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

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.

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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

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 him from his focus. However having signs would allow the user to quickly scan the content as he 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 him more time to react. However if he was 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 him 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 his line of sight he 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 him 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.