Q&A: Urban Crashes

October 2012

1 How significant is the problem of urban crashes?

More crash deaths occur in rural areas. In 2010, 61 percent of crash deaths occurred in rural areas. However, vehicle miles traveled are increasing rapidly in urban areas, where most of the U.S. population lives. From 1985 to 2010, total vehicle miles traveled rose 68 percent, but miles traveled in urban areas grew 90 percent. Federal Highway Administration. 2012. Highway Statistics, 2010. Washington, DC: US Department of Transportation. Federal Highway Administration. 1986. Highway Statistics, 1985. Washington, DC: US Department of Transportation.

Pedestrian deaths and injuries are more prevalent in urban areas. Seventy-three percent of pedestrian deaths in 2010 occurred in urban settings.

Crashes that cause nonfatal injuries and/or property damage also are more frequent in urban areas. A 2008 Insurance Research Council study of a national sample of automobile crash injury claims found that 80 percent of the crashes occurred in urban areas. Insurance Research Council. 2008. Auto injury insurance claims: countrywide patterns in treatment, cost, and compensation. Malvern, PA.  The Highway Loss Data Institute examined insurance losses by vehicle density for 2003-05 model year vehicles. Highway Loss Data Institute. 2006. Insurance losses by vehicle density. Insurance special report A-71. Arlington, VA.  The collision claim frequency rates (claims for vehicle and other property damage per insured vehicle years) and the injury claim frequency rates were higher in the densest areas compared to the least dense ones.

2 What are the most common types of urban crashes?

A 1995 Institute study of more than 4,500 crashes in 4 urban areas found that 22 percent involved drivers who ran red lights or other traffic controls such as stop or yield signs. Retting, R.A.; Williams, A.F.; Preusser, D.F.; and Weinstein, H.B. 1995. Classifying urban crashes for countermeasure development.Accident Analysis and Prevention 27:283-94.  Eighteen percent occurred when a vehicle that was stopped or in the process of stopping was struck from the rear. The 3 other leading types of urban crashes included running off the road and striking an object (14 percent), swerving into another occupied lane (13 percent), and turning left and colliding with an oncoming vehicle (9 percent). These 5 crash types accounted for three-fourths of all urban crashes included in the Institute's study and 83 percent of crashes involving injuries.

3 Which crashes are most likely to cause injuries?

According to the 1995 Institute study of urban crashes, 41 percent of urban crashes associated with a vehicle turning left into oncoming traffic involved injuries. These crashes often involved a side impact. More than a third of the crashes typed as running a red light or other traffic control, rear ending a stopped or stopping vehicle, and running off the road involved injury. In contrast, injuries occurred in only 16 percent of crashes involving lane change maneuvers. Retting, R.A.; Williams, A.F.; Preusser, D.F.; and Weinstein, H.B. 1995. Classifying urban crashes for countermeasure development.Accident Analysis and Prevention 27:283-94.

4 Where and when do urban crashes occur?

Nearly half of all urban crashes occur at intersections. Based on a nationally representative sample of crashes, an estimated 1.2 million urban crashes occurred at intersections or were deemed intersection-related in 2010. Of these crashes, 53 percent occurred at traffic signals. An additional 20 percent occurred at stop signs. 

Pedestrians in urban areas are more likely to be injured in crashes at intersections than at other locations. In 2010, 56 percent of urban crashes involving injured pedestrians occurred at intersections. A 1993 study of fatal pedestrian crashes in 4 U.S. cities found that 40 percent of the crashes involving vehicles other than large trucks and 51 percent of the crashes involving large trucks occurred at intersections. Retting, R.A. 1993. A study of fatal crashes involving pedestrians and trucks in four cities. Journal of Safety Research 24:195-203.

Most urban crashes happen during the afternoon or early evening. In 2010, 61 percent of urban crashes occurred between noon and 9 p.m. An additional 26 percent were between 6 a.m. and noon. While only a small percentage (5 percent) of urban crashes occurred between midnight and 6 a.m., 24 percent of fatal urban crashes happened during these hours.

5 What traffic engineering countermeasures can officials implement?

Institute research has found that crashes on urban roads often are concentrated at specific locations and occur in patterns that can be reduced through relatively simple and inexpensive engineering modifications. Retting, R.A.; Weinstein, H.B.; Williams, A.F.; and Preusser, D.F. 2001. A simple method for identifying and correcting crash problems on urban arterial streets. Accident Analysis and Prevention 33:723-34.  An evaluation of low-cost engineering countermeasures implemented at six locations with significant clusters of specific types of crashes found large reductions in the targeted crash types. Retting, R.A.; Farmer, C.M.; Ferguson, S.A.; and Weinstein, H.B. 2006. Reducing urban arterial intersection crashes through crash typing analysis: a case study. ITE Journal 76:18-23.  At two intersections where protected left-turn signals were installed (to permit left turns only when opposing traffic is stopped), left-turn crashes were eliminated compared with left-turn crashes that occurred frequently prior to the signal changes.

