What is Speed Limit in Highway? 5 Important Points

By November 22, 2020June 1st, 2021Civil Engineering, Highway Engineering
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Imagine you are driving on a road and suddenly you realise that you don’t know the speed limit of that road, what would you do?

That raises an important question, what is the speed limit on the highway? Why there is a speed limit on the highway, how it is specified, who sets it, etc.

Please read this interesting blog for details:

What is Speed Limit in Highway?

Speed Limit simply means the maximum permitted posted speed of any vehicle on a road or highway. What you see on the speed signboard, is the upper limit of the road in Kph or Mph depending on the national speed unit standard.

How to find the Speed Limit of a Road?

Normally National Roads, Motorway and Freeways provide high-speed limits and the posted limits are placed at every entry point of those roads and, also with the repeater speed signposts at regular interval. Generally, the speed limits are quite constant along the whole stretch of that road until a reduced speed is proposed due to some temporary traffic management during ongoing road maintenance or construction.

In the urban street, it is generally recommended to drive at 30 KPH or lower speed, unless the posted signs recommend otherwise.

In the UK, Smart motorways come with Variable Speed Limit Signs (VMS). The VMS signs are the latest speed management systems to control the speed of the motorways in a real-time situation.

What I mean here that these motorways are fully monitored with a CCTV control system and at any section of the motorway, if a road accident takes place, then CCTV control unit reports it and that particular lane is prohibited to use for a very long length and remaining lanes are set for a lower speed limit.

By doing this, the whole situation of traffic jams for a longer duration is minimised and traffic continues to run but at a slower pace.

Who sets the speed limits?

Speed limits are set by national governing laws and applied to all roads of the countries. However, the speed of local roads can be reduced by local authorities to meet local circumstances such as high pedestrianised area, high streets, school zone, hospitals, etc.

When a new road is planned to develop, it’s very important to identify the design and posted speed of the proposed road to meet the local environment. For example, if it is planned to develop a local single carriageway road in a rural environment, how do you set the design and posted speed?

UK DMRB, TD9, recommends identifying the design speed by analysing Layout Constraint (Lc) and Alignment Constraints (Ac) of the local environment and proposed link road.

Please see the link for details.

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Speed Limits of various type of Roads:

Speed limits of one type of road are different from others. It means, speed limits are to consider the type of road as the primary factor:

  • Urban Streets
  • Single Carriageway Road
  • Dual Carriageway Road with Physical Central Median
  • Motorways or Freeways
  • Autobahns

Urban Streets

Urban Streets are primarily designed for a low-speed environment considering pedestrians, cyclist as preferred road users. These streets are designed with wider footways, cycle tracks with narrow carriageways with traffic calming measures.

Generally, the maximum speed on these urban streets is 30Kph or 20Mph, although it varies to meet the local environment needs.

Single Carriageway Road

Single Carriageway Road caters to inbound and outbound traffic through an undivided central median which is delineated by white road markings. Generally, the speed limit on a single carriageway road is 50Kph or 50Mph on compliant sections and lower than that on sharp bends or sharp crest or valleys when a compliant design alignment is not designed and constructed.

Although it is important to note that drivers need to make decisions considering Real-time environments. For example, in adverse weather conditions, low visibility due to darkness or foggy conditions, drivers should drive slower than posted speed as per their driving ability and safety conditions.

Dual Carriageway Road

In dual carriageway Road, two lanes for outbound and two lanes for inbound traffic are provided along with a central reserve made of steel barrier or wire rope barrier or concrete barrier to make uninterrupted traffic flow.

Generally, the speed limit on dual carriageway road is 80Kph or 70mph on compliant section and less than that on non-compliant section based on site constraints such as bends, visibility obstruction, etc. Central Barrier itself become an obstruction along curves and as a result, central median widening is proposed to provide desirable minimum SSD.

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Motorways or Freeways or Autobahns

Motorway normally comes with three lanes inbound and three outbound lanes along with central median made of steel barrier or concrete barrier.  Motorways are the highest category of road in the UK as for as Road Speed is concerned, which means it comes with the highest speed limit which is 70 to 80Mph. In a similar manner, the US provides Freeways and Germany provides Autobahns.

To know more please read the blog below for details:

Is there any Minimum Speed limit in Highway?

Although there are no minimum speed limit criteria in most of the countries but driving too slowly would create a safety risk to other drivers including the one who drives very slowly.

In a high-speed road, if a driver drives far slower than the required speed, then it is not anticipated by other drivers (who would be driving equal to or around the maximum posted speed), result front to back end collision would be quite likely to happen. Countries like the USA, Ireland, Japan provide minimum speed limitation to their roads.

Therefore, it is recommended to drive more or less close to the posted speed limit and consistent with other running vehicles to make an easy and predictable ride for other road users.

Hope this blog gives you enough inside of speed limit, why it is provided, who set the limits, how to set the limits, etc. Please feel free to like, comment and share it.


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