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Road Management Journal
Copyright © 1997 by TranSafety, Inc.
October 1, 1997
TranSafety, Inc.
(360) 683-6276
Fax: (360) 335-6402

Iowa Field Study Documented Successful Heat-Straightening Repair of a Steel Bridge by In-House Personnel
Sealers Shown to Lengthen the Service Life of Concrete Bridges Exposed to Chloride
Large Trucks a Significant Factor in Major Freeway Incidents in Houston, Texas
Warnings Combined with Enforcement Can Reduce Speeding

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Large Trucks a Significant Factor in Major Freeway Incidents in Houston, Texas

Research has shown that almost 60 percent of traffic congestion in the United States is due to incidents (collisions, disabled vehicles, hazardous material spills, etc.). These incidents are particularly significant on freeways in large urban areas, where they can affect traffic conditions for hours. A study in Texas (1986-1992) examined the impact on traffic conditions of major freeway incidents in Houston. Gerald L. Ullman and Michael A. Ogden presented the results of the study in "Analysis of Major Freeway Incidents in Houston, Texas" (Transportation Research Record 1554). They reported that 81 percent of major freeway incidents in Houston involved large trucks. They also found that most major incidents required more than one response agency, the median time required to clear an incident was 2.5 hours, and incidents involving four or more response agencies took significantly longer to clear.

Research studies on freeway incidents generally categorize the data into lane-blocking and shoulder incidents, but there was insufficient data on the number and characteristics of lane-blocking incidents involving multiple response agencies. Therefore, this study analyzed previously unreported data on such incidents in Houston, Texas. Access to such information could help justify the effort and money needed to improve response to these incidents.


The data for the study originated with the solo motorcycle patrol division of the Houston Police Department (HPD), which patrols approximately 150 miles of freeway in the Houston area. The Houston freeway system accommodates between 100,000 and 225,000 vehicles per day; many (up to 14 percent) are large tractor-trailer trucks connected with Houston's numerous petrochemical refineries or with its commercial port facility. Between 4:00 a.m. and 10:00 p.m., the motorcycle division responds to all freeway lane-blocking incidents expected to close at least one lane of traffic for 45 minutes or more. During the seven-year span of the study, the patrol officers recorded 612 major freeway incidents--approximately 87 every year, or about 1.7 every week. For each incident, the officers recorded the location, the number and types of vehicles involved, the type of incident, the number of response agencies, and the number of travel lanes closed. In addition, they wrote down the time the HPD was notified, arrived, and left the scene. Finally, they estimated the amount of time that travel lanes were blocked.


Data collected by the HPD indicated that major incidents on Houston freeways occurred at a rate of 0.68 incidents per 100 MVK (million vehicle kilometers), which is equivalent to a major incident happening every 147 MVK. Data also showed that major incidents were nearly 3.5 times as likely to occur within freeway interchanges as between them. (A freeway-to-freeway interchange is "the distance from the first exit ramp approaching the interchange to the last entrance ramp leaving the interchange.") This meant that while major incidents happened at a rate of 0.51 per 100 MVK between freeway-to-freeway interchanges (equivalent to one major incident for every 196 MVK driven), their frequency rate rose to 1.80 per 100 MVK within the interchange area (equivalent to an incident every 56 MVK driven).

Large trucks accounted for only 7.7 percent of the total vehicle-kilometers traveled on Houston's freeways, yet they were involved in 81 percent (498 of 612) of the major incidents. Of the 612 incidents, 406 (66 percent) involved single trucks, 92 (15 percent) involved more than one truck, 104 (17 percent) involved a truck and an automobile, and 10 (2 percent) involved automobiles only. While large trucks were involved in the majority of freeway incidents classified as major, it should not be assumed that all truck incidents were likely to be major. In fact, most truck mishaps on the freeways were not major incidents. However, when a truck was involved in a major incident, it was often due to a lost or spilled load, an overturned truck, or both. These three occurrences accounted for 57 percent of the major incidents on Houston's freeways.

The severity classification of incidents related to the number of fatalities and injuries, the number of vehicles involved, and the number of response agencies. Four percent (23) of the 612 major incidents involved fatalities, which translated into a fatal accident rate of 0.026 per 100 MVK. Thirty-four percent (206) involved some type of injury, for an injury rate of 0.228 per 100 MVK. Both rates were lower than national statistics for urban freeways, possibly because many fatal incidents occurred late at night, when HPD motorcycle officers were off duty, or because death occurred after victims were transported to a hospital and was, therefore, not noted on the HPD form.

Seventy percent of incidents involved single vehicles, 18 percent involved two vehicles, 7 percent involved three vehicles, and 5 percent involved four or more vehicles. Most of the incidents also involved more than one response agency; only 25 percent of the incidents required one agency, which was the HPD. In addition to the HPD, the agency that most frequently responded was one of the private wrecker services. The next- most-frequent responder was the Texas Department of Transportation (TxDOT), followed by the Houston Fire Department.

Determining the duration of major incidents was a significant goal of the study. Of particular importance were the clearance time (the time from the HPD's arrival on the scene to its departure) and the length of time travel lanes were actually blocked. Median clearance times were 2.5 hours, and the lanes were closed for a median duration time of just over 1.5 hours. However, nearly 19 percent (1 in 5) required four or more hours to clear, and 8 percent (1 in 12) took six or more hours. There was little appreciable clearance time difference among the three types of incidents (truck-only, automobile-only, or truck-automobile). However, clearance times were somewhat longer for overturned trucks (3-plus hours as compared to 2.5 hours). Proportionally, a greater number of overturned trucks took several hours to clear--35 percent required four or more hours, and 14 percent required six or more hours. Lane-blockage times also increased for these incidents.

The number of vehicles involved in a major incident had little effect on either clearance or lane-blockage times; but, as might be expected, more severe or complex incidents involved more response agencies, which in turn increased both the clearance and lane-blockage times. However, both times were essentially consistent when one to three agencies were involved in response and cleanup. When the incident required four or more agencies, both clearance and lane-blockage times "increased significantly." And while the number of interchange incidents was significantly greater than the number of incidents between interchanges, the average time needed to clear the interchange incidents was only about 18 minutes longer.

One of the study's objectives was estimating the impact of major freeway incidents on traffic, especially the relationship between the number of lanes closed and traffic delays. Unfortunately, the HPD did not routinely document this information, so analysis of "special traffic management techniques employed during the incidents" was not possible. Some incident reports did show the number of lanes closed. Thirty-three percent blocked one lane, 39 percent blocked two lanes, 22 percent blocked three lanes, 7 percent blocked four or five lanes, and 6 percent blocked all lanes.


Based on information the HPD gathered over this seven-year period, results indicated that in the Houston area freeway-to-freeway interchanges produced more major freeway incidents than the areas between interchanges. In addition, large trucks were involved in the majority of major freeway incidents, whereas automobile-only involvement was low. The majority of incidents involved a single vehicle and multiple agency response, for which the median clearance time was slightly less than 2.5 hours. The duration of incidents was essentially equal for automobiles and trucks, but overturned trucks increased the incident time. Finally, the number of vehicles involved in an incident had almost no bearing on clearance times, while incidents involving four or more response agencies significantly increased those times.

Copyright © 1997 by TranSafety, Inc.

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