Speed Zone Study 85^th Percentile Speed

Speed Zone Study - To reduce the speed limit below the maximum speed limit law 22349 vc, a speed zone study must be conducted, and the speed limit must be set according to the 85^th percentile speed.

Calculating the 85th Percentile Speed or Critical Speed

To properly calculate the 85^th percentile speed a data sample of at least 100 vehicles is needed. The data for the 100 vehicle speeds must be plotted in such a way so that the 85^th percentile speed can be determined by looking at the speed of the 15th vehicle down from the top speed.  Fifteen percent of the vehicles in the sample data are traveling faster than this speed, and eighty five percent are traveling at or below this speed.

Examples for calculating the 85^th percentile speed

Example 1:

• The speed of 100 vehicles was measured.
• 0.85 X 100 = 85
• Therefore, 85 vehicles represent the 85^th percentile speed. 15 vehicles were driving in excess of the 85^th percentile speed.
• If 85 vehicles were driving at speeds below 31 mph, then the most appropriate speed limit would be 30 mph or 25 mph depending on conditions not readily apparent.

Example 2:

• The speed of 60 vehicles was measured.
• 0.85 x 60 = 51

• Therefore, 51 vehicles represent the 85th percentile speed. 9 vehicles were driving in excess of the 85^th percentile speed.

• If 51 vehicles were driving at speeds below 48 mph, then the most appropriate speed limit would be 50 mph or 45 mph depending on conditions not readily apparent.

Accident Data Analysis for Setting Speed Limit

Accident data is analyzed in setting speed limit on roadways in collisions per million vehicle miles (acc/mvm) and average acc/mvm for the most recent three years (if available) or the most recent data available for the most up to date roadway reconfiguration. 

Higher than average accident rates should be investigated to determine how many of the accidents are related to speed. Speeding related accidents should be investigated to determine if California Vehicle Code Section 22350 Basic Speed Law, also referred to as excessive speed for conditions, is the cited section.  Speeding ticket for violation of Section 22350 vc does not necessarily indicate that the posted speed limit was violated. When reviewing accident date it is important to see if any of the accidents occurred at intersections and in stop and go traffic at speeds much lower than the posted limit.

Relying on accident rate to justify a change of speed limit requires an analysis showing that the proposed speed change would reduce the accident rate or severity. Accident rate should not be used as justification to lower the speed limit an additional 5 mph if the speed limit has already been lowered 5 mph for conditions not readily apparent. Other appropriate counter measures should also be considered to reduce collision rates. Therefore if the speed limit was reduced an additional 5 mph that could indicate an error in setting the speed limit too low and issuing unfair speeding tickets.

Existing Conditions Analysis for Setting Speed Limit

The limits of the speed zone study segment should be reviewed in relation to the 85th percentile speed to determine local land uses, traffic control devices (ie, traffic signals, stop sign etc), number of traffic lanes, pavement markings and striping, existing posted speed limit signs. Traffic flow should be evaluated to determine areas of where there may be conflicts, high pedestrian/bicycle movements and other driver behaviors.

Conditions not readily apparent to the driver is a situation where, if the drivers were aware, they would adjust the speed of the vehicle accordingly.  Drivers are aware of the street width, curvature, grade, and surface conditions. California Vehicle Code Section 22358.5 vc prohibits lowering the speed limit for conditions readily apparent to the driver. Special events, construction, or congestion are also apparent when present. The basic speed law Section 22350 vc is sufficient regulation for such conditions.  If readily apparent conditions are of great concern, advisory speed signs may be posted for specific roadway conditions.

Non-apparent conditions include those high volume traffic generators (vehicular, bicycle or pedestrian) not visible and access points that are not visible to the driver. Warning signs can be placed for such conditions however, this is not always adequate.  Every attempt should be made to make the driver aware of non-apparent conditions before lowering the speed limit to levels below the 85th percentile speed.  Non-apparent conditions to the driver should not be the reason to lower the speed limit an additional 5 mph if the speed limit is also being lowered for a higher than average accident rate.

In California to use electronic speed detection devices such as radar or lidar for speed enforcement, a valid speed zone survey is required. In most cases the violation code 22350 vc is used for roadways where the speed limit has been reduced below the maximum speed limit. If the speed zone survey is not valid or it does not follow the requirements of the California Vehicle Code, then a speeding ticket for 22350 vc can be contested. If you have a speeding ticket and if you would like to fight your speeding ticket without going to court using a trial by written declaration, we can help. Signup for our service and take advantage of our 100% money back guarantee. We provide trial by written declaration samples and examples to fight speeding tickets.