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42 Type 3b--Conventional Median Opening With One Left-Turn Lane at Three-Leg Intersection Advantages The presence of a left-turn lane in one direction of travel reduces potential for rear-end collisions between left-turn or U-turn vehicles and following through vehicles for one direction of travel only on the major road. The presence of a left-turn lane in the direction of travel mitigates the problem of U-turn vehicles in that direction encroaching on the adjacent lane and interfering with through traffic while waiting for a gap in the opposing traffic. Vehicles on the minor road can make left and right turns onto the major road. Vehicles on the major road can make left and right turns onto the minor road and make U-turns on the major road. Delays will be less than at a four-leg intersection. No additional VMT is incurred by vehicles making left-, right-, or U- turning maneuvers. Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. There are only 11 conflict points, which is less than at a four-leg intersection. Accident rates at three-leg intersections are lower than at four-leg intersections. Disadvantages U-turn vehicles in the direction of travel without a left-turn lane forced to stop in the median opening may encroach on adjacent lanes and interfere with through traffic. Narrow medians may not provide enough space for larger vehicles to negotiate a U-turn. Vehicles on the major road in the direction of travel without a left-turn lane must initiate U-turn maneuvers from the through lane, which may delay through vehicles and create the potential for rear-end collisions. With no directional island, opposing U-turn vehicles may have to overlap. Minimum 6- to 8-second gaps are needed to cross traffic, possibly even 10 sec where six-lane or eight-lane arterials must be crossed. These gaps will be limited during busy travel period on heavily used arterials. Where peak-hour traffic exceeds critical volumes, vehicles entering the major road from the minor road may experience long delays. Figure 26. Advantages and disadvantages of median opening type 3b--conventional median opening with one left-turn lane at three-leg intersection. Figures 34 and 35 illustrate these two median opening cation system identifies what turning movements will occur designs and their advantages and disadvantages. at the median opening. Another set of factors influence how a particular median opening design will operate at a particular location. Such factors are related to the roadway environment, operational FACTORS THAT INFLUENCE THE SAFETY AND OPERATIONAL PERFORMANCE demands, physical constraints, roadway geometrics, and OF MEDIAN OPENINGS nearby features. Table 14 identifies the major factors that influence the safety and operational performance of median The first part of this chapter of the report presents a clas- openings and constrain the choice of median openings sification system for median opening designs. This classifi- designs.

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43 Type 3c--Conventional Median Opening With Two Left-Turn Lanes at Three-Leg Intersection Advantages The presence of left-turn lanes reduces potential for rear-end collisions between left-turn or U-turn vehicles and following through vehicles for both directions of travel on the major road. The presence of left-turn lanes mitigates the problem of U-turn vehicles encroaching on adjacent lanes and interfering with through traffic while waiting for a gap in the opposing traffic. Vehicles on the minor road can make both left and right turns onto the major road. Vehicles on the major road can make left and right turns onto the minor road and make U-turns on the major road. Delays will be less than at a four-leg intersection. No additional VMT is incurred by vehicles making left-, right-, or U- turning maneuvers. Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. There are only 11 conflict points, which is less than at a four-leg intersection. Accident rates at three-leg intersections are lower than at four-leg intersections. Disadvantages U-turn vehicles entering the through lanes may delay full-speed through traffic. Narrow medians may not provide enough space for larger vehicles to negotiate a U-turn. With no directional island, opposing U-turn vehicles may have to overlap. Minimum 6- to 8-sec gaps are needed to cross traffic, possibly even 10 sec where 6-lane or 8-lane arterials must be crossed. These gaps will be limited during busy travel period on heavily used arterials. Where peak-hour traffic exceeds critical volumes, vehicles entering the major road from the minor road may experience long delays. Figure 27. Advantages and disadvantages of median opening type 3c--conventional median opening with two left-turn lanes at three-leg intersection. Roadway Environment umes it will experience and the turning volumes that will need to be accommodated. A median opening in an urban area Environmental factors that influence the operational and probably will be surrounded by businesses that serve as traf- safety performance of a median opening include degree of fic generators, creating a greater need for U-turns and left urbanization, speed, and access density. These three factors turns through the median opening. Median openings in rural are interrelated to a great extent. For example, urban areas are areas, however, may only have to accommodate an occasional typically characterized by lower speeds and greater access U-turn or left-turn vehicle because of the greater through- density, whereas rural areas are typically characterized by traffic demand of the roadway. higher speeds and greater control of access. Degree of Urbanization Operating Speed The type of area in which a median opening is designed Operating speed is another environmental factor that influ- (i.e., urban, suburban, or rural) also determines the traffic vol- ences the safety and operational performance of a median

