Do signs printed on the road offer a significant advantage for the user over signs on a post?

Question

I was driving earlier today and I came across a stretch of road that didn't have signs for the speed limit but instead had it written on the road like so:

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It got me thinking that it seems like it would make sense to do this all the time as you're constantly looking at the road (hopefully) while driving. It would be something similar to how turn lanes are done.

Image source

Question(s)

Is there a particular reason the speed limits aren't written on the road? Does it give a better user experience to have them all on signs?

Answer 1

I would say there are a couple of aspects here

1. Line of sight : Though while driving your line of sight is mainly on the road, the main point of focus is at object ahead of you straight ahead (e.g. a vehicle going ahead). Hence writing the speed signs on the road would have to require the person driving to focus down and assimilate the information which would deviate them from their focus. However having signs would allow the user to quickly scan the content as they goes past it while not getting too distracted.

2. Speed : The example you gave above would work if the user was driving at a low speed say 25 MPH as that gives them more time to react. However if they were driving on the highway in a 60 MPH zone, the reaction time take to read the text on the road and also keep track of the vehicles in front of them would be less and hence there is potential scope of accidents. However in the case of a sign, since its at the eye level of the user and within their line of sight they can quickly scan the content without deviating focus from the road.

3. Eye level : The signs are raised and hence a better eye level as compared to the text on the road which requires the user to focus down. Hence a quick glance would enable them to read the information. That said, there are strict guidelines on the positioning of signs to ensure the user doesnt have to scan too far to read the sign as shown in this article

The radar speed sign should be installed no more than 5 feet from the road curb. At more than 6 feet from the road, the sign will take the eyes of the driver an unsafe distance from the roadway.

Answer 2

Here in Finland the main reason is this:

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Road markings are used to denote speed limits but never as a primary mean. And as Jung Lee points out, re-applying road paint is labor intensive, especially here, as studded tyres usually erode most of the paint in one or two winters.

Edit: The Finnish law actually states that road markings, such as the one for speed limit, are used to amplify or clarify the effect of traffic signs. Not to be used on their own.

And if I remember correctly, the law states that even though traffic signs and / or road markings are obscured, you can be penalized for breaking the law. That is because you are assumed to be aware of for example how fast you can drive. Being a tourist is not an excuse.

Answer 3

Do signs printed on the road offer a significant advantage for the user over signs on a post?

Let's make a matrix:

-------------------------------------------------------------------------------
                                 |  On Road          |  On Sign                
---------------------------------|-------------------|-------------------------
   readable with snow            |  no               |  yes
   readable with heavy traffic   |  no               |  usually
   readable at high speeds       |  maybe?           |  yes
   read normally (top to bottom) |  sometimes        |  yes
   reflective at night           |  somewhat         |  yes
   resilience to wear            |  at mercy of road |  quite resilient
   cost to manufacture           |  lower (paint)    |  higher (metal, concrete)
   interferes with traffic 
   during install?               |  usually          |  not usually
   at normal sight lines         |  arguably         |  yes
   motorcycle hazard             |  high             |  requires good aim
-------------------------------------------------------------------------------

Granted, none of the above is scientific. Merely a quick heuristic evaluation. But, in general, I don't see strong arguments for on-road signage having a significant advantage. It may very well be a nice addition to signs, but I don't see it as being a huge advantage.

The one exception that comes to mind is: Turn lanes. Signage is usually good for this, but sometimes that's just not practical (due to them usually having to be installed overhead). And since intersections are typically where people (at least) should slow down a bit and be paying attention, combined with the fact that turn lane markings are usually simple iconic arrows, they seem to work well.

ADDENDUM:

Now that I've spend time driving around some cities I was unfamiliar with, I rescind my comment that turn lanes work well when painted on roads. They work horribly. By the time you spot the lane restrictions, I'm usually already in the wrong lane and due to traffic creeping to a stop, I have no room or time to switch. So, I do believe they are still done this way because they are much cheaper, but I would not argue they work well.

Answer 4

Besides the snow-covered roads—a very clear reason but only applicable in a small number of cases—there are several vision-related explanations.

The first one is simply geometrical. On an empty road, a traffic sign remains for a long time close to the centre of the driver's field of view (where vision is sharpest) and can be seen from far away. It gets off the centre but at the same time bigger and more clear as the car approaches. It can even be well visible at a slightly curved road. A painted sign would only be visible when you are close to it, giving you less time to react.

Another reason is optical. Although a road pavement is rough, at grazing incidence (i.e. when you are looking almost parallel to its surface) it becomes highly reflective (you can experiment at home by holding a dark-covered book perpendicular to the window and looking parallel to the book). Light from the sky is therefore well reflected by the road surface, making the painting practically invisible. The effect is even worse if the road is wet.

Then, imagine a situation when information to be provided is more than just one number of the speed limit. Multiple signs or an extended text can be easily mounted on a single pole and would occupy a relatively small area. The same information positioned on the road surface would require 1/sin(a) more space, a being the angle between the road and the line of sight. It makes the painted signs huge and increases the probability that at least part of it will be obscured by other cars. Therefore large signs need to be positioned on the poles. And since some of the signs are already there, drivers are used to that position, making it optimal for smaller signs too.

