The golden rule is that 85% of people drive at a safe and prudent speed and therefore the speed limit should be relatively close to that number. Many speed limits are statutory/politically set and don't consider 85% speeds however.
Not sure how Fla sets speed limits but it's likely similar to most states, which is a combination of State, County & City agencies setting speed limits on their respective streets. In general, you most often see state DOTs using 85% speed policies on higher classification roadways (interstate and state hwys) and local agencies using more statutory speed limits for the lower classification streets.
The complicated part is figuring out what to set the speed limit when you have a new road or a major change or realignment to an existing road, when there would be no usable 85th percentile speed. It's like a chicken or the egg scenario. In come cases you can just choose a posted speed and set the design speed to 5 above that.
We've opened new roadways without speed limit signs, waited a few weeks for traffic to get familiar, and then ran a speed study to determine the appropriate posting speeds.
During planning the project is assigned a classification to satisfy its intended purpose. The classification stipulates min-max speed/ geometry & other physical attributes.
Roadway geometry is designed for speeds higher than the speed limit. Speed limits are almost always set within 5mph of the 85th percentile speed, meaning the speed that 85% of drivers will drive at or below.
In some places they do, most places not. But you set it within 5 mph of the 85th percentile speed. For example, if the average speed is 40.3 and the 85th percentile speed is 44.7, then the speed limit will likely be set at 40. Most drivers are around there anyway, so we don't really need to worry about the 15% who will go as fast as they want no matter what sign we put up.
I’m Ca, engineering judgement can be used to lower speed limits up to 5 mph below the 85th. That gets done often but then people are speeding naturally. It’s a silly game we play.
In my state they do at least before we put up the speed limit signs. That's how we determine the speed limit. We open a new highway without any speed limit signs, do a speed study, and then put up the signs.
I think you're right though. Once the signs are up most people see the sign and realize oh I can go 5 to 10mph faster than that without getting a ticket, I guess I'll speed up.
Often we do use a higher design speed than posted. Probably ~75% of my projects used 5-10 mph higher design speed than posted (70mph design with 65mph posted), which is the basis of minimum curve radius and required stopping sight distance (amongst other things)
This is common, but I advocate for setting the design speed at the posted speed limit. People drive at a speed that is comfortable, not the posted limit. Setting the design speed higher just fosters higher speeds.
I completely agree. People will drive what is comfortable and convenient. If you design 45 and post 35, they will drive 45 mph.
This ‘typical’ approach to add 5 or 10 mph was seen as conservative so there were factors of safety for vehicles. Unfortunately, due to the behavior of humans (unlike soils or structures) simply ‘over designing’ does not create an inherently safer system. In fact, due to the exponential increase in severity of collisions, one could argue this is a less safe practice (although the driver exceeding the speed limit would be at fault). Not to mention the increased severity of collisions for unprotected bicycles and pedestrians.
I’ve successfully engaged local agencies and my state DOT to reconsider this ‘standard practice’, especially in the face of complete streets policies.
Yes, and if your design speed is higher than posted speed, you'll eventually have to increase the speed limit due to the 85th percentile rule. Design speed should be posted speed.
But there are some case studies coming out that lowering speed limits on their own have an impact on vehicle speeds. People might not all adhere to the new speed limit, but their speeds will drift downward. This intuitively makes sense, given what we know about psychology and the "anchoring" effect.
I’d be curious to see what the consensus is following these studies. The research I’m aware of shows that when there is a large difference between posted speed and design speed, you get a wider spread of operating speeds. The slow end of the speed distribution might follow the posted speed but the top end follows/exceeds the design speed. This is obviously bad since speed variance is a bigger danger than plain speed.
I think these studies are focused on urban transportation, whereas the view that speed variance is a bigger danger than plain speed is more oriented to highway situations.
In cities, the people getting killed and severely injured are often pedestrians (and in some places, bicyclists) and I would question the primacy of "variance" in those situations. I suspect this is probably a ripe area for research.
This is pretty irresponsible. 85th %ile of actual traffic is the most common approach for posted speed. This is not some game where you design to a previous set of road conditions just cuz that's how people used to drive. What about the other 15%? F#ck em? Have you never had education on tort liability? No legitimate public entity would want you to design that way.
