Most major urban and suburban streets weren't designed to accommodate today's heavy traffic. Instead they've evolved as traffic has increased — and it has increased a lot. Now motorists travel 2 billion miles every day on 150,000 miles of arterial roads in and near U.S. cities. More than 8,000 fatal and a million nonfatal injuries occur each year. The crashes aren't happening haphazardly. Many of them occur in predictable locations and involve predictable sequences of contributing events. Remedies can involve costly and disruptive re-engineering of the roadways.
Summary of improvements:
targeted crashes per year (average) at Leesburg Pike intersections before/after application of crash reduction measures
But not always. Some relatively simple and inexpensive roadway changes can be effective. It's a matter of studying an urban arterial systematically to pinpoint where crashes are occurring frequently and then identifying potential solutions, looking first for less costly ones that can be implemented more quickly than major re-engineering projects. Solutions might involve, for example, simply moving a bus stop a few hundred feet up or down a street to reduce rear-end crashes. At a problem intersection, it might help to add a left-turn arrow to a traffic light to make turning less risky or, at another location, to mark the pavement to warn approaching drivers of potential conflicts ahead.
"Not every urban traffic problem is amenable to these kinds of solutions, but lots of the problems are," says Richard Retting, the Institute's senior transportation engineer and the lead researcher in a multiyear program to reduce crashes along Leesburg Pike, a major arterial in suburban Fairfax County, Va., near Washington, D.C. Working with officials at the Virginia Department of Transportation, Retting developed a step-by-step plan for identifying specific problems that were contributing to crashes. He also identified appropriate countermeasures, worked with the state officials to implement the countermeasures, and then evaluated the results.
At two of the six intersections where remedies were applied, crash types that previously were overrepresented were reduced to zero. At four other intersections, the targeted crashes were significantly reduced.
"We applied some straightforward engineering improvements that were effective and not very costly," says Connie Sorrell, the Virginia Department of Transportation's chief of system operations. "We're looking to use these and similar measures elsewhere."
Police crash reports include a wealth of information about crash patterns. Retting and his colleagues first gathered reports over 3 years on crashes that had occurred at 14 intersections along 6 miles of Leesburg Pike and sorted them by location.
Next the researchers classified the crashes (except bicycle and pedestrian crashes) according to type — a vehicle was struck by oncoming traffic during a left turn, a motorist failed to heed a traffic signal and collided with another vehicle, a motorist moved out of a travel lane and collided with another vehicle, or a stopped or very slowly moving vehicle was struck in the rear. Where specific types of crashes were clustered at particular intersections, the researchers calculated the expected numbers of such crashes, based on data from the whole arterial, to determine whether a crash type was overrepresented. This revealed six problem intersections where crashes of a specific type were occurring more frequently than expected.
The third step was to inspect police crash reports and observe the six intersections where crashes were clustered to see why the excessive crashes were occurring. Were rear-end crashes occurring because turning vehicles weren't adequately accommodated? Was the sight distance adequate to make a turn safely? Asking these kinds of questions led to identification of appropriate countermeasures.
"This is highly individualized work," Retting points out, "because crash types and circumstances vary widely from one location to another, even along the same road. So what you have to do is tailor the countermeasures to the specific reasons the crashes are occurring."
Making it easier to turn
For example, westbound motorists on Leesburg Pike used to find it difficult to turn left across several lanes of heavy traffic at the intersection with Lewinsville Road. The speed limit for oncoming traffic is 45 mph, so vehicles often approach this intersection at relatively high speeds. To make matters worse, the view of oncoming traffic often is obscured by eastbound vehicles, especially trucks and buses waiting to turn left. These factors contributed to an overrepresentation of potentially serious side impacts involving left-turning vehicles.
"The crashes can be serious, but reducing them didn't involve costly and complex solutions," Retting points out. "We worked with Virginia Department of Transportation officials to install a protected left-turn signal for westbound motorists. Plus the timing of the signal light was adjusted for better traffic flow."
The results are dramatic. Westbound motorists at Lewinsville Road had been experiencing an annual average of 4.6 left-turn crashes before the interventions. During the next 31 months, not one such crash occurred. Similar measures were applied at Ring Road, with equally dramatic results. An average of 8.7 left-turn crashes had been occurring annually before the interventions. These were reduced to zero during the subsequent 2 years.
Reducing the risks associated with merging traffic
Long queues used to form among southbound motorists on Westpark Drive waiting in the far right lane to merge onto Leesburg Pike. Problems included a short merge area and heavy, fast-moving traffic that left few gaps for the merging vehicles. The result was that the queuing motorists on Westpark experienced a lot of rear-end crashes. To address this, transportation officials extended the merge lane. This led to shorter traffic queues, smoother merges, and a dramatic decline in crashes — from an annual average of 8.2 rear-end crashes before the lane extension to 0.9 afterward.
At the intersection of Leesburg Pike and Patterson Road, rear-end crashes into stopped or stopping vehicles were associated with a bus stop. Researchers took note of another nearby bus stop and suggested eliminating the one at Patterson. Once this stop was removed, rear-end crashes declined from an average of 3.5 per year to 1.4. A similar improvement was accomplished at Magarity Road by widening the shoulder of Leesburg Pike two feet to accommodate buses. This way bus drivers no longer have to stop in traffic and interrupt the flow of vehicles. Rear-end crashes have declined from 4.3 per year to 2.5.
Before bus stop was moved: 3.5 average crashes per year
After bus stop was moved: 1.4 average crashes per year
Traffic is heavy turning right onto Marshall Drive from eastbound Leesburg Pike. There's no turn lane to accommodate this traffic, so motorists waiting to turn queue on Leesburg Pike. Rear-end crashes used to occur frequently when fast-moving eastbound motorists unexpectedly encountered the queue.
"One solution would have been to construct a right-turn lane. This might be the best idea in the long run, but until it can be constructed Virginia officials have marked the pavement on Leesburg Pike to warn motorists of potential conflicts with right-turning vehicles," Retting points out. This has reduced rear-end crashes from an average of 8.3 per year to 3.
Model for the nation
"We didn't set out just to improve Leesburg Pike," Retting points out. "We wanted to create a model for local officials nationwide to improve safety and traffic flow in their own areas. Officials can systematically study their arterials and use the four-step procedure to identify problems. They should seek remedies besides the ones we've spelled out for Leesburg Pike because every urban arterial is different."
Alternative remedies might include installing red light cameras, for example. At other locations constructing roundabouts might be appropriate. These approaches are known to be effective (see Status Report special issue: automated enforcement, May 4, 2002, and "Roundabouts reduce traffic backups and crashes, too," July 28, 2001).