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25 Construction of driveways along acceleration and decel- spacing. The various studies point to one consistent finding: eration lanes and tapers is discouraged because of the an increase in the number of access points translates into potential for vehicular weaving conflicts. higher accident rates. The specific relationships vary, reflect- Driveways across from median openings shall be con- ing differences in road geometry, operating speeds, and drive- solidated whenever feasible to coordinate access at the way and intersection traffic volumes. Figure 12 shows the median opening. composite accident rates as presented in NCHRP Report 420. In addition to the review of safety studies, a comprehensive In a research study by Lall et al. (63), guidelines were safety analysis was performed using accident data from eight developed for an access management program for the Ore- states. Accident rates were derived for various unsignalized gon DOT. An analysis was performed on a 47-km (29-mi) access spacings and median types. The analysis showed that section of Oregon Coast Highway 9 to determine the rela- accident rates increase with total access points per mile in tionship between access density and accident experience and urban and rural areas. The authors concluded that in urban and severity. The analysis demonstrated a relationship between suburban areas, each additional access point (or driveway) on frequency of accidents and density of access points. The undivided highways increases the annual accident rate by 0.07 results showed that the number of accidents increased as the to 0.11 accidents per million veh-km (0.11 to 0.18 accidents number of access points increased along the highway. per million veh-mi) traveled. Each additional access point on Brown and Tarko developed impact models to predict crash highways with TWLTLs or nontraversable medians increases frequencies based on the geometric and access control charac- the annual accident rate by 0.06 to 0.08 accidents per million teristics of a roadway (64). Negative binomial regression mod- veh-km (0.09 to 0.13 accidents per million veh-mi) traveled. els were developed to predict the total number of crashes, In rural areas, each additional access point (or driveway) number of property-damage-only crashes, and number of fatal increases the annual accident rate by 0.01 to 0.04 accidents per and injury crashes. The significant factors included density of million veh-km (0.02 and 0.07 accidents per million veh-mi) access points, proportion of signalized access points, presence traveled on undivided highways and on highways with of an outside shoulder, presence of a TWLTL, and presence of TWLTLs or nontraversable medians, respectively. a median with no openings between signals. The results indi- TRB Circular 456 (6) presents a compilation of the cur- cated that access control has a beneficial effect on safety. rent state and local practices for designing streets and high- The need to address the safety effects of U-turns at unsignal- ways from an access management perspective. The circular ized median openings is a direct result of increased attention illustrates the basic considerations for spacing standards and to access management. Highway agencies are installing more raised medians on arterials in response to access management guidelines. Median installation generally increases U-turn vol- umes and necessitates effective design of unsignalized median openings. SPACING BETWEEN ACCESS POINTS Access spacing is a key element of access management. An earlier portion of Chapter 2 addressed the effect on safety of the spacing between median openings. However, the spacing of access points between median openings is also an impor- tant aspect of access management. Access points introduce conflicts and friction into the traffic stream. Vehicles entering and leaving the main roadway often slow the through traffic, and the difference in speeds between through and turning traf- fic increases accident potential. The Green Book (3) states that "Driveways are, in effect, intersections . . . The number of crashes is disproportionately higher at driveways than at other intersections; thus their design and location merit spe- cial consideration." It is believed that increasing the spacing between access points improves arterial flow and safety by reducing the number of conflicts per mile, by providing greater distance to anticipate and recover from turning maneuvers, and by providing opportunities for use of turn lanes. NCHRP Report 420 (4) presents an extensive summary of Figure 12. Composite accident rates as a function of access the safety research and experience associated with access point density (4).