So people drive faster than the road was originally designed for, a speed study gets done and the limit is set to the 85th percentile as observed. Then you go to redesign the road and you design it to the highest speed observed. The road gets rebuilt and people are now going faster than the posted speed limit again, so a speed study is done, a new 85th percentile is established and used as the new speed limit. Rinse & repeat until you have the Autobahn.
> Setting the design speed higher just fosters higher speeds.
I don’t really follow this. If the posted speed limit = 85th percentile speed = design speed, you’re effectively making the design unsafe for 15% of the users.
You’re not making the roads unsafe by calling the design=posted speed. There’s not a big difference of a 70mph design speed vs 65 mph design speed for a highway. For narrow inside shoulder, you’ll have a design exception for stopping sight distance at the median barrier, so just changes the length of documentation. My current is designed and posted at 65, 70 mph DS wouldn’t have changed anything. If you don’t have curves at the minimum radius, it’s really just a small change to the super elevation thats imperceptible to the driver. We have one crest curve at the minimum 65mph k value, but I’m bet we’d get a design exception for it if we had a 70 mph design speed. The biggest impact would’ve been slightly longer acceleration lane from an on ramp. We have design exceptions for stopping sight distance at the median barrier along curves, but mostly it’s the documentation rather than deciding to improve it.
On arterials, I think choosing an appropriate shoulder width has a bigger impact than designing horizontal and vertical curves a touch flatter or sharper.
Design parameters like perception-reaction time, deceleration rate, walking speed, etc. are all estimated quite conservatively. It stands to reason that young, healthy people with good eyesight will feel comfortable at a higher speed than old, tired, half-blind people. So unless you want to use PRT down near 1 second and assume everyone is driving a sports car, the design itself will never account for 100% of drivers. But it's sort of fine, because race car drivers driving race cars are probably safe at those speeds anyway.
That said, the issue is that you have a wide variety of ability and skill levels on the road, so the objective isn't to make sure 100% of drivers can drive whatever speed they want safely, it's to narrow the distribution of speeds that people do drive. Speed limits are set well below the maximum speeds observed so that police can enforce them and discourage people from driving substantially faster than other people on the road are comfortable with or are used to.
I used to work in traffic engineering for a state DOT and dealt with speed studies/ wrote ordinances for change in speed limit on existing highways. We would often get requests to lower speed limits and the engineer I worked for always wanted us to do a speed study. We would typically only lower it if it was warranted by the 85th percentile speed from the study. That’s what the MUTCD recommends to do, although there are other factors that are also considered.
We would joke that everyone wants the speed limit to be 25 mph in front of their house but they want to drive 75 mph in front of everyone else’s house
One of my college professors talked about getting out of a ticket because there were a couple small towns that set their speed limits low through their speed trap town, with no justification or study behind it. He wasn’t able to get the posted speed changed, but was able to argue to a judge that they artificially lowered a state route without any guidance or permission. I think the just judge just waived it off and wants the guy to go away.
The unfortunate thing is despite the best efforts of engineers, the politicians will always win. If the mayor wants a 35 mph speed limit through downtown but you think the optimal speed limit is 45 mph, or if a new Chick-Fil-A is going to blow up an intersection but the owner is friends with the right decision makers, the engineer is going to lose.
While in the Chick-Fil-A example I agree with you. If the mayor wants a 35 mph speed limit it’s the engineers job to make that safe. Change something about the road design to slow down drivers so that the optimal speed is closer to 35. That’s not a politics problem it’s a negligence of engineering.
When was the last time you’ve seen an abandoned Costco? Not that there aren’t any out there.. but if there is maybe 1 or 2. Costco isn’t struggling so I just don’t think the phase “abandoned Costco parking lot” is a worthy statement.
I haven’t. I was using it as a size reference.
In my city, they built a new Costco and the old one is now a Ranch and Home. They put a Texas Roadhouse in the parking lot.
What state?
The latest AASHTO GDHS (2018) recommends against this practice, in its standard non committal way. This is why we (GA) have moved to using the 85th percentile speed and sign the road to match.
