1. Adjusting Speed Limits
Jon Cheney, P.E.
July 7, 2016

2. Adjusting Speed Limits
Agenda:
What Influences Driver Speed?
Florida Statute (2)
Florida DOT Criteria
How VCTE Handles Speed Request
Speed Study Example
Staff Recommendations
Questions

3. What Influences Driver Speed?
Time of day/purpose of trip
Pavement wetness
Ambient light
Weather
Familiarity of driver with the road
Type of vehicle
Vehicle parking
Condition of vehicle
Road width
Urgency of trip
Lane width
Emotional condition of driver
Traffic volume
Driver skill
Adjacent land use and development
Personality of driver
Speed of other vehicles
Pavement roughness
Driver - late or on time
Shoulder width and condition
Presence and/or history of enforcement
Pavement type and condition
Road geometrics
Form
Road geometrics   
Road width
Lane width
On street parking
Adjacent land use/development
Length of trip
Activity
Traffic volume (Including presence of pedestrians & Bicycles)
Florida Department of Transportation

4. Adjusting Speed Limits
Florida Statute (2)
SPEED ON COUNTY ROADS
After an investigation determines:
- change is reasonable AND
- conforms to Florida DOT criteria, then
Speed may be set by the County Council

5. Adjusting Speed Limits
Florida DOT Criteria
Found in Speed Zoning for Highways,
Roads and Streets in Florida, 2010
Investigation determines:
- 85th percentile of speed
- 10 MPH pace speed
May adjust +/- 3 MPH in investigation
Adjusting -4 to -8 MPH requires supplemental investigation into:
- Roadway Length - Traffic Volume Alignment - Crash Experience Roadway Width - Sight Distance Surface Condition - Design Speed Development Speed in Curves

6. Adjusting Speed Limits
HOW VCTE HANDLES SPEED REQUEST
Receive Request from Variety of Sources:
Residents
Businesses
Government staff (i.e., city/county/state)
Elected officials (i.e., city/county/state/federal)
Screen Each Individual Requests
No further action-document request, when:  Last speed study performed along roadway w/in past several years  Post speed is same as roadway design speed
If law enforcement issue or cut through traffic problem:  Will use radar feedback sign  Will refer them to local law enforcement agency
Will perform full Speed Study after:  Field review of roadside development & roadway alignment  Review crash history

7. Adjusting Speed Limits - Example
Residential Local Street
¼ MILE LENGTH
Posted Speed – 30 MPH
FIELD REVIEW
Roadway Alignment: - 2 horizontal curves
Roadside Development - Driveway spacing - Parked cars adjacent to road

8. 100% Adjusting Speed Limits - Example FIELD STUDY
Frequency Distribution of Spot Speeds
100%
Participation by every user driving on the road over 48-hour period
Existing Speed Limit – 30 MPH
Accident rate is 3x normal rate for this type road
NUMBER of VEHICLES
VEHICLE SPEED (MPH)

9. Adjusting Speed Limits - Example Frequency Distribution of Spot Speeds
FDOT Criteria
Investigation determines:
- 10 MPH pace speed
Frequency Distribution of Spot Speeds
10 MPH Pace: MPH
Existing Speed Limit – 30 MPH
Accident rate is 3x normal rate for this type road
NUMBER of VEHICLES
VEHICLE SPEED (MPH)

10. Adjusting Speed Limits - Example
FDOT Criteria
Investigation determines:
- 85th percentile of speed
CONVERT
Frequency Distribution of Spot Speeds TO
Cumulative Speed Distribution
DETERMINE
85th Percentile of Speed
85th %tile Speed: 27.1 MPH
CUMULATIVE % FREQUENCY
Existing Speed Limit – 30 MPH
VEHICLE SPEED (MPH)
Cumulative Speed Distribution

11. Adjusting Speed Limits - Example Cumulative Speed Distribution
FDOT Criteria
May adjust +/- 3 MPH
85th %tile Speed: 27.1 MPH
Upper 10 MPH Pace: 25 MPH
+/- 3 MPH: MPH
CUMULATIVE % FREQUENCY
+/- 3 MPH: MPH
Existing Speed Limit – 30 MPH
VEHICLE SPEED (MPH)
Cumulative Speed Distribution

12. Adjusting Speed Limits - Example Cumulative Speed Distribution
FDOT Criteria
May adjust +/- 3 MPH
Potential Speed 20 – 30 MPH
+/- 3 MPH: MPH
CUMULATIVE % FREQUENCY
+/- 3 MPH: MPH
Existing Speed Limit – 30 MPH
VEHICLE SPEED (MPH)
Cumulative Speed Distribution

13. Adjusting Speed Limits - Example Cumulative Speed Distribution
FDOT Criteria
May adjust +/- 3 MPH
Potential Speed 20 – 30 MPH
Study recommends adjusting speed from 30 mph to 25 mph
CUMULATIVE % FREQUENCY
Existing Speed Limit – 30 MPH
85th %tile Speed MPH
VEHICLE SPEED (MPH)
Cumulative Speed Distribution

14. Adjusting Speed Limits
RECOMMENDATIONS:
Going to County Council as extra precaution
Biannually
Consent agenda item
Adjusting speeds on ALL road types: - Arterials - Collectors - Local residential streets

15. QUESTIONS?