Finally, traffic signs are more effective with headlights. A traffic sign is positioned almost perpendicular to the headlight beam and thus optimally reflects light back to the driver. To the contrary, the road surface is oriented almost parallel to the headlight beam and reflects back very little light. A simple paint therefore would be barely visible at night. One could use a retroreflecting paint, but it only became available recently and is rather expensive to be constantly replaced.

Answer 5

In addition to the ones mentioned by others, here are additional considerations:

1. Road paint is more labor intensive to install initially.

2. Road paint is more labor intensive to maintain as well because road needs to be repaved every few years and/or potholes filled.

3. But more importantly, road paint is slippery in wet weather. (well known concerns among bikers) It's one thing to have road paint as lane dividers. But it's another thing to have it painted in the middle of road that has high speed limit and traffic volumne.

Answer 6

Couple of reasons:

1. When driving there are often cars ahead, so anything written on the road can be obscured. The same is true with street signs, but it's not nearly as bad.
2. Drivers look forward and straight. Signs are usually off to either side of the road, where both a driver and passenger can see. Thus the signs are a little easier to view for all passengers, front or back.
3. Text on the signs isn't stretched or deformed, making it easier to read. On the road, text is stretched so it's easier to read at speed and from an angle. However, that means there's an optimal speed and angle to read it, and thus an unlimited number of less than optimal viewing angles and distances. Not so for road signs (at least it's not nearly as different from day-to-day sign reading, because it's only distance based).

Answer 7

Extreme Environmental Conditions

Similar to conditions in Finland, as shown by @locationunknown, locations in India suffer problems as well. Where monsoon damages roads every year. Imagine speed limits and other markings on road would never be seen on these roads.

1. 

2. 

Other constraints here in India, why speed limits are not on road is the sheer environmental conditions. Heat, Rain etc posing challenges all year round.

Answer 8

Also, information printed on the road can only be read when there is little traffic, or at least no cars blocking it (or other obstacles such as weather as already previously stated). In rush hour, for example, your speed limit on the road would likely be missed by everyone. The same would happen even if you were driving at the speed limit and there was a car a reasonable distance in front of you.

Answer 9

Where I live in Australia (a place very rarely affected by snow cover), speed limits are painted on the roads only when they change:

^{Image from Teach Yourself Driving in Australia}

Regular speed limit signs are also posted both at the point where the limit changes and many times along a roadway as a simple reminder or for commuters that have joined a roadway from a side street.

The advantage of this approach is that the speed limit painted on the road is a sign that you need to modify your driving in some way (consider people with cruise control enabled, for instance).

This also makes it relatively cheap to change the speed limit in an area. Australia is very spread out and so we have very long roads that are often a single lane each way; changing the speed limits for those roads would generally require only diverting traffic for a short time in one place each direction, and then you can change the signs beside the road more methodically along the way without having to stop traffic at all.

Fixed road signs do have their own advantages, though. For instance, they provide the benefit of being variable in different weather or traffic conditions:

^{Image from Teach Yourself Driving in Australia}

One other consideration is that this means maintaining two separate systems, which can lead to contradictory information being posted:

^{Image from KatieKat}

Answer 10

A more technological solution would be for the speed limit to appear as part of a heads up display for the driver. This could enable the sign to be given extra emphasis when the speed limit is being approached or broken. More simply the car could just be fitted with a speed limiter that adjusts to the local restriction so that no speed limit signs would be needed at all. Mind you, when we're in driverless cars we won't need any of this...

Answer 11

Apparently (I have been unable to locate the research), A UK study showed that in slow, dense traffic drivers would spend more time looking at the road surface directly ahead of their vehicle but in light, fast-moving traffic would spend more time looking further ahead. In the first case this would offer a lower view and the in the second a higher view. As a result of this study, in areas that were prone to 'shunt' accidents (running into the back of another car), more signs were painted on the road surface and in areas that were prone to 'drift' (running into the side of another car, out of lane or off the road) accidents, more signs were used above or to the side of the road. As far as I can remember (again, I can't find the research right now) this reduced the number of accidents in both cases whereas trying more signage in 'shunt' spots and more road markings in 'drift' spots made no difference.

Answer 12

In Phoenix, we don't have the MPH written on the road in our highways, but we do have the highway that this is an exit-only lane for written on the road. For example, you're travelling on the 202W, and there's an exit for the 101N coming up. Before the exit, there's an on-ramp from another street and after that on-ramp (and before the exit) there's an off-ramp for a different street.

Of course, there are signs above with yellow sections and black arrows pointing down at which lanes are for what--which is an exit-only for the new off-ramp, which are exit-only for the 101N, and which will allow you to continue on the 202W. I think it's safe to say there's a lot going on here.

In addition to this signage, there's also writing in the lane that says:

101 N

or

202 W

as you drive along, so it's clear that you're in the proper (or improper!) lane. This is much more clear, and I think that of all the use cases for writing on the highway, this is probably one of the most useful.

I've tried to find a picture of it, but apparently all of us here in Phoenix take it for granted! :)

Next time I'm driving on the highway as a passenger I'll try to snap a picture of it for you to see.

Anyway, it doesn't have to do with speed limits, but it does have to do with a different kind of signage so I thought it might be interesting to chip in.