The Boston Region Metropolitan Planning Organization (MPO) analyzes safety concerns using its Mobility Management System, corridor studies, and safety data (such as crash records). These analyses are used in evaluating and selecting projects for the long-range Transportation Plan and the Transportation Improvement Program.

All transportation improvements should result in a facility that safely accommodates its users: some projects are specifically designed to address safety problems, while others improve safety conditions as an integral part of making other improvements.

Roadway Safety Concerns

The most common causes of vehicle crashes are human factors (such as speeding, other traffic violations, alcohol impairment, and driver error). Roadway factors (such as road design, hazards, and conditions) are the second-most prevalent cause of crashes, and vehicle factors (such as tire and brake failure) contribute less often to crashes than the other two categories. However, there is rarely a single cause, as many factors often combine to lead to a crash.

A large proportion of the total number of crashes—and more than 50 percent of the fatal and injury crashes—take place at intersections and interchanges, where vehicles make conflicting turning and weaving movements. In addition, a number of studies correlate motor vehicle speed with the severity of collisions. For vehicle-pedestrian collisions, the severity of injuries increases sharply as vehicle speed increases.

 

What safety factors are considered in roadway design?

Factors considered in assessing safety in the existing system or in new project design are:

Sight Distance
Sight distance is the length of roadway ahead that is visible to roadway users as they stop, pass, turn, or execute other maneuvers. The greater the distance visible to the driver, the more time he or she has to make decisions.

Exposure of Pedestrians and Bicyclists
For vehicle-pedestrian collisions, the severity of injuries increases sharply as vehicle speed increases. Reducing the exposure of bicyclists and pedestrians to motor vehicle traffic reduces both the likelihood and the severity of a crash.

Along a roadway, shoulders, bicycle lanes, or buffered sidewalks can provide greater separation of motorized and nonmotorized users. At crossings, the exposure of bicyclists and pedestrians to motor vehicle traffic can be mitigated through signal-controlled crossings, grade separation, or installation of crossing islands or medians.

Turning Lanes
Removing turning vehicles from through lanes by providing turning lanes reduces the conflicts associated with the speed changes necessary to make turns.

The majority of driveway-related crashes involve turns onto or from the major road.

The safety benefits of left-turn lanes are well documented. The median crash rate reduction resulting from installation of left-turn lanes is 50 percent. Left-turn lanes also benefit highway operations by reducing delays for through traffic.

Access Management
Vehicles entering or leaving the road via a driveway generally operate at slower speeds than the prevailing traffic, which increases crash potential and slows roadway travel. Safety studies demonstrate that crash rates increase as the spacing of unsignalized access points decreases. Managing driveway spacing often enhances operations and safety for the entire corridor. Other tools of access management include signal spacing and timing, shared driveways, managing left-turning traffic, and land use controls.

Traffic Calming
Traffic calming is the reduction of vehicle speeds through such measures as narrowing the real or apparent width of the street, deflecting the vehicle path, preventing certain vehicle movements, and enforcing traffic regulations. When traffic-calming measures are applied in an appropriate setting, the resulting reduction in vehicle speeds reduces the frequency and severity of collisions.