Statistics of Speed Limits Aren Cambre EMIS 7370 Fall 2003.

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Presentation transcript:

Statistics of Speed Limits Aren Cambre EMIS 7370 Fall 2003

Agenda Fundamental Concepts Actual Practice Speed and fatalities? That IIHS study

Fundamental Concept #1 Speed variance increases crashes

Solomon (1964 & Cirillo 1968)

West and Dunn (1971)

Other Studies Fildes, Rumbold, and Leening (1991) –Above mean speed = higher crash risk –Below mean = insignificant Harkey, Robertson, and Davis (1990) –Found same U-shaped speed/crash relationship as Solomon and Cirillo

Fundamental Concept #2 Crash severity increases exponentially with speed.

Crash Severity Graph Note how fatality probability declines over time

Fundamental Concept #3 Speed limits have little effect on actual speeds. Urban areas: no effect. Rural areas: modest effect. Most comprehensive studies of 1995 repeal of federal speed limit controls: no significant speed changes

Contemporary Example Environmental Speed Limits in 9- county DFW area: –All 70 and 65 MPH limits reduced by 5 MPH –1 year later Speed checks show average 1.6 MPH speed drop Much of drop attributable to suburban development

Fundamental Concept #4 Traffic enforcement has an effect on speeds and safety.

However! Effect limited to vicinity of enforcement activity. Halo effect. Laws must appear reasonable to everyday drivers.

Setting Speed Limits Speed limit should only criminalize speeds associated with excessive crash risk.

Important Observations Excessive crash risk in top 5%-10% of drivers Speeds are normally distributed 85 th percentile speed = top speed of “flow of traffic”

Texas speed zoning procedure Measure passenger vehicle speeds Determine 85 th percentile speed Unusual crash or roadway problem? Lower speed by up to 7 MPH Lower speed by up to 10 MPH crash roadway Set speed limit TxDOT intends for this path to be taken the vast majority of the time, but that’s not what actually happens. no problem

State Law Speed limits capped 70 MPH in most cases, 75 MPH in particular desolate counties. All speed limits prima facie.

Analysis of Dallas-area statistics Speed limits on highways outside I- 635/I-20 loop are 30 th percentile

If properly set… RoadwayCurrent LimitCorrect Limit I-35E between downtown and MPH70 MPH I-635 west of I-35E 60 MPH75 MPH Ennis 65 MPH80 MPH (?) US-75 in Collin County 65 MPH75 MPH Centennial Blvd. in Richardson 40 MPH45 MPH

Excessive Speed Related Fatalities? Common sense says that speed causes most fatalities. Is it true?

Excessive Speed Related Fatalities? In Dallas County, 88 fatalities in first 4 months of (47%) were “speed-related.”

Dallas Speed-Related Fatalities

Speed was not primary factor in 95% of Dallas-area highway deaths! NHSTA definition of speed-related fatality: –Driving too fast for conditions –Exceeding posted limit Under-posted limits = more “speeders” = more speed-related crashes!

The recent big news

My Own Analysis

Summary Statistics is highly relevant to speed limits. The statistics are surprising.