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44 Type 3d--Conventional Median Opening With Left-Turn Lanes and Loons at Three-Leg Intersection Advantages Widening on the far side of the turn makes it possible to make a U-turn without stopping or backing and reduces interference between U-turn and through traffic, particularly for large vehicles. Without performing major reconstruction, additional space can be provided to facilitate the larger turning path of commercial vehicles along narrow medians. The presence of left-turn lanes reduces potential for rear-end collisions between left-turn or U-turn vehicles and following through vehicles for both directions of travel on the major road. The presence of left-turn lanes mitigates the problem of U-turn vehicles encroaching on adjacent lanes and interfering with through traffic while waiting for a gap in the opposing traffic. Vehicles on the minor road can make left and right turns onto the major road. Vehicles on the major road can make left and right turns onto the cross street and make U-turns on the major road. Delays will be less than at a four-leg intersection. No additional VMT is incurred by vehicles making left-, right-, or U- turning maneuvers. Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. Accident rates at three-leg intersections are lower than at four-leg intersections. Disadvantages U-turn vehicles entering the through lanes may delay full-speed through traffic. Narrow medians may not provide enough space for larger vehicles to negotiate a U-turn. With no directional island, opposing U-turn vehicles may have to overlap. Minimum 6- to 8-sec gaps are needed to cross traffic, possibly even 10 sec where 6-lane or 8-lane arterials must be crossed. These gaps will be limited during busy travel period on heavily used arterials. Where peak-hour traffic exceeds critical volumes, vehicles entering the major road from the minor road may experience long delays. The presence of a loon may make snow removal and other maintenance work more difficult. Figure 28. Advantages and disadvantages of median opening type 3d--conventional median opening with left-turn lanes and loons at three-leg intersection. opening. For example, the speed of through vehicles on the differentials between turning vehicles and following through major road influences left-turn lane requirements, gap avail- traffic. Research has shown that accident potential increases ability, and weaving maneuvers. as the difference in speeds between vehicles in a traffic stream The safety and operational performance of a median open- increases. Furthermore, drivers begin to slow down a con- ing without a left-turn lane is largely dependent on the oper- siderable distance upstream of a median opening. Thus, large ating speeds of the vehicles on the roadway. Median open- speed differentials are created a considerable distance in ings, and other intersections, can experience large speed advance of the location at which the turn is made. Left-turn

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45 Type 4a--Directional Median Opening for Left Turns From Major Road at Three-Leg Intersection Advantages The presence of a left-turn lane reduces potential for rear-end collisions between left-turn or U-turn vehicles and following through vehicles on the major road. The presence of a left-turn lane mitigates the problem of U-turn vehicles encroaching on adjacent lanes and interfering with through traffic while waiting for a gap in the opposing traffic. Vehicles on the major road have direct left-turn access to the minor road. Vehicles on the major road can make left turns onto the minor road without having to accommodate minor-road vehicles making left-turn maneuvers through the median opening. Vehicles on the major road can make U-turn maneuvers in one direction of travel. Vehicles making right-turn maneuvers from the minor road do not experience delay associated with waiting behind a left-turning vehicle. For those maneuvers permitted, delays will be less than at a conventional median opening at a three-leg intersection. Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. There are only six conflict points, which is less than at a conventional median opening at a three-leg intersection. Accident rates at three-leg intersections are lower than at four-leg intersections. Disadvantages Minor road vehicles do not have direct left-turn access to the major road. U-turn vehicles entering the through lanes may delay full-speed through traffic. Additional VMT will be incurred by vehicles whose drivers desire to turn left from the minor road onto the major road. Figure 29. Advantages and disadvantages of median opening type 4a--directional median opening for left turns from major road at three- leg intersection. lanes are the only means of effectively controlling the speed in order to merge into the inside lane, move laterally into the differential between turning vehicles and other traffic on major left-turn lane, and then come to a stop. The operating speeds roadways. of through vehicles on the major roadway not only affect the Because gaps are time based, rather than distance based, the available gaps for merging from the outside lane to the inside availability of gaps for crossing and turning vehicles on a high- lane, but determine the speed at which the weaving vehicle speed roadway is less than on a lower speed roadway with will enter the left-turn lane. Thus, a lower speed environment comparable traffic volumes. The reduced gap availability may is more desirable for weaving maneuvers. cause drivers to accept smaller gaps, thus taking greater risks to perform a crossing or turning maneuver. The presence of shorter gaps can also influence the storage length require- Access Density ments of any turn lanes that are present at a median opening. That is, the difficulty turning vehicles experience in com- Access density is another environmental factor that influ- pleting a U-turn or left turn probably will create longer queues ences the safety and operational performance of a median in a left-turn lane. opening. Access points introduce conflicts and friction into Finally, speed influences the ease with which a vehicle can the traffic stream. Vehicles entering and leaving the main perform a weaving maneuver from an upstream intersection roadway often slow the through traffic. The differences in to the median opening. Desirable conditions would permit a speeds between through and turning vehicles increase acci- driver to merge into the outside lane, select an acceptable gap dent potential.

