem doing that
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### Author:  [@Atamasama](https://boards.straightdope.com/u/Atamasama)
#### Post date: [August 23, 2016, 2:49pm UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/18 "2016-08-23T14:49:12Z")
Washington State RCW 46.61.425:
**Minimum speed regulation—Passing slow moving vehicle.**
(1) No person shall drive a motor vehicle at such a slow speed as to impede the normal and reasonable movement of traffic except when reduced speed is necessary for safe operation or in compliance with law: PROVIDED, That a person following a vehicle driving at less than the legal maximum speed and desiring to pass such vehicle may exceed the speed limit, subject to the provisions of RCW 46.61.120 on highways having only one lane of traffic in each direction, at only such a speed and for only such a distance as is necessary to complete the pass with a reasonable margin of safety.
(2) Whenever the secretary of transportation or local authorities within their respective jurisdictions determine on the basis of an engineering and traffic investigation that slow speeds on any part of a highway unreasonably impede the normal movement of traffic, the secretary or such local authority may determine and declare a minimum speed limit thereat which shall be effective when appropriate signs giving notice thereof are erected. No person shall drive a vehicle slower than such minimum speed limit except when necessary for safe operation or in compliance with law.
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### Author:  [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)
#### Post date: [August 23, 2016, 3:04pm UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/19 "2016-08-23T15:04:48Z")
> [@joe.russell](#):
>
> Iowa has a law of a min of 45 mph on all interstates. Mostly because of farm equipment I believe. There is also a grade limit on all(?) Interstates of x° where a heavy vehicle should have no problem doing that
There are hills in Iowa? 😉
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### Author:  [@Bullitt](https://boards.straightdope.com/u/Bullitt)
#### Post date: [August 23, 2016, 3:07pm UTC](https://boards.straightdope.com/t/what-is-are-the-minimum-speed-on-the-usa-interstate-highway-system/763683/20 "2016-08-23T15:07:39Z")
> [@kenobi\_65](#):
>
> There are hills in Iowa? 😉
In Florida, it’s the same. But the rest of us call them overpasses.
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