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Access management slows incidence of traffic accidents.

This is how an accident happens. You are on your way home and in a hurry. You have stayed at work longer than you intended and you are going to be late for your son's Little League game. As you leave the building, rush hour is beginning. You decide to make a left turn across two lanes of oncoming traffic to save time. If you had not been concentrating on the traffic moving toward you, you probably would have seen the car coming from the other direction. You turn right in front of it.

Could this accident have been avoided? You were in a hurry, distracted, and should not have attempted a difficult turn in busy traffic. You could have turned right and traveled around the block. Why was it so easy to make a potentially dangerous turn? If a raised median strip had been present, or if the driveway exit had been restricted to right turns, you would not be facing expensive car repair bills and increased insurance rates.

We think of high-speed freeways or urban intersections as the most likely sites for automobile accidents, but entrances/exits of commercial or residential property can be equally hazardous to the motorist.

Driveways also can be sources of congestion - friction points in the flow of traffic. When you exit a driveway and attempt a left turn across traffic, other cars may need to slow down to avoid you. When drivers slow down to enter a driveway, that also slows down the traffic behind them. If other cars are already in the driveway, then an entire lane of traffic comes to a stop.

Over the past decade, transportation engineers and planners have made entering and exiting the roadway safer and more efficient with a technique called access management. Access management consists of traffic engineering tools that can not only make roads safer, but increase roadway capacity and ease traffic congestion. Besides driveway location and design, techniques include the use of raised medians, turn lanes, and proper spacing of traffic signals.

Access Management Techniques

Access management techniques fall into two groups: techniques used on public roads, which cities or other government agencies can implement through roadway design and regulations, and methods applied to design and spacing of private driveways, which agencies can implement through a driveway permit program.

Examples of access management techniques related to highway design and operation:

* Install raised medians prohibiting direct left-turn access

* Install traffic signals at high-volume driveways

* Channelize raised median openings to control path of traffic

* Install curbs to prevent uncontrolled access to property frontage

* Restrict parking on roadway next to driveways

* Install a two-way continuous or alternating left-turn lane

* Increase capacity of existing left turn bay

Examples of access management techniques related to driveway location and operation:

* Offset opposing driveways

* Install two one-way driveways in place of one two-way driveway

* Regulate the maximum number of driveways per property footage

* Consolidate existing access points when separate parcels of land are assembled under one usage or where cross-access between properties is feasible

* Install supplementary access onto adjacent intersecting streets

* Install a driveway channelizing island to prevent left turns

Although the number of vehicles on the road has more than doubled in the last 25 years, the number of paved road lanes has increased by only a third. That means traffic congestion.

"Access management is attracting attention from cities facing serious capacity problems on their roads," said Joe Hart, P.E., transportation manager in the Denver, Colorado office of Fort Worth, Texas-based Carter and Burgess, Inc. "Often there's no room to widen a congested road. It's an expensive alternative to build another, parallel road. Access management is a cost-effective way to make the existing road more efficient for through traffic - and improve safety at the same time."

Most of us are familiar with the restricted access to and from interstate highways or urban freeways. And many other busy roads use raised medians, left turn lanes, and restricted access to adjoining properties to speed the flow of traffic and reduce traffic conflicts. But much of the nation's roadway mileage consists of two- or four-lane roads with unrestricted driveways, unrestricted turns or, in small towns and rural areas, no curbs and the freedom to pull off the road wherever the driver desires.

With no ordinances to control access to roads, small towns can find themselves overwhelmed when suburban growth reaches their borders. "We are working with two small cities north of Austin, Texas, to develop regulations on driveway spacing," said Mike Weaver, a principal with Carter and Burgess. "Accidents in one city were increasing. When the city planned ten subdivisions in one year, they realized that having inadequate rules for building roads could cause even greater traffic problems."

Benefits of Access Management

Statistical research has verified that controlling access reduces vehicle accidents. In one study, driveway accidents along routes with raised medians were found to be only a third as great as along routes without raised medians. A study conducted in Arapahoe County, Colorado, found that the accident rate on typical uncontrolled arterials was more than double that of arterials with intensive use of access management.

Research also has established that access management increases traffic flow. The Arapahoe County study measured average travel speeds during afternoon peak hours. The study found that the average speed on uncontrolled arterials was half the speed that cars achieved on the arterials with intensive access management. A typical four-lane arterial road with a high level of access management can handle almost 10,000 more vehicles per day than the same road without access controls. Also, because access management can relieve congestion, it may contribute to improving local air quality.

Designing a new development or building a new road are the easiest situations in which to apply access management because a plan usually can be developed with the involvement of adjacent land-owners. Another time to consider improving access design is during roadway expansions or improvements. A local or state agency generally has regulatory power over traffic laws and highway configurations and can include access controls in a road improvement project.

Municipalities can regulate location and design of new or improved driveways through a driveway permit program.

"Regulations which affect property are going to be controversial," said Hart. "Many commercial property owners think that access management will be a detriment to their site, but when driveways are well defined and safe, that can actually result in more traffic to their front door."

Heavily controlled access is not automatically the best approach. "A developer asked us about a road widening next to his property," said Hart. "The project included a raised median, which would restrict turns to the property. The county proposed to consolidate the main entrance with a driveway to a gas station next door. We collected data on the turning movements into and out of both properties, added projected growth at the shopping center, and established that the shared driveway would become overloaded. We showed the county that maintaining separate driveways and changing the design of the median to include two breaks with limited turns resulted in better traffic flow."

Computer software often can provide conclusive evidence that an access management plan will work. "A developer asked us to design access for properties along a road with a median barrier," said Weaver. "We proposed a series of limited purpose median breaks, but the highway department was skeptical of the idea. By modeling the whole corridor, we established that traffic queues would not build up at the access breaks, and the department accepted the change."

Historically, roads have two functions: the first, to get people and goods from place to place. The second, no less important, is to provide access to property that borders the road. An inherent conflict between these two purposes arises when traffic on a road increases. It is not possible to have completely free access and, at the same time, unhindered traffic flow. Access management offers a way to strike a balance between the needs of landowners and the traveling public.
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Publication:Public Works
Date:Feb 1, 1995
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