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46 Type 4b--Directional Median Opening for Left Turns Onto Major Road at Three-Leg Intersection Advantages Vehicles on the minor road have direct left-turn access to the major road. Vehicles on the major road in one direction of travel can make U-turn maneuvers. Right-turn maneuvers are unaffected by this treatment. Delays will be less than at a conventional median opening at a three-leg intersection. Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. There are only six conflict points, which is less than at a conventional median opening at a three-leg intersection. Accident rates at three-leg intersections are lower than at four-leg intersections. Disadvantages Vehicles on the major road do not have direct left-turn access to the cross street. The absence of a left-turn lane on the major road increases potential for rear- end collisions between U-turn vehicles and following through vehicles. U-turn vehicles forced to stop in the median opening may encroach on adjacent lanes and interfere with through traffic. U-turn vehicles entering the through lanes may delay full-speed through traffic. Additional VMT will be incurred by vehicles whose drivers desire to turn left from the major road onto the minor road. Figure 30. Advantages and disadvantages of median opening type 4b-- directional median opening for left turns onto major road at three-leg intersection. Operational Demands crossing maneuvers from the median opening. Large major- road volumes limit the number of gaps available for turning The number of vehicles seeking to perform each traffic and crossing maneuvers and, thus, create a greater potential movement accommodated by a median opening influences for angle collisions. Major-road volume also influences weav- such operational and safety factors as gap availability, number ing maneuvers in advance of the median opening. In this case, of conflicts, and left-turn storage. Specifically, major-road vol- large major-road volumes limit the number of gaps available ume, minor-road volume, left-turn volume, and U-turn volume for vehicles on an upstream minor-road or driveway to merge represent operational demands that are placed on a median with the major-road traffic, change lanes from the outside lane opening and influence its operational and safety performance. to the inside lane, and safely perform a turning maneuver at All of the volume types combined determine the number of the median opening. This can lead to increased potential for conflicts at a median opening. That is, a median opening on sideswipe collisions between weaving vehicles and through a low-volume road with minimal turning volumes has fewer traffic. potential conflicts than a median opening on a high-volume roadway with large turning volumes. Minor-Road Volume Major-Road Volume Minor-road volume plays a key role in the number of Major-road volume plays a large role in the gap availabil- crossing conflicts at a median opening. To reduce the poten- ity on the major road for vehicles performing turning and tial for angle collisions, some median opening designs pro-

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47 Type 5a--Conventional Median Opening Without Left-Turn Lanes at Four-Leg Intersection Advantages Vehicles on the minor road can either cross the major road or make left and right turns onto the major road. Vehicles on the major road in both directions of travel can make left and right turns onto the minor road and make U-turns on the major road. No additional VMT is incurred by vehicles making left-, right-, or U- turning maneuvers. Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. Disadvantages The absence of left-turn lanes increases potential for rear-end collisions between U-turn vehicles and following through vehicles. U-turn vehicles forced to stop in the median opening may encroach on adjacent lanes and interfere with through traffic. U-turn vehicles entering the through lanes may delay full-speed through traffic. With no directional island, opposing U-turn vehicles may have to overlap. Minimum 6- to 8-sec gaps are needed to cross traffic, possibly even 10 sec where 6-lane or 8-lane arterials must be crossed. These gaps will be limited during busy travel period on heavily used arterials. Where peak-hour traffic exceeds critical volumes, vehicles entering the major road from the cross street may experience long delays. There are 32 potential conflict points with both minor-road and major- road vehicles entering the median opening. Accident rates at four-leg intersections are higher than at three-leg intersections. Figure 31. Advantages and disadvantages of median opening Type 5a--conventional median opening without left-turn lanes at four-leg intersection. hibit crossing maneuvers by providing directional channel- any median opening, with or without a left-turn lane, left- ization in the median opening and restricting the minor road turn volume affects the number of potential crossing conflicts to right turns only. between left-turn vehicles and opposing through vehicles, and U-turn volume affects the number of potential merging con- flicts between U-turn vehicles and opposing through vehicles. Left-Turn and U-Turn Volumes Left-turn volume and U-turn volume influence the safety Physical Constraints and operational performance of a median opening. The extent of this influence is based on the presence of a median left- Three constraints that influence the operational and safety turn lane and its storage capacity. The absence of a left-turn performance of a median opening are sight distance, right-of- lane increases the potential for rear-end collisions between way width, and building setback. U-turn or left-turn vehicles and following through vehicles. At a median opening with a left-turn lane, left-turn and U-turn volumes influence the length of the turning lane necessary to Sight Distance accommodate the storage requirements. A queue of vehicles extending beyond the left-turn lane interferes with through Intersection sight distance (ISD) is an important safety and traffic and increases the potential for rear-end collisions. At operational consideration in the design of a median opening.