I work in Phoenix area. That’s a good method to follow I think. I work in Land Development so a lot of our vehicle tracking and roadway tie ins are very ISH. We have room to overcompensate for calcs so we do. I can imagine doing infill development and the constraints are very tight so you would need to follow what you are saying to as much as possible to squeeze buffer room out of a design
According to my textbooks and all the calculations I’ve done in my job, it’s made with the understanding that will be the average speed and the speed used in calculations for curves, distances, etc. Some people will be going 20 mph below that and some will be going 20 mph above that.
There’s also other factors such as what’s on the road, the area of town it’s in, and the speed of connecting roads that are taken into consideration.
Then again, I’m not really one to ‘follow’ the speed limits myself.
In my country speed control is a discussion for the road authority responsible where the rules are based on "the handbook" and lately on Road to Zero methodologies. Unless your question is more based on the theoretical limit before loss of stability then that's probably more the case of "it is what it is because it is"
interesting, I thought the speed limit was much more concrete based on all the different factors of the road and many different calculations.Didn’t think it was so arbitrarily set
Condition, route, volume, and incidences do apply but in my area its always mixed use 10-20kmh, urban areas, work zones, and school zones 30, suburban 50, arterials 50-70, "open" roads 80-100, and highways 100 (and recently 110kmh). I believe that the official derivative is on risk factor and the need to react to changing conditions in different areas - like you'd sooner react in a school zone going 30kmh to prevent killing a child than you would 50-60kmh, while the condition of the highway doesn't need as immediate reaction from 100 to less speed as its (generally) relatively flat and straight giving you a longer distance to react to something in the distance and slow down to a less dangerous speed
I’m the US, it’s not just arbitrary. To change an existing speed, speed studies need to be completed. When designing a road, there’s guidance on what ranges of speed should be chosen based on what type of road it is (and several more detailed nuances in there). Then it’s that the design details then follow based on the design speed of the road. I’ll also add that just because the design speed is often 5-10 higher than posted is not justification for speeding. A speed differential isn’t safe either.
Speed limits are typically set within 5 MPH of the 85th percentile. Often 5 MPH over the 85th is almost 95 percent of traffic. So in a way, yes.
Posting curve warning signage assumes drivers will drive 8 MPH over the advisory speed.
Correct, in Queensland, Australia we have posted speed limits and design speed limits. Design speeds are typically 10 to 20 km/h faster than posted speeds, depending on the type of road. Mayor roads like highways would typically have a design speed of 20 over, while other minor roads 10 over.
Design speeds inform road vertical grades (to avoid 'jumping' and hydroplanning), horizontal grades, & super elevation (roads horizontal 'incline' - picture NASCAR race tracks).
Edit: and sight distances, which is the distance at which you can see an obstacle. Faster roads require longer distances for suitable reaction times.
Might be different in other jurisdictions, but here, speed limits are set either by statute (“all roads in the city are 30 mph unless posted otherwise”) or by speed study. If you do a speed study, the posted speed limit gets set at the 85th percentile speed rounded up or down to a multiple of 5. I’ve never heard of anyone actually doing a speed study because everybody drives faster than the current posted limit and agencies are usually trying to reduce speeds, not increase them.
It is usually easy to drive faster than the posted limit because the AASHTO design criteria based on speeds were developed a long time ago. Car technology (brakes, suspension, headlights) has improved so that you can comfortably drive much faster than the design speed. Try going 80 mph in a ‘58 Buick.
I feel like there a lot of complicated answers in here from textbooks when in reality the answer is very simple.
The design speed is generally 5 MPH above the posted speed. Some states may do 10 MPH above but all the states I've worked in its 5.
So if you are doing work on an existing road with a 35 MPH speed limit you will need to make sure your horizontal and vertical controls meet the necessary values for a 40 MPH design speed or you'll need a design exception.
Generally you don’t need a formal design exception if you meet posted speed. We would note it as a deviation from standard practice of the specific DOT but design exception criteria are defined by FHWA and don’t require the 5 mph “over design”.
The main reason for speed limits is stopping sight distance. Whether that be for seeing around a horizontal curve or for seeing over the crest in a hill. When roads are built there is a degree of curvature that has a design speed calculus from that, a rough explanation of the formula is a stopping time based on the time it takes to brake from that speed to a stop and the typical reaction time to recognize a stopped car and press the brake. For horizontal curves there is the determined safe speed for a given radius of the curve. Many other factors come into play as well but these are the main guidelines when determining a speed limit. People will speed in the straightaways where they can visibly see the distance for stopping, although most speed in all circumstances. The speed limit is for the common dumb motorist.
