A Transportation Plan for the Greenleaf Terrace Neighborhood

Neighborhood concerns about speeding problems appear to be oriented to those motorists
whose speed is significantly higher—generally more than 10 MPH—over the posted limit. Such
speeds are the main cause of concern, and usually are the ones that create accident potential.
Moreover, speeding problems should generally prevail throughout several blocks of a street or
along several streets of a neighborhood for the problem to be considered for traffic calming
treatments.
The criteria to be used to determine the presence and seriousness of speeding problems is given
in Figure 3. It relates the observed 85 th percentile speed to the posted speed limit. If the speed
studies reveal the 85 th percentile speed to be equal to or less than the posted limit, a tolerable
condition will be found to exist. Actions to reduce the prevailing speed are generally not
considered to be warranted when the relationship between the 85 th percentile speed and the
posted speed limit is below the lower threshold line on the chart.
Figure 3: Criteria for determining presence and severity of speeding problems.
When the 85 th percentile speed exceeds the posted speed limit, it will be concluded that a
speeding problem exists. The problem will be classed as “moderate” if the 85 th percentile/posted
speed limit comparison falls between the two threshold lines on Figure 3 and “serious” when the
comparison exceeds the upper threshold value line. Often, jurisdictions will adjust the posted
speed limit based upon the percentage of motorists traveling (usually 10) mph over the 85 th
percentile speed, based on spot speed study evaluations.
City of Charlottesville
Traffic Calming Device Implementation Guidebook