The most common crash type — a driver running a traffic control — might be reduced by lengthening signal clearance intervals, improving signal and sign visibility, increasing sight distances, and reducing vehicle speeds near intersections. A 2002 Institute study of modified yellow and all-red traffic signal timing at urban intersections reported that injury crashes were reduced by 12 percent at experimental sites relative to control sites. Retting, R.A.; Chapline, J.F.; and Williams, A.F. 2002. Changes in crash risk following re-timing of traffic signal change intervals.Accident Analysis and Prevention 34:215-20.  A 2002 Federal Highway Administration study reported that adding left-turn and right-turn lanes can reduce intersection crashes. The addition of left-turn lanes at urban unsignalized intersections reduced crashes by about 30 percent. Harwood, D.W.; Bauer, K.M.; Potts, I.B.; Torbic, D.J.; Richard, K.R.; Rabbani, E.R.K.; et al. 2002. Safety effectiveness of intersection left- and right-turn lanes. Report no. FHWA RD-02-089. Washington, DC: Federal Highway Administration. Intersection crashes also can be substantially reduced by installing modern roundabouts in place of signals and stop signs. A 2001 Institute study of 23 intersections reported that converting from traffic signals or stop signs to roundabouts reduced injury crashes by 80 percent and all crashes by 40 percent. Persaud, B.N.; Retting, R.A.; Garder, P.E.; and Lord, D. 2001. Safety effect of roundabout conversions in the United States: empirical Bayes observational before-after study. Transportation Research Record 1751:1-8.  At signalized intersections with low traffic volumes, crashes can be reduced by installing multiway stop sign control. A 1997 Institute study reported that multiway stop sign control in place of traffic signals reduced crashes by 24 percent. Persaud, B.N.; Hauer, E.J.; Retting, R.A.; Vallurupalli, R.; and Mucsi, K. 1997. Crash reductions related to traffic signal removal in Philadelphia. Accident Analysis and Prevention 29:803-10.

6 Can technology such as speed and red light cameras help reduce urban crashes?

Common traffic infractions such as speeding and red light running are associated with many urban crashes. Speed cameras and red light cameras can help police enforce traffic laws in dense urban areas where traffic pursuits and stops can be dangerous to officers and to other motorists and pedestrians.

Institute research indicates that red light camera enforcement generally reduces violations by 40-50 percent and injury crashe s by 25-30 percent. Retting, R.A.; Williams, A.F.; Farmer, C.M.; and Feldman, A.F. 1999. Evaluation of red light camera enforcement in Fairfax, Va., USA. ITE Journal 69:30-34. Retting, R.A.; Williams, A.F.; Farmer, C.M.; and Feldman, A.F. 1999. Evaluation of red light camera enforcement in Oxnard, California. Accident Analysis and Prevention 31:169-74. Retting, R.A.; Ferguson, S.A.; and Hakkert, A.S. 2003. Effects of red light cameras on violations and crashes: a review of the international literature. Traffic Injury Prevention 4:17-23.

Speed cameras, used to enforce speed limits, have been used on a limited basis in the United States but are used extensively throughout Europe and in Australia. The Institute examined the effect of speed cameras in the District of Columbia, Scottsdale, Ariz.., and Montgomery County, Md., and found that the proportion of drivers traveling more than 10 mph faster than posted speed limits declined 70 percent to 88 percent. Retting, R.A. and Farmer, C.M. 2003. Evaluation of speed camera enforcement in the District of Columbia. Transportation Research Record 1830:34-37. Retting, R.A.; Kyrychenko, S.Y.; and McCartt, A.T. 2008. Evaluation of automated speed enforcement on Loop 101 freeway in Scottsdale, Arizona. Accident Analysis and Prevention 9:440-45. Retting, R.A. and Farmer, C.F. 2008. Evaluation of automated speed enforcement in Montgomery County, Maryland. Traffic Injury Prevention 40:1506-12.  The effects of speed cameras on crashes have been the subject of considerable research, as summarized in two recent systematic reviews. Pilkington and Kinra (2005) reviewed 14 studies that found crash reductions in the immediate vicinities of camera sites ranging from 5 to 69 percent for all crashes, 12 to 65 percent for injury crashes, and 17 to 71 percent for fatal crashes. Pilkington, P. and Kinra, S. 2008. Effectiveness of speed cameras in preventing road traffic collisions and related casualties: systematic review. British Medical Journal 330:331-34.  Wilson et al. (2006) reviewed 21 studies that found reductions ranging from 14 to 72 percent for all crashes, 8 to 46 percent for injury crashes, and 40 to 45 percent for crashes involving fatalities and serious injuries. Wilson, C.; Willis, C.; Hendrikz, J.K.; and Bellamy, N. 2006. Speed enforcement detection devices for preventing road traffic injuries. Cochrane Database of Systematic Reviews 2006, Issue 2, Art. no. CD004607. Oxfordshire, England: The Chochrane Collaboration.