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48 Type 5b--Conventional Median Opening With Left-Turn Lanes at Four-Leg Intersection Advantages The presence of left-turn lanes reduces potential for rear-end collisions between left-turn or U-turn vehicles and following through vehicles for both directions of travel on the major road. The presence of left-turn lanes mitigates the problem of U-turn vehicles encroaching on adjacent lanes and interfering with through traffic while waiting for a gap in the opposing traffic. Vehicles on the minor road can either cross the major road or make left and right turns onto the major road. Vehicles on the major road can make left and right turns onto the minor road and make U-turns on the major road. No additional VMT is incurred by vehicles making left-, right-, or U- turning maneuvers. Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. Disadvantages U-turn vehicles entering the through lanes may delay full-speed through traffic. With no directional island, opposing U-turn vehicles may have to overlap. Minimum 6- to 8-sec gaps are needed to cross traffic, possibly even 10 sec where 6-lane or 8-lane arterials must be crossed. These gaps will be limited during busy travel period on heavily used arterials. Where peak-hour traffic exceeds critical volumes, vehicles entering the major road from the cross street may experience long delays. There are 32 potential conflict points with both cross-street and major-road vehicles entering the median opening. Accident rates at four-leg intersections are higher than at three-leg intersections. Figure 32. Advantages and disadvantages of median opening type 5b--conventional median opening with left-turn lanes at four-leg intersection. NCHRP Report 375 states that ISD at median openings is Either behavior may cause other vehicles to perform evasive complicated by the presence of the median itself, which may or braking maneuvers. increase the ISD requirements at some locations or may con- tain sight obstructions that reduce the ISD. If a median is Right-of-Way Width and Building Setback wide enough to store a vehicle, the sight distance require- ments of the intersection can be determined separately for Right-of-way width and building setback can limit the each directional roadway. Insufficient ISD has both safety design options of a median opening. Local and highways and operational problems. From a safety standpoint, drivers agencies frequently are unable to acquire additional right-of- with insufficient ISD may be unable to anticipate and avoid way because of financial or political limitations. Further- potential collisions. From an operational standpoint, drivers more, the acquisition of buildings is usually avoided unless with insufficient ISD may either extend their vehicle into the absolutely necessary. These constraints often result in a nar- traffic stream in an attempt to improve their view of the road- row median, limiting the design options to indirect left-turn way or accept less than desirable gaps in the traffic stream. designs, jughandles, or medians without a left-turn lane.

