Speed Limit Signs: When do you have to slow down? Do you have to be at the reduced speed at the sign or slow down at the sign?

Factual Questions
21

IN THEORY:
•If the posted speed limit increases, you may not accelerate to the new faster speed before your rear bumper passes the sign.
•If the posted speed limit decreases, you must decelerate to the new slower speed before your front bumper passes the sign.

IN PRACTICE:
Strictness of enforcement is subject to the judgment of the officer in question.

CAVEAT:
Speed limits apply for perfect daytime driving conditions. At night or under adverse weather, you must reduce your speed appropriately.

22

I’m assuming that applies for speeding up, as well.

23

Yes. A lot of people have this belief of if they can see it they can drive it. But that isn’t so.

24

Do the rules take into account Lorentzian contraction resulting from the car’s velocity or acceleration? Note that the driver of the car will not observe himself (or any part of his car) passing the sign at the same moment the observing police officer will.

If the vehicle is traveling faster than the speed of light, will the observing officer see the rear bumper pass the sign before the front bumper? Or what if the vehicle is simply driving in reverse?

If the law is to be taken seriously, these kinds of details must be clearly specified.

25

In the UK, Yes.

Same in Australia, you definitely have to be going the slower speed when you pass the sign

26

I don’t know if other states do it this way, but in California as you’re leaving a town and the speed limit increases from say 35 to 55 mph, there will be a sign that says something like “End 35 mph zone” and then another “Speed limit 55” sign a little farther down. I always assumed you’re allowed to start accelerating to 55 mph after you pass the first sign.

27

Ah good ol’ “End [number] limit”, the least-useful traffic sign in the world.

28

The speed limit starts at the sign which is to be posted at the point where the limit changes.

Here’s some federal law on the signage (2009 edition of the Manual on Uniform Traffic Control Devices)

https://mutcd.fhwa.dot.gov/pdfs/2009r1r2/part2b.pdf:

Section 2B.13 Speed Limit Sign (R2-1) Standard: 01 Speed zones (other than statutory speed limits) shall only be established on the basis of an engineering study that has been performed in accordance with traffic engineering practices. The engineering study shall include an analysis of the current speed distribution of free-flowing vehicles. 02 The Speed Limit (R2-1) sign (see Figure 2B-3) shall display the limit established by law, ordinance, regulation, or as adopted by the authorized agency based on the engineering study. The speed limits displayed shall be in multiples of 5 mph. 03 Speed Limit (R2-1) signs, indicating speed limits for which posting is required by law, shall be located at the points of change from one speed limit to another

04 At the downstream end of the section to which a speed limit applies, a Speed Limit sign showing the next speed limit shall be installed. Additional Speed Limit signs shall be installed beyond major intersections and at other locations where it is necessary to remind road users of the speed limit that is applicable. 05 Speed Limit signs indicating the statutory speed limits shall be installed at entrances to the State and, where appropriate, at jurisdictional boundaries in urban areas. Support: 06 In general, the maximum speed limits applicable to rural and urban roads are established: A. Statutorily – a maximum speed limit applicable to a particular class of road, such as freeways or city streets, that is established by State law; or B. As altered speed zones – based on engineering studies. 07 State statutory limits might restrict the maximum speed limit that can be established on a particular road, notwithstanding what an engineering study might indicate. Option: 08 If a jurisdiction has a policy of installing Speed Limit signs in accordance with statutory requirements only on the streets that enter a city, neighborhood, or residential area to indicate the speed limit that is applicable to the entire city, neighborhood, or residential area unless otherwise posted, a CITYWIDE (R2-5aP), NEIGHBORHOOD (R2-5bP), or RESIDENTIAL (R2-5cP) plaque may be mounted above the Speed Limit sign and an UNLESS OTHERWISE POSTED (R2-5P) plaque may be mounted below the Speed Limit sign (see Figure 2B-3). Figure 2B-3. Speed Limit and Photo Enforcement Signs and Plaques R2-1 R2-2P R2-4P R2-4a R2-5aP R2-5bP R2-5cP R2-6P R2-6aP R2-6bP R2-3P R2-5P R2-10 R2-11 R10-18 R10-19P R10-19aP Page 58 2009 Edition Sect. 2B.13 to 2B.15 December 2009 Guidance: 09 A Reduced Speed Limit Ahead (W3-5 or W3-5a) sign (see Section 2C.38) should be used to inform road users of a reduced speed zone where the speed limit is being reduced by more than 10 mph, or where engineering judgment indicates the need for advance notice to comply with the posted speed limit ahead. 10 States and local agencies should conduct engineering studies to reevaluate non-statutory speed limits on segments of their roadways that have undergone significant changes since the last review, such as the addition or elimination of parking or driveways, changes in the number of travel lanes, changes in the configuration of bicycle lanes, changes in traffic control signal coordination, or significant changes in traffic volumes. 11 No more than three speed limits should be displayed on any one Speed Limit sign or assembly. 12 When a speed limit within a speed zone is posted, it should be within 5 mph of the 85th-percentile speed of free-flowing traffic. 13 Speed studies for signalized intersection approaches should be taken outside the influence area of the traffic control signal, which is generally considered to be approximately 1/2 mile, to avoid obtaining skewed results for the 85th-percentile speed. Support: 14 Advance warning signs and other traffic control devices to attract the motorist’s attention to a signalized intersection are usually more effective than a reduced speed limit zone. Guidance: 15 An advisory speed plaque (see Section 2C.08) mounted below a warning sign should be used to warn road users of an advisory speed for a roadway condition. A Speed Limit sign should not be used for this situation. Option: 16 Other factors that may be considered when establishing or reevaluating speed limits are the following: A. Road characteristics, shoulder condition, grade, alignment, and sight distance; B. The pace; C. Roadside development and environment; D. Parking practices and pedestrian activity; and E. Reported crash experience for at least a 12-month period. 17 Two types of Speed Limit signs may be used: one to designate passenger car speeds, including any nighttime information or minimum speed limit that might apply; and the other to show any special speed limits for trucks and other vehicles. 18 A changeable message sign that changes the speed limit for traffic and ambient conditions may be installed provided that the appropriate speed limit is displayed at the proper times. 19 A changeable message sign that displays to approaching drivers the speed at which they are traveling may be installed in conjunction with a Speed Limit sign. Guidance: 20 If a changeable message sign displaying approach speeds is installed, the legend YOUR SPEED XX MPH or such similar legend should be displayed. The color of the changeable message legend should be a yellow legend on a black background or the reverse of these colors. Support: 21 Advisory Speed signs and plaques are discussed in Sections 2C.08 and 2C.14. Temporary Traffic Control Zone Speed signs are discussed in Part 6. The WORK ZONE (G20-5aP) plaque intended for installation above a Speed