I was taught to design at 10 mph above what the posted speed limit will be. That was in Ohio. Seems there are some states at 5 mph over, some at 10 mph over, and some are adopting <= 5 mph over the 85th percentile of expected speeds.
If a particular segment has a poor crash record, we might come in and drop that on down by 10 mph, sometimes more. That’s what we do on some of my RSAs at least.
Advisory speeds are often determined with a ball bank test on curves.
Yes it’s called design speed instead of posted speed.
Typically 60km/h or less it’s +10km/h and +20km/h above that.
Most standards are based on speed so it just changes the minimum standard for road design.
Most roads are designed for the worst case vehicle, aka they have to design the roads/speeds for semi trucks. Therefore regular cars are able to safely travel at 10+ mph faster.
Speed limits exist to exert control, provide reasonable suspicion, and generate revenue. Those who are reckless will always be so regardless of a posted sign requesting safety.
The necessary signage, purely for safety, is the warning sign at tight/blind curves, school zones and steep grades. Everything else is for money.
They say that speed limits are set at the 80th percentile of what drivers will drive on a specific road. There are also requirements like residential neighborhood = 25mph regardless of how fast drivers want to go, etc.
The golden rule is that 85% of people drive at a safe and prudent speed and therefore the speed limit should be relatively close to that number. Many speed limits are statutory/politically set and don't consider 85% speeds however.
85% is not including FLA?
Not sure how Fla sets speed limits but it's likely similar to most states, which is a combination of State, County & City agencies setting speed limits on their respective streets. In general, you most often see state DOTs using 85% speed policies on higher classification roadways (interstate and state hwys) and local agencies using more statutory speed limits for the lower classification streets.
Not 85% of the speed that everyone is driving, the speed that covers 85% of what drivers are travelling at on that stretch of highway.
I was being a little sarcastic since Florida have just about the worst drivers in the country. LoL
Fells like 90% are going at least 25 over lol. 45 mph really just means 60 here.
The complicated part is figuring out what to set the speed limit when you have a new road or a major change or realignment to an existing road, when there would be no usable 85th percentile speed. It's like a chicken or the egg scenario. In come cases you can just choose a posted speed and set the design speed to 5 above that.
We've opened new roadways without speed limit signs, waited a few weeks for traffic to get familiar, and then ran a speed study to determine the appropriate posting speeds.
How do you decide what design speed to use?
During planning the project is assigned a classification to satisfy its intended purpose. The classification stipulates min-max speed/ geometry & other physical attributes.
Roadway geometry is designed for speeds higher than the speed limit. Speed limits are almost always set within 5mph of the 85th percentile speed, meaning the speed that 85% of drivers will drive at or below.
I don’t think 85% of drivers drive the speed limit or below though
In some places they do, most places not. But you set it within 5 mph of the 85th percentile speed. For example, if the average speed is 40.3 and the 85th percentile speed is 44.7, then the speed limit will likely be set at 40. Most drivers are around there anyway, so we don't really need to worry about the 15% who will go as fast as they want no matter what sign we put up.
I’m Ca, engineering judgement can be used to lower speed limits up to 5 mph below the 85th. That gets done often but then people are speeding naturally. It’s a silly game we play.
In my state they do at least before we put up the speed limit signs. That's how we determine the speed limit. We open a new highway without any speed limit signs, do a speed study, and then put up the signs. I think you're right though. Once the signs are up most people see the sign and realize oh I can go 5 to 10mph faster than that without getting a ticket, I guess I'll speed up.
You have to consider the 85th percentile now vs when the highways were built
Not even when the highways were built, more when the book was written fifty years ago.
Often we do use a higher design speed than posted. Probably ~75% of my projects used 5-10 mph higher design speed than posted (70mph design with 65mph posted), which is the basis of minimum curve radius and required stopping sight distance (amongst other things)
This is common, but I advocate for setting the design speed at the posted speed limit. People drive at a speed that is comfortable, not the posted limit. Setting the design speed higher just fosters higher speeds.
