Vehicle speed limits

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Speed limits are used in most countries as maximum or minimum value of speed that allowed to appear in a road. In most case, speed limit is the maximum. Most jurisdictions use the metric speed unit of kilometers per hour or miles per hour for speed limits. The value of speed limits may be variable and in some places it may not be installed. The 85th-percentile speed is commonly used as the speed limit on a road. In general, when the purpose of the study is to establish posted speed limits, to observe speed trends, or to collect basic data, it is recommended that the study be conducted when traffic is free-flowing, usually during off-peak hours.



Conceptual Example

The fundamental aim of speed limit is improve safety.

We install speed limit to make sure that the vehicles running on the road neither too fast nor too slow. The different between fast car and slow car can be very great since people may have no conception about speed without speed limit and drive with the speed they prefer. In other words, the variety of speed is high which will cause more crashes. To avoid this situation, speed limit is set up to keep the speed within an acceptable range.

Without speed limit, some drivers may drive too fast to stop immediately when there are some emergencies, such as people or animals crossing the road, vehicle performance problems, severe weather and so on.

Specific Example

Maximum speed limits are usually set to inform drivers of the highest speed that it is safe and appropriate for ideal traffic, road and weather conditions. There are many previous studies show that relaxing speed limits may cause negatively affect safety, especially with regard to fatal and injury crashes.

In 2013, the British Columbia Ministry of Transportation and Infrastructure initiated a speed limits review. The review found that the 85th percentile speed on many highway segments was 10 km/h higher than corresponding posted speed limits and 1300 km of rural provincial highway segments were recommended for higher speed limits. Most of the highway segments had 10 km/h speed limit increase with a small section having 20 km/h speed limit increase. Changing speed limits led to a statistically significant increase in fatal-plus-injury (severe) crashes of 11.1%.



Tradeoffs of implementing this policy may include:

  1. Decline the number of crash since speed limit control the variabilities of speed.
  2. Give drivers a caution about how fast they can drive.
  3. Minimum speed limits make some people that prefer low speed feel nervous.
  4. Speed limits can cause negative effects to the efficiency of the road since it impose restrictions on traffic flow.
  5. Give dirvers more opportunity to make correct decision to figure out emergencies.
  6. Assure the pedestrian’s security.
Compatibility Assessment

Compatibility Assessment.png

If answered yes, the following questions indicate conditions under which the policy may be most effectively implemented:

  1. Are there roadways with significant amounts of traffic that are not subject to a speed limit?
  2. Is the lack of specific speed limits resulting in a wide variation vehicle speeds?
  3. Is there a significant demand among pedestrians to cross the road?
  4. Is the capacity of the road lower than the demand?
  5. Is the variation in unregulated roads creating safety issues and resulting in accidents?


Assuming that a jurisdiction has decided to adopt the policy, the following questions will need to be answered when determining how to implement this policy:

  1. Do a road need both maximum speed limits and minimum speed limits?
    1. If attempting to improve the efficiency and safety, both of two need to be taken into concerned.
    2. The range between maximum speed limits and minimum speed limits can not be too large or too small.
  2. What factors will impact speed limits?
    1. The condition of the road.
    2. The traffic flow.
    3. The demand of user.
  3. Under what conditions will speed limits be altered?
    1. Most of the vehicle can not meet the requirement of the speed limit.
    2. The traffic condition on the road is unobstructed in normal case.
  4. What will the penalty be for drivers who fail to adhere to the speed limit(s)?
    1. Warning and fine is necessary.
    2. If the drivers make serious adverse event, they may need detention.
  5. How will the speed limit(s) be enforced?
    1. Using marked sign warns drivers what the limit is.
    2. Institution of punitory regulations



  • Notable entities who have implemented or adopted this policy include:
    • City of Northern Territory: The Northern Territory had no blanket speed limits outside major towns until January 2007 when a general rural speed limit of 110 km/h (68 mph) was introduced although four major highways had higher 130 km/h (81 mph) zones. [1]
    • Isle of Man: The Isle of Man has a general speed limits 60 mph and of 70 mph on Mountain Road for safety reasons were not progressed following consultation. Measured travel speeds on the island are relatively low. [2]
    • Country of Switzerland : Switzerland  imposed a temporary 100 km/h (62 mph) maximum limit in quick response to higher fuel prices; the limit on motorways was increased to 130 km/h (81 mph) later in 1974. [3]
    • State of South Dakota: Annual surveys of speed on South Dakota Interstate roads show that from 2000 to 2011, the average speed rose from 71 to 74 mph; South Dakota increased its maximum speed limit from 65 to 75 mph (120 km/h) in 1996. [4]




  • Advocates - Automobile Safety. Assumption: Set up speed limit control vehcles’ speed so that it can improve the automobile safety.
  • Constituent Groups - Seniors. Assumption: Seniors may need more time when they are crossing the road. Speed limit controls the speed of vehicles and gives them more protection.
  • Advocates - Sober Driving. Assumption: Since car will never running at very high speed, speed limit can relieve grave incidents.
  • Labor Unions - Truck Drivers. Assumption: Heavy vehicles are easy to lose control and the speed limits give drivers warning. Therefore, speed limits decrease the possibility of crash.


  • Labor Unions - Taxi Drivers. Assumption: With the speed limit, driver can not driver too fast even if there is no car. Therefore, taxi drivers may feel it is a waste of time.
  • Labor Unions - Construction Workers. Assumption: Speed limit may decrease the number of people driving cars and the demand of parking. Therefore, the job opportunity will be reduced.
  • Constituent Groups - Local Businesses. Assumption: Speed limit may negatively impact of efficiency and cause needless delay. In this cause, local businesses will be damaged.
  • Constituent Groups - Tourists. Assumption: Tourists may prefer lower speed since they are not familiar with the local road. The minimum speed limit may cause pressure on them.







When it’s all horizontal and drifts, it kills the visibility, and we have a horrible time trying to keep people on the road. [5]

Driver at the 50% percentile has a greater crash risk than the 85th percentile driver. Below the 30th percentile crash risk is significantly increased and these speeds tend to be used by less skilled and competent drivers. [6]

Those who are against the motorist and will do all they can to constrain him, and those who oppose constraints unless they are relatively minor in their effects. [7]

Initial results of a Transfund Research project being undertaken to evaluate the effectiveness of Variable Message Speed Signs (VMSS) within the Ngauranga Active Traffic Management System ( NATMS). [8]

  1. [1].
  2. [2].
  4. [3].
  5. Land Line Magazine. October 15, 2010. Retrieved 2010-10-16. Available at: [4].
  6. Safe Speed. Retrieved 2010-04-17. Available at:[].
  7. Criminal on the road. Tavistock Publications. 1964. p. 102. ISBN 978-0-415-26416-7. Retrieved 2010-04-27. Available at:[].
  8. Andrew W Fergus and David J Turner (MWH NZ Ltd) (22–25 September 2002). Available at:[].
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