Signalization and Safety - Teach America Corporation · Signalization and Safety A Study of the...

Signalization and Safety

A Study of the Safety Effects of SignalizingIntersections on Colorado State Highways

Presentation by Richard G. Sarchet, P.E.

of the Colorado Department of Transportation

at the TRB National Roundabout Conference

in Vail, Colorado

May 2005

PWhy do we Install Traffic Signals?< Because They Reduce Delay? < Because The Location Meets Warrants?< Because They Improve Safety?

PWhy do we Install Traffic Signals?< Because They Reduce Delay?

– Replacing a 2-way stop with a Signal generally reducesdelay on the minor road.

– Replacing a 2-way stop with a Signal almost alwaysincreases total delay.– At certain side road volumes 2-way stop fails. Signal causes less

delay than all-way stop.– Delay at Signals is distributed more equitably than at 2-way stops.

– Delay at a Roundabout is usually less than at a Signal .

PWhy do we Install Traffic Signals?< Because They Reduce Delay?...Sometimes

PWhy do we Install Traffic Signals?< Because The Location Meets Warrants?

– Every New Signal Studied was “Warranted”– Engineers, Politicians, Press and Public Fret Over

Planned Signals that are “Warranted but Unfunded”– MUTCD Says:

– “The satisfaction of a traffic signal warrant or warrantsshall not in itself require the installation of a traffic signal”

– and “A traffic control signal should not be installed unlessan engineering study indicates that installing a trafficcontrol signal will improve the overall safety and/oroperation of the intersection.”

PWhy do we Install Traffic Signals?< Because The Location Meets Warrants?...Maybe

PWhy do we Install Traffic Signals?< Because They Improve Safety?

– ITE Traffic Engineering Handbook says, “TrafficEngineers know that a traffic signal is not a panaceaand can actually contribute to collisions, congestion,delay, and speeding.”

– Thomas and Smith of Iowa State University foundrear end and “left turn” accidents increase with newsignals, but overall crashes decrease slightly. (2001)

– Voss of Kansas DOT found that new signals shouldbe assigned an ARF of 45%. (1997)

PWhy do we Install Traffic Signals?< Because They Improve Safety?

Do Traffic Signals Improve Safety?

PStudy Locations on Colorado Highways< Intersections that became signalized< Where data is available

PCompare 3 years Before and 3 years After< Consider traffic volume growth

Signalization and SafetyA Study of the Safety Effects of SignalizingIntersections on Colorado State Highways

PUsed Video Log to Identify Locations< Found 112 Locations w/ Signals in 2002, Not in 1992< Which to Use for Sample?

PALL OF THEM!

Site Selection

PData from CDOT Accident History DatabaseAnalysis

All Accidents

Broadside Accidents

Rear End Accidents

Approach Turn (Left Turn)Accidents

Property Damage Accidents

Fatal Accidents

Injury Accidents

PThese Decreased in the After Period< Fatal Crashes< Persons Killed< School Aged Pedestrian Struck< Broadside< Overtaking Turn< Bicycle Struck< Dark, Not Lighted< Motor Home (At Fault)< Motorcycle (At Fault)< Driver Emotionally Upset< Driver Evading Law Enforcement< Driver Physically Disabled< Driver Under Influence of Alcohol and Drugs

Analysis

PThese Were Unchanged in the After Period< Bicycle (At Fault)< Driver Under Influence of Illegal Drugs

PAll Others Were Increased (49 Attributes)

Analysis

P95% Confidence Intervals Were Constructed< These DECREASED by an Amount Significantly Different Than Zero

– Broadside– Overtaking Turn

Analysis

P95% Confidence Intervals Were Constructed< 31 Attributes Were INCREASED by an Amount Significantly Different

Than Zero.

< 9 Attributes INCREASED Significantly More Than 50%

– Property Damage Only Crashes– Crashes Involving 3 or More Vehicles– Crashes on the Roadway– Rear Ends– Approach Turns– Dark, Lighted– Pickup Truck or Utility Van (At Fault)– Driver Inexperienced

Analysis

P95% Confidence Intervals Were Constructed< 31 Attributes Were INCREASED by an Amount Significantly Different

Than Zero.

