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Simple Road Improvements Can Yield Big Results

August 3, 2005
Most crash deaths occur on rural roads, but motorists drive 2 billion miles every day on urban arterial roads as well and an increasing number of crashes are occurring there as well.

IIHS-Auto Safety • Acura TSX Luxury Sedan Top Safety Pick • Nissan Murano Leads SUV Safety Test • Lexus EX35, Cadillac CTS Named Top Safety Picks • Nissan Quest Worst Performer in Minivan Bumper Test • Study: Crash Tests Predict Fatalities In Cars, Not Trucks • Top Safety Awards Go to 34 Cars, SUVs and a Pickup • Midsize SUVs Perform Poorly in Side Impacts • BMW 5 Series Performs Poorly in New Impact Test • Luxury Cars a "Mediocre Lot" in Low-Speed Collisions • Light Trucks Vulnerable to Whiplash --- • More IIHS Tests

The Insurance Institute for Highway Safety (IIHS) reports that about 8,000 deaths and more than 1 million injuries occur each year on suburban and urban roads. “The crashes aren't occurring haphazardly. Many of them happen in predictable locations and involve predictable sequences of contributing events,” according to IIHS.

Remedies can involve costly and disruptive re-engineering of the roadways — but not always. Some relatively simple and inexpensive roadway changes can be effective.

"Urban arterials weren't built to accommodate today's heavy traffic. They've evolved as traffic has increased, and they haven't always evolved in the best way to enhance safety and ensure a smooth flow of traffic. So it's a matter of studying urban arterials to pinpoint where crashes are occurring frequently and then identifying potential solutions, looking first for less costly measures that can be implemented more quickly than major re-engineering," said Richard Retting, senior transportation engineer at IIHS.

Demonstration Project

To demonstrate how to reduce crashes by applying less costly and complex measures, Richard Retting, senior transportation engineer at IIHS led a project involving a major arterial in suburban Fairfax County, Virginia, near Washington, D.C.

The researchers studied traffic on Leesburg Pike, identified problems that were causing crashes, and then applied measures like adding protected left-turn signals at problem intersections or moving a bus stop a few hundred feet.

At two of the six locations where such measures were applied, crash types that had been over-represented were reduced to zero. At four other intersections, the targeted crashes were significantly reduced.

How the project was conducted: 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 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 over represented. This revealed six problem intersections where crashes of a specific type were occurring more frequently than expected.

The researchers then inspected police crash reports and observed 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 individualized work," Retting points out, "because crash types and circumstances vary widely from one location to another, even along the same road. What you have to do is tailor your countermeasures to the specific reasons the crashes are occurring."

Results of simple and relatively inexpensive changes are dramatic: 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.

Reducing these crashes didn't involve costly and complex solutions. The Virginia Department of Transportation installed a protected left-turn signal for westbound motorists and adjusted the timing of the signal light 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 before the interventions were reduced to zero during the subsequent 2 years.

Other crashes were reduced by extending a merge lane from Westpark Drive onto Leesburg Pike. This reduced the queue of motorists waiting to merge and cut rear-end crashes from an average of 8.2 annually before the merge lane was extended 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.

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