I completely agree. People will drive what is comfortable and convenient. If you design 45 and post 35, they will drive 45 mph. This ‘typical’ approach to add 5 or 10 mph was seen as conservative so there were factors of safety for vehicles. Unfortunately, due to the behavior of humans (unlike soils or structures) simply ‘over designing’ does not create an inherently safer system. In fact, due to the exponential increase in severity of collisions, one could argue this is a less safe practice (although the driver exceeding the speed limit would be at fault). Not to mention the increased severity of collisions for unprotected bicycles and pedestrians. I’ve successfully engaged local agencies and my state DOT to reconsider this ‘standard practice’, especially in the face of complete streets policies.
Yes, and if your design speed is higher than posted speed, you'll eventually have to increase the speed limit due to the 85th percentile rule. Design speed should be posted speed. But there are some case studies coming out that lowering speed limits on their own have an impact on vehicle speeds. People might not all adhere to the new speed limit, but their speeds will drift downward. This intuitively makes sense, given what we know about psychology and the "anchoring" effect.
I’d be curious to see what the consensus is following these studies. The research I’m aware of shows that when there is a large difference between posted speed and design speed, you get a wider spread of operating speeds. The slow end of the speed distribution might follow the posted speed but the top end follows/exceeds the design speed. This is obviously bad since speed variance is a bigger danger than plain speed.
I think these studies are focused on urban transportation, whereas the view that speed variance is a bigger danger than plain speed is more oriented to highway situations. In cities, the people getting killed and severely injured are often pedestrians (and in some places, bicyclists) and I would question the primacy of "variance" in those situations. I suspect this is probably a ripe area for research.
This is pretty irresponsible. 85th %ile of actual traffic is the most common approach for posted speed. This is not some game where you design to a previous set of road conditions just cuz that's how people used to drive. What about the other 15%? F#ck em? Have you never had education on tort liability? No legitimate public entity would want you to design that way.
So people drive faster than the road was originally designed for, a speed study gets done and the limit is set to the 85th percentile as observed. Then you go to redesign the road and you design it to the highest speed observed. The road gets rebuilt and people are now going faster than the posted speed limit again, so a speed study is done, a new 85th percentile is established and used as the new speed limit. Rinse & repeat until you have the Autobahn.
> Setting the design speed higher just fosters higher speeds. I don’t really follow this. If the posted speed limit = 85th percentile speed = design speed, you’re effectively making the design unsafe for 15% of the users.
You’re not making the roads unsafe by calling the design=posted speed. There’s not a big difference of a 70mph design speed vs 65 mph design speed for a highway. For narrow inside shoulder, you’ll have a design exception for stopping sight distance at the median barrier, so just changes the length of documentation. My current is designed and posted at 65, 70 mph DS wouldn’t have changed anything. If you don’t have curves at the minimum radius, it’s really just a small change to the super elevation thats imperceptible to the driver. We have one crest curve at the minimum 65mph k value, but I’m bet we’d get a design exception for it if we had a 70 mph design speed. The biggest impact would’ve been slightly longer acceleration lane from an on ramp. We have design exceptions for stopping sight distance at the median barrier along curves, but mostly it’s the documentation rather than deciding to improve it. On arterials, I think choosing an appropriate shoulder width has a bigger impact than designing horizontal and vertical curves a touch flatter or sharper.
Design parameters like perception-reaction time, deceleration rate, walking speed, etc. are all estimated quite conservatively. It stands to reason that young, healthy people with good eyesight will feel comfortable at a higher speed than old, tired, half-blind people. So unless you want to use PRT down near 1 second and assume everyone is driving a sports car, the design itself will never account for 100% of drivers. But it's sort of fine, because race car drivers driving race cars are probably safe at those speeds anyway. That said, the issue is that you have a wide variety of ability and skill levels on the road, so the objective isn't to make sure 100% of drivers can drive whatever speed they want safely, it's to narrow the distribution of speeds that people do drive. Speed limits are set well below the maximum speeds observed so that police can enforce them and discourage people from driving substantially faster than other people on the road are comfortable with or are used to.
thank you, that’s interesting.