29

What does this mean?

30

Maximum, minimum, and truck speeds?

31

I’m deeply offended that no one mentioned one of the worst speed traps of all time - New Rome , Ohio, a microscopic village that parlayed its 1000 feet of U.S. 40 into a $400,000 a year ticket revenue machine.

If you didn’t immediately spot and react to the sign suddenly reducing the speed limit from 45 to 35 mph, you were liable to be nailed for a $90 ticket, plus whatever bogus violations they could tack on, like having dusty tailights.

The Ohio Legislature finally cracked down and dissolved the village of New Rome, which got absorbed by a neighboring township.

The kicker is that the Ohio House has passed a bill to allow townships with as few as 5,000 people to send police officers to patrol Interstates that pass through their territory, a license for speed trap abuse.
The bill has not made it through the Ohio Senate - yet.

32

We used to see maximum, truck, and night speed limits. The nighttime limit was white lettering on a black background; it was the only reflective part of the sign so at night only the night speed limit showed up. We don’t have night limits anymore; headlights must be better.

33

“On March 1 [2021] the city of Chicago enacted a law saying if you go over 6 mph above the speed limit you can get a ticket from a speed camera.”
Chicago speed cameras issue more than 1 million tickets in 4 months - ABC7 Chicago

This is only for the cameras, but as long as I’ve been driving, I’ve always stayed within that limit. Since this was new, obviously it was more previously, but I haven’t kept track.

34

At what speed would the car have to be traveling to create the effect? I remembered this joke:

…about the physicist who tried to beat a ticket for running a red light by telling the judge that at the speed he was approaching the signal, the red light was Doppler-shifted so it appeared green. The judge pondered this for a few minutes and tore up the red light ticket. Then, seeing as the physicist would have to be driving about a quarter of the speed of light to see a red signal as green, the judge fined him 269 million dollars for speeding, one dollar for each kilometer per hour over the limit.

35

We used to have a small town that required 30mph. It changed to 45 and then 55 as the highway went beyond the city limits.

The small town cop sat in that 45mph area to catch any traveller’s that resumed highway speeds to quickly.

The state finally built a bypass around the town. So highway traffic flows uninterrupted.

It’s interesting that they caught more drivers exiting the town. People naturally slowed entering the area. I used to stop, refill my gas, and buy a snack. You naturally want to resume highway speeds as you leave town.

36

I lived next to a very, very small town. Every street in the town had a 20 mph speed limit…except one which was 10 mph. Guess where the police hung out? Even my school bus driver got ticketed (no small thing since the fine was multiplied (or added to somehow) for every child on the bus). I remember the cop coming on the bus and counting us.

37

There’s a whole lot of rural roads around here, and if it is unposted, then it is the state limit of 55mph.

People sometimes get confused as to what the sign “Speed Zone Ahead” means. It means that there is a posted speed limit ahead, even if sometimes that posted limit is still 55. But I have known people to believe that it means that it is an area where it is okay to speed.

38

Well most western countries have a similar leeway and i thought another argument for it might be overtaking.

For example, let’s say you’re traveling at 45 in a 50 mph zone and you encounter a long vehicle traveling at 40. You’d be side by side with that vehicle for a long time at just +5 mph, so virtually all drivers would accelerate at least a bit for a few seconds.
Seems pretty harsh if you’re ticketed at that precise moment for going slightly over 50.

39

Parking rules are vigorously enforced in Philadelphia. And as that story explains, if you leave your car on the street for more than a bare overnight, there is serious risk of it not being there when you return.

However, moving violation enforcement seems lax to me. No one is giving tickets at the point of a slowdown speed limit sign.

40

Those cities are run by amateurs. Let me introduce you to Tremont City, Ohio. I drive through it at least twice a week, albeit very carefully.

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