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Explanation of Speed Surveys

A speed survey is the tool we use to determine what the speed limit should be for a particular section of roadway. Speed limits provide notice to drivers of the maximum speed limit on a roadway and are an enforcement tool to assist police in separating violators from the reasonable majority. Roads are typically split into sections that have similar physical attributes. Please note that not all roadways are required to have speed surveys done to determine the speed limit.  For information on the setting of speed limits click here.


This explanation discusses items in the order they appear in on the newer type of speed survey form we use, but the majority of the terms and elements are present on all speed surveys we have used in the past.

Description of Roadway

Surveys include a description of the physical attributes of the roadway including what roadside development is present. The number of lanes, length, and road curvature is also discussed in this section.  (Our newer surveys also include a map of the particular road segment.)


Prevailing Speeds

This is the most important part of the survey and data from this section is the major factor in determining the posted speed limit. 

The most important number is the called either the critical speed or the 85th percentile speed. This speed (we refer to it as the critical speed) is the speed at which 85 percent of the vehicles sampled travel at or below. For example if we sampled 100 vehicles and the critical speed was 37 mph than 85 vehicles were traveling at or below 37 mph.

Other numbers given include the 50th percentile speed or median speed. This is the speed at which 50 percent of the vehicles sampled are traveling at or below. 

The 10 mph pace speed is also listed. This is the 10 mph range in which the highest percent of the vehicles are going. For example if 100 cars are sampled and the slowest car was traveling 20 mph and the fastest 50 mph but 80 cars were traveling between 31 mph and 40 mph the pace would be 31 mph to 40 mph with a percent in pace of 80 percent. 


Traffic Volume

Data is given for the average daily traffic (ADT) for this particular segment on an average day. 


Collison Records

Collision records are gone through for a two year period to determine a collision rate. The collision rate is calculated in million vehicle miles as typically collision rates are low enough that measuring them in this scale is more manageable.  A segment’s collision rate is then compared to an average of similar roads in Pleasanton.


Highway, Traffic and Road Conditions That Are Not Readily Apparent to the Drive

This section discusses elements a driver may not realize about the segment being traveled.  These elements include horizontal and vertical curves, higher than average collision rates, pedestrian patterns and facilities available, nearby pedestrian generators including schools or parks, nearby senior housing or facilities, any hidden driveways, etc.


Recommended Speed Limit

The recommended speed limit (this is the posted speed limit) is given near the end of the newer speed surveys and in the middle of the older type.  This limit is obtained through the examination of the critical speed (85th percentile speed).  State and Federal law requires that the speed limit is set at the 5 mph increment closest to the critical speed. 

A 5 mph speed limit reduction is only allowed at locations with conditions not readily apparent to the driver.

Here are two examples of this process:

If a critical speed is 32 mph the law requires that the speed limit is set at 30 mph.  If there were unusual conditions not readily apparent to the driver that warranted a 5 mph reduction in the speed limit, the speed limit may be legally reduced to 25 mph.

If a critical speed was 33 mph the law requires that the speed limit is set at 35 mph.  If there were unusual conditions not readily apparent to the driver that warranted a 5 mph reduction in the speed limit, the speed limit may be legally reduced to 30 mph.

Citizens frequently ask why we don't install lower speed limit signs to control speed.

Speed limits in most areas of town are determined by an engineering and traffic study that establishes speed limits based on the speed of the vehicles on the roadway. Most drivers drive at a speed that they consider comfortable, regardless of the posted speed limit. Historically “before and after” studies have shown that there are no significant changes in average vehicle speeds following the posting of new or revised speed limits. Furthermore, research has found no direct relationship between posted speed limits and collision frequency.

If posted speed limits are set artificially low, disrespect is created for the limits, and since they are not set in accordance with the law, they are not enforceable by law. The state maximum speed limit would then apply. The occasional driver attempting to obey the posted speed limit may decrease safety by causing additional lane changes, passing and tailgating.

Setting speed limits near the prevailing speed of traffic results in a greater percentage of traffic flowing at uniform speeds. Correctly set speed limits can increase safety and lead to fewer collisions.

If you have further questions about a speed survey please call us at (925) 931-5677.




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