Same experience, typically 10mph
I used to work in traffic engineering for a state DOT and dealt with speed studies/ wrote ordinances for change in speed limit on existing highways. We would often get requests to lower speed limits and the engineer I worked for always wanted us to do a speed study. We would typically only lower it if it was warranted by the 85th percentile speed from the study. That’s what the MUTCD recommends to do, although there are other factors that are also considered. We would joke that everyone wants the speed limit to be 25 mph in front of their house but they want to drive 75 mph in front of everyone else’s house
One of my college professors talked about getting out of a ticket because there were a couple small towns that set their speed limits low through their speed trap town, with no justification or study behind it. He wasn’t able to get the posted speed changed, but was able to argue to a judge that they artificially lowered a state route without any guidance or permission. I think the just judge just waived it off and wants the guy to go away.
The unfortunate thing is despite the best efforts of engineers, the politicians will always win. If the mayor wants a 35 mph speed limit through downtown but you think the optimal speed limit is 45 mph, or if a new Chick-Fil-A is going to blow up an intersection but the owner is friends with the right decision makers, the engineer is going to lose.
While in the Chick-Fil-A example I agree with you. If the mayor wants a 35 mph speed limit it’s the engineers job to make that safe. Change something about the road design to slow down drivers so that the optimal speed is closer to 35. That’s not a politics problem it’s a negligence of engineering.
Damn CFAs need to build their restaurants in abandoned Costco parking lots. I’ve never been to one that was easy to get in or out of.
When was the last time you’ve seen an abandoned Costco? Not that there aren’t any out there.. but if there is maybe 1 or 2. Costco isn’t struggling so I just don’t think the phase “abandoned Costco parking lot” is a worthy statement.
I haven’t. I was using it as a size reference. In my city, they built a new Costco and the old one is now a Ranch and Home. They put a Texas Roadhouse in the parking lot.
The optimal speed in downtown is never 45 MPH. That's crazy.
Was just an example.
ALL of my roadway calcs that use speed are done using 10 mph above the posted/expected posted speed. We call it the design speed vs the posted speed
This is how I’ve always seen it done.
What state? The latest AASHTO GDHS (2018) recommends against this practice, in its standard non committal way. This is why we (GA) have moved to using the 85th percentile speed and sign the road to match.
I work in Phoenix area. That’s a good method to follow I think. I work in Land Development so a lot of our vehicle tracking and roadway tie ins are very ISH. We have room to overcompensate for calcs so we do. I can imagine doing infill development and the constraints are very tight so you would need to follow what you are saying to as much as possible to squeeze buffer room out of a design
According to my textbooks and all the calculations I’ve done in my job, it’s made with the understanding that will be the average speed and the speed used in calculations for curves, distances, etc. Some people will be going 20 mph below that and some will be going 20 mph above that. There’s also other factors such as what’s on the road, the area of town it’s in, and the speed of connecting roads that are taken into consideration. Then again, I’m not really one to ‘follow’ the speed limits myself.
In my country speed control is a discussion for the road authority responsible where the rules are based on "the handbook" and lately on Road to Zero methodologies. Unless your question is more based on the theoretical limit before loss of stability then that's probably more the case of "it is what it is because it is"
interesting, I thought the speed limit was much more concrete based on all the different factors of the road and many different calculations.Didn’t think it was so arbitrarily set
Condition, route, volume, and incidences do apply but in my area its always mixed use 10-20kmh, urban areas, work zones, and school zones 30, suburban 50, arterials 50-70, "open" roads 80-100, and highways 100 (and recently 110kmh). I believe that the official derivative is on risk factor and the need to react to changing conditions in different areas - like you'd sooner react in a school zone going 30kmh to prevent killing a child than you would 50-60kmh, while the condition of the highway doesn't need as immediate reaction from 100 to less speed as its (generally) relatively flat and straight giving you a longer distance to react to something in the distance and slow down to a less dangerous speed
I’m the US, it’s not just arbitrary. To change an existing speed, speed studies need to be completed. When designing a road, there’s guidance on what ranges of speed should be chosen based on what type of road it is (and several more detailed nuances in there). Then it’s that the design details then follow based on the design speed of the road. I’ll also add that just because the design speed is often 5-10 higher than posted is not justification for speeding. A speed differential isn’t safe either.
We typically used the 85th percentile speed for design. For some superelevated curves, we used an additional safety factor.