7 Can vehicle design changes help reduce urban crashes?

Yes. Many vehicle features can reduce urban crashes or their consequences. Head restraints can prevent whiplash injuries in relatively minor rear-end crashes, the kind that occur frequently on urban streets. Rear-end collisions account for about 18 percent of all urban crashes, second only to crashes involving running a traffic control. Retting, R.A.; Williams, A.F.; Preusser, D.F.; and Weinstein, H.B. 1995. Classifying urban crashes for countermeasure development.Accident Analysis and Prevention 27:283-94.  Neck injuries, particularly whiplash, occur with significantly higher frequency in rear-end crashes than in other crash configurations. A 1999 Institute study found that 26 percent of rear-struck vehicle drivers reported neck injuries. Farmer, C.M.; Wells, J.K.; and Werner, J.V. 1999. Relationship of head restraint positioning to driver neck injury in rear-end crashes.Accident Analysis and Prevention 31:719-28.  The Institute publishes head restraint ratings for a range of passenger vehicles based on geometry and dynamic testing. A 2008 Institute study examined insurance claims for evidence of driver neck injury for cars and SUVs struck in the rear by the front of another passenger vehicle. Driver neck injury rates were 15 percent lower for vehicles with seats rated good in Institute dynamic tests compared with vehicles with seats rated poor. Farmer, C.M.; Zuby, D.S.; Wells, J.K.; and Hellinga, L.A. 2008. Relationship of dynamic seat ratings to real-world neck injury rates. Presented at the World Congress on Neck Pain, Los Angeles, California. Arlington, VA: Insurance Institute for Highway Safety.

High center-mounted stop lights — a third brake light usually located in a vehicle's rear window — are standard on passenger vehicles. By prominently alerting following drivers that the vehicle in front is braking, the lights have helped reduce the incidence of rear-end collisions by about 5 percent. Farmer, C.M. 1996. Effectiveness estimates for center high mounted stop lamps: a six-year study. Accident Analysis and Prevention28:201-08. Another crash avoidance feature is daytime running lights. Activated by the ignition switch, daytime running lights increase vehicle conspicuity during daylight hours. A 2002 Institute study reported a 3 percent decline in daytime multiple-vehicle crash risk in 9 U.S. states concurrent with the introduction of daytime running lights. Farmer, C.M. and Williams, A.F. 2002. Effects of daytime running lights on multiple-vehicle daylight crashes in the United States.Accident Analysis and Prevention 34:197-203.  Federal researchers, using data collected nationwide from 1995-2001, found a 5 percent decline in daytime, two-vehicle, opposite-direction crashes associated with daytime running lights. Tessmer, J.M. 2004. An assessment of the crash-reducing effectiveness of passenger vehicle daytime running lamps (DRLs). Report no. DOT HS-809-760. Washington, DC: National Highway Traffic Safety Administration.  Studies of the effects on fatal crashes with pedestrians and bicyclists have found mixed results. Tessmer, J.M. 2004. An assessment of the crash-reducing effectiveness of passenger vehicle daytime running lamps (DRLs). Report no. DOT HS-809-760. Washington, DC: National Highway Traffic Safety Administration. Wang, J.S. 2008. The effectiveness of daytime running lights for passenger vehicles. Report no. DOT HS-811-029. Washington, DC: National Highway Traffic Safety Administration.  Good bumpers will not help drivers avoid crashes, but they can reduce repair costs associated with low-speed crashes, which occur frequently on urban streets and in parking lots. An Institute study of vehicles brought to insurance drive-in claims centers in a major metropolitan area during 2001-02 found that about 14 percent of all claims for auto damage involved parking lot collisions — just the kinds of impacts in which strong bumpers could reduce or eliminate much of the damage. McCartt, A.T. and Hellinga, L.A. 2003. Types and extent of damage to passenger vehicles in low-speed front and rear crashes. Arlington, VA: Insurance Institute for Highway Safety.  Yet bumpers are not nearly as damage resistant as they should be because federal requirements for car bumpers are weak, and no requirements apply to the bumpers on pickups, minivans, and SUVs.

About one-fourth of fatal crashes in urban areas are angle crashes, which often involve side impacts and result in more than 3,000 deaths annually. Burgess, M. 2005. Contrasting rural and urban fatal crashes 1994-2003. Report no. DOT HS-809-896. Washington, DC: National Highway Traffic Safety Administration.  Side airbags can reduce serious injuries and deaths in side-impact crashes. An Institute study found that side airbag designs that include protection for car drivers' heads reduce the risk of driver fatalities in driver-side crashes by 37 percent. Airbags that protect the torso but not the head reduce deaths by 26 percent. The death risk for drivers of SUVs involved in driver-side collisions was reduced by 52 percent with head-protecting side airbags and by 30 percent with torso-only airbags. McCartt, A.T. and Kyrychenko, S.Y. 2007. Efficacy of side airbags in reducing driver deaths in driver-side car and SUV collisions.Traffic Injury Prevention 8:162-70.

Advanced crash avoidance technologies may also help prevent or mitigate the severity of urban crashes. For example, forward collision warning systems continuously monitor traffic in front of vehicles and warn drivers of potential collisions. Some systems automatically apply the brakes when a crash is imminent. Most current systems are designed to operate at higher speeds but some newer systems operate at speeds as low as 2 mph. For example, Volvo's City Safety system is specifically designed to address low-speed urban crashes. Active between 2 and 19 mph, the system engages the brakes if it judges that a collision is imminent.