< 9 Attributes INCREASED Significantly More Than 50%

– Property Damage Only Crashes– Crashes Involving 3 or More Vehicles– Crashes on the Roadway– Rear Ends– Approach Turns– Dark, Lighted– Pickup Truck or Utility Van (At Fault)– Driver Inexperienced– Total Number of Crashes

Analysis

PTraffic Volume (AADT) Increased by 19.30% (Average) from Before to After

P95% Confidence Intervals Compared to 19.30% (Rather Than Zero)< Attributes Significantly Reduced Relative to Change in Highway Traffic Volume

– Collisions Involving a School Aged Pedestrian– Broadside Collisions– Overtaking Turn Collisions– Motor Home as the At-Fault Vehicle

Analysis

P95% Confidence Intervals Compared to 19.30% (Rather Than Zero)< Attributes Significantly Reduced Relative to Change in Highway Traffic Volume

– Collisions Involving a School Aged Pedestrian– Broadside Collisions– Overtaking Turn Collisions– Motor Home as the At-Fault Vehicle

Analysis

P95% Confidence Intervals Compared to 19.30% (Rather Than Zero)<Severity Measures Increased Significantly Beyond Change in Highway Traffic Volume

– Property Damage Only Collisions– Injury Collisions– Total Persons Injured

Analysis

P95% Confidence Intervals Compared to 19.30% (Rather Than Zero)< Collisions Under The Following Conditions Increased Significantly Beyond Change in

Highway Traffic Volume– Daylight– Dawn or Dusk– Darkness, at Illuminated Locations– Good Weather– Rain– Snow, Sleet or Hail– Dry Road– Wet Road

Analysis

P95% Confidence Intervals Compared to 19.30% (Rather Than Zero)< Collisions Where Drivers of the Following Vehicle Types Were At Fault Increased

Significantly Beyond Change in Highway Traffic Volume– Passenger Cars and Vans– Pickups and Utility Vehicles– Heavy Trucks and Busses– Unknown (Hit and Run) Vehicles

Analysis

P95% Confidence Intervals Compared to 19.30% (Rather Than Zero)< Collisions Involving The Following Apparent Human Factors Increased Significantly

Beyond Change in Highway Traffic Volume– No Apparent Contributing Human Factor– Driver Inexperience– Driver Preoccupied

Analysis

P95% Confidence Intervals Compared to 19.30% (Rather Than Zero)< Collisions Types Increased Significantly Beyond Change in Highway Traffic Volume

– Rear End– Approach Turn (Left Turning)

Analysis

PSafety Was Generally Not Improved< Accidents Increased at 75% of Locations< Accidents Increased by 74.6% while AADT Increased 19.3%< 26 Attributes Were Increased Significantly More Than AADT< Only 4 Attributes Were Decreased Significantly Relative to

P Increases Followed Signalization< Signalization Isn’t Necessarily Cause of Each Increase

– Lacking other arguments, signalization is the most likely culprit.

Conclusions

PThe Bad News is Good News< Rear Ends Increased at 77% of Locations< Rear Ends Increased by 165%< Increase in Rear Ends = 64% of Increase in Total Accidents< Approach Turns Increased at 58% of Locations< Approach Turns Increased by 150%< Increase in Approach Turns = 34% of Total Increase

PHow is That Good?< Rear End Countermeasures (Dilemma Prevention, Signal

Progression) Approach 50% Reduction< Fully Protected Lefts Reduce Approach Turn by 90%+< Roundabouts Don’t Have Approach Turn and Reduce Rear End

Conclusions

P MUTCD says, “A traffic signal should not be installedunless an engineering study indicates that installing atraffic signal will improve the overall safety and/oroperation of the intersection.”

P Traffic Engineers should strive to improve safety ANDoperation (though many of the studied signals apparentlyimproved neither).

P By thoughtfully considering HCM, available counter-measures for the (now expected) safety impacts andappropriate alternatives, we should be able to improveboth.

Conclusions

Thank You!

Signalization and Safety - Teach America Corporation · Signalization and Safety A Study of the...

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