FHWA Speed Demo

In Cooperation With The Federal Highway Administration And The South Central Safe Community Partnership

SCPDC received a grant from the Federal Highway Administration (FHWA) in September 2002 to conduct speed studies on various roads throughout its six parish region Assumption, Lafourche, St. Charles, St. James, St John the Baptist and Terrebonne. This is one of many such studies conducted throughout the Untied States by the FHWA to demonstrate the use of the 85th Percentile method in setting speed limits. The grant involves collection of data on certain specified streets, analysis and recommendation regarding the 85th percentile, funding to enforce any newly enacted speed limits and provision of education about the 85th Percentile Method.

The 85th Percentile method for establishing speed limits is recognized by AASHTO and incorporated in the Manual on Uniform Traffic Control Devices. This standard is used by the federal government and all 50 States to post proper road signage, signalization and to determine the proper speed limit for different road segments through the 85th Percentile.

The 85th Percentile is the speed at which 85% of drivers feel they can reasonably travel in a safe manner. The rational behind this has been developed through many decades of scientific study regarding motorist behavior and highway design. Most drivers will travel at speeds at which they feel comfortable and safe while still maintaining a travel time that suits their need. Other factors are taken into consideration including surrounding land use that might create congestion, number of cross streets, pedestrian activity, proximity to schools and residential neighborhoods, road surface quality, road geometry and so forth.

As a result of constant testing and analysis throughout the nation, the FHWA has learned that most drivers disregard posted speed limits because they perceive them as being set artificially low through political pressure. Their research has shown that lowering speed limits through means other than the rational method does not have the desired impact of decreasing motorist speed and reducing crashes. Rather, motorists disregard the speed signs and other road advisories because they are perceived as having no rational basis. Law enforcement personnel find it difficult to enforce speed limits that are not based upon the 85th Percentile because the posted speed limit does not distinguish safe from unsafe drivers. Using the 85th percentile method legalizes the behavior of the majority of drivers who are acting in a reasonable and safe manner.

Research by the FHWA demonstrates that the majority of drivers are traveling at a speed within 10 miles per hour of each other. This is called the pace speed and the safest roadways are those where motorist are traveling at a uniform speed. Most crashes occur at speeds that are well above the 85th percentile or far below it. Slower motorists can contribute as much to the crash problem as those who drive excessive speeds far above the 85th percentile. The SCPDC region was chosen as another location to study the impact of the 85th percentile in helping to set safe speed limits and monitor the effect on driver behavior.

At the end of the year the road data was reviewed to determine the 85th percentile. Data collected by SCPDC on State or Federal roads was turned over to the Louisiana Department of Transportation and Development District Traffic Engineer for review and analyses. For local roads, SCPDC contracted with an experienced licensed traffic engineer to provide analysis and recommendations. Many were found to already be set at the 85th percentile a credit to our region’s state and local traffic engineers. Several streets were also dropped because of changes in road conditions that made further study for the purpose of the demonstration grant less desirable. This includes areas for which construction is now planned or such similar reasons. Other roads had to be dropped because although the 85th Percentile exceeded the posted speed limit, there was a statutory limit on the maximum speed that could be posted. And, in still other cases, the traffic engineers working with SCPDC on the study recommended speed limits not be changed due to road surface, geometric, or other conditions.

If speed limits are approved for change, the road segment will enter the next phase of the study. For one year after new speed limits are posted, the SCPDC will continue to collect data and coordinate review of it with the traffic engineers. Area law enforcement agencies will be working with SCPDC in this phase of the program by strictly enforcing the new posted speed limits. SCPDC will also be working throughout the region to educate and inform the public about the Federal Highway Administration Program and how the 85th Percentile works.

FINAL STREET LIST: A Total Of 13 Streets

Local Streets:

  • St. Charles Parish
  • River Road in Norco between the Bonnet Carre Spillway and Apple Street
  • Terrebonne Parish
  • Bayou Gardens Boulevard Map
  • Corporate Drive between Hollywood Road and Martin Luther King Boulevard (south side) Map
  • East Woodland Ranch Road Map
  • Industrial Boulevard Map
  • St. Louis Canal Road Map
  • South Hollywood Road between Valhi Boulevard and Martin Luther King Boulevard Map

State Highways

Assumption Parish

  • La. Hwy. 662 (Bayou L�Ourse Hwy.) between .3 miles south of La. Hwy. 663 and .35 miles north of La. Hwy. 662 Map

Lafourche Parish

  • La. Hwy. 1 between Bowie Road and St. Charles Bypass Map

St. Charles Parish

  • La. Hwy. 48 (River Road) between Apple Street and 1.37 miles south east of Apple Map

Terrebonne Parish

  • La. Hwy. 661 (South Van Avenue) between Laban St. and the Houma Navigation Canal Bridge Map
  • La. Hwy. 311 between Lafayette Street and Hollywood Road Map
  • La. Hwy. 57 (Grand Caillou Road) between Bayou Terrebonne and La. Hwy. 3040 (East Tunnel Boulevard) Map
  • La. Hwy. 24 between 1 mile north of Gray and La. Hwy. 3185 Map

Final Street List

FHWA Speed Management Website

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