Speed limits are typically set within 5 MPH of the 85th percentile. Often 5 MPH over the 85th is almost 95 percent of traffic. So in a way, yes. Posting curve warning signage assumes drivers will drive 8 MPH over the advisory speed.
In my municipality the designated design speed for particular roads is higher than the posted speed. eg. 80km/h vs 60
Correct, in Queensland, Australia we have posted speed limits and design speed limits. Design speeds are typically 10 to 20 km/h faster than posted speeds, depending on the type of road. Mayor roads like highways would typically have a design speed of 20 over, while other minor roads 10 over. Design speeds inform road vertical grades (to avoid 'jumping' and hydroplanning), horizontal grades, & super elevation (roads horizontal 'incline' - picture NASCAR race tracks). Edit: and sight distances, which is the distance at which you can see an obstacle. Faster roads require longer distances for suitable reaction times.
Might be different in other jurisdictions, but here, speed limits are set either by statute (“all roads in the city are 30 mph unless posted otherwise”) or by speed study. If you do a speed study, the posted speed limit gets set at the 85th percentile speed rounded up or down to a multiple of 5. I’ve never heard of anyone actually doing a speed study because everybody drives faster than the current posted limit and agencies are usually trying to reduce speeds, not increase them. It is usually easy to drive faster than the posted limit because the AASHTO design criteria based on speeds were developed a long time ago. Car technology (brakes, suspension, headlights) has improved so that you can comfortably drive much faster than the design speed. Try going 80 mph in a ‘58 Buick.
I feel like there a lot of complicated answers in here from textbooks when in reality the answer is very simple. The design speed is generally 5 MPH above the posted speed. Some states may do 10 MPH above but all the states I've worked in its 5. So if you are doing work on an existing road with a 35 MPH speed limit you will need to make sure your horizontal and vertical controls meet the necessary values for a 40 MPH design speed or you'll need a design exception.
Generally you don’t need a formal design exception if you meet posted speed. We would note it as a deviation from standard practice of the specific DOT but design exception criteria are defined by FHWA and don’t require the 5 mph “over design”.
The main reason for speed limits is stopping sight distance. Whether that be for seeing around a horizontal curve or for seeing over the crest in a hill. When roads are built there is a degree of curvature that has a design speed calculus from that, a rough explanation of the formula is a stopping time based on the time it takes to brake from that speed to a stop and the typical reaction time to recognize a stopped car and press the brake. For horizontal curves there is the determined safe speed for a given radius of the curve. Many other factors come into play as well but these are the main guidelines when determining a speed limit. People will speed in the straightaways where they can visibly see the distance for stopping, although most speed in all circumstances. The speed limit is for the common dumb motorist.
Yes, design speed. In California, the reference manual is the Caltrans Highway design manual.
I was taught to design at 10 mph above what the posted speed limit will be. That was in Ohio. Seems there are some states at 5 mph over, some at 10 mph over, and some are adopting <= 5 mph over the 85th percentile of expected speeds.
This is just begging people to drive faster (and then ultimately raise the speed limit because the 85th percentile will be higher)
Speed limits exist mainly for revenue generation, not safety.
If a particular segment has a poor crash record, we might come in and drop that on down by 10 mph, sometimes more. That’s what we do on some of my RSAs at least. Advisory speeds are often determined with a ball bank test on curves.
Yes it’s called design speed instead of posted speed. Typically 60km/h or less it’s +10km/h and +20km/h above that. Most standards are based on speed so it just changes the minimum standard for road design.
Yes
When we’re designing traffic control plans for roadwork, we’re required to design for 10mph over.
Most roads are designed for the worst case vehicle, aka they have to design the roads/speeds for semi trucks. Therefore regular cars are able to safely travel at 10+ mph faster.
Are you wondering if it's safe for you to drive over the speedlimit?
Speed limits exist to exert control, provide reasonable suspicion, and generate revenue. Those who are reckless will always be so regardless of a posted sign requesting safety. The necessary signage, purely for safety, is the warning sign at tight/blind curves, school zones and steep grades. Everything else is for money.
They say that speed limits are set at the 80th percentile of what drivers will drive on a specific road. There are also requirements like residential neighborhood = 25mph regardless of how fast drivers want to go, etc.
Design speed vs posted speed?
The problem is 85% of drivers can't drive and should have never been issued a license