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49 Type 6a--Directional Median Opening for Left Turn From Major Road at Four-Leg Intersection Advantages The presence of left-turn lanes reduces potential for rear-end collisions between left-turn or U-turn vehicles and following through vehicles for both directions of travel on the major road. The presence of left-turn lanes mitigates the problem of U-turn vehicles encroaching on adjacent lanes and interfering with through traffic while waiting for a gap in the opposing traffic. Vehicles on the major road have direct left-turn access to the minor road. Vehicles on the major road can make left turns onto the minor road without having to accommodate minor-road vehicles making left-turn maneuvers through the median opening. Vehicles on the major road in both directions of travel can make U-turn maneuvers. Vehicles making right-turn maneuvers from the minor road do not experience delay associated with waiting behind a left-turning vehicle. Delays will be less than at a conventional median opening at a four-leg intersection. Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. There are only 12 conflict points, which is less than at a conventional median opening at a four-leg intersection. Accident rates at directional four-leg median openings are lower than at conventional four-leg median openings. Disadvantages Minor-road vehicles do not have direct left-turn access to the major road. U-turn vehicles entering the through lanes may delay full-speed through traffic. Accident rates at four-leg intersections are higher than at three-leg intersections. Direct crossing maneuvers on the minor road are not permitted. Additional VMT will be incurred by vehicles whose drivers desire to make crossing maneuvers on the minor road and left-turn maneuvers from the minor road to the major road. Figure 33. Advantages and disadvantages of median opening type 6a-- directional median opening for left turn from major road at four-leg intersection. Roadway Geometrics From an operational standpoint, the number of lanes on an arterial affects the ability of vehicles to complete U-turn Principal roadway geometrics that influence the opera- maneuvers and is particularly critical for U-turns on arterials tional and safety performance of a median opening are the with narrow medians. On a four-lane arterial, it may be neces- number of lanes and the median width. sary to provide a loon or a jughandle to accommodate U-turns. However, loons or jughandles may not be necessary on a six- lane arterial, regardless of the median width. Number of Lanes The number of lanes on an arterial affects both the safety and operational performance of a median opening. From a Median Width safety standpoint, the travel distance across opposing traffic increases for crossing and turning vehicles as the number of Median width affects the safety and operational perfor- lanes increase. For example, a vehicle making a left turn from mance of a median opening. From a safety perspective, median a six-lane arterial has a greater distance to travel across widths that are either too narrow or too wide can cause vari- opposing traffic to complete the turn than on a four-lane arte- ous types of undesirable driving behavior that can lead to rial. Therefore, the exposure to conflicting traffic is greater. potential safety problems. NCHRP Report 375 (8) reports the

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50 Type 7a--Midblock Jughandle to the Left for U-Turn Maneuvers Advantages Midblock access is provided for vehicles to (a) make a U-turn and (b) reach driveways on the opposite side of the street. Since only major-road traffic is involved, delays to vehicles making U-turns will be less than where an intersection is present. If U-turn traffic would otherwise proceed downstream to an intersection with a median opening, then this treatment should reduce VMT. There are only three conflict points, which is considerably lower than at an intersection. This design places vehicles in a better position for merging with major-road traffic than a conventional median opening design. The removal of left turns from a downstream intersection will increase the capacity of that downstream intersection. The presence of a left-turn lane reduces potential for rear-end collisions between left-turn or U-turn vehicles and following through vehicles for both directions of travel on the major road. The presence of a left-turn lane mitigates the problem of U-turn vehicles encroaching on adjacent lanes and interfering with through traffic while waiting for a gap in the opposing traffic. Disadvantages A minimum 4.6-m (15-ft) median is required for this design. U-turn vehicles must make a crossing maneuver, which is not required at conventional or directional midblock median openings. Figure 34. Advantages and disadvantages of median opening type 7a--midblock jughandle to the left for U-turn maneuvers. following observed types of undesirable driving behavior Angle stopping--Another undesirable driving situation related to median width: occurs when a vehicle stops on the median roadway at some angle other than perpendicular to the through lanes of the divided highway. In some cases, where the median Side-by-side queuing--When one vehicle is waiting in is very narrow or a driver decides to cut a corner, the the median opening for an opportunity to cross or enter driver of a single vehicle may stop at an angle to the major the far roadway and a second vehicle arrives in the same road. Alternatively, when the median roadway is already travel direction, the second driver may stop his or her occupied by one or more vehicles in the same direction vehicle beside rather than behind the first vehicle. Side- of travel, a driver of another vehicle entering the median by-side queuing is a concern because it can lead to opening may find it necessary to stop at an angle to avoid driver confusion about which of the two vehicles is to encroaching on the through lanes of the major road. In proceed first and, thus, can lead to conflicts. This poten- either case, stopping at an unusual angle is undesirable tial safety problem tends to occur where the median because the vehicle may be hit by another vehicle from width is less than the length of two vehicles. Side-by-side any of several directions and because other drivers may queuing can also become a problem where a median be confused about the intended path of that vehicle. opening is wide enough to store two or more vehicles Encroachment on through lanes--Encroachment on a one behind another. In this situation, the driver of a sec- through lane by either the front or rear of a vehicle may ond vehicle may be tempted to pull beside the first vehi- occur if the median width is less than the length of a cle to avoid delay. Again, this maneuver has the poten- vehicle and the driver enters the median when there is tial for the drivers to become confused about which no available gap to cross or enter the far lanes of the vehicle is going to proceed first. divided highway.

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51 Type 7b--Midblock Jughandle to the Right for U-Turn Maneuvers Advantages Midblock access is provided for vehicles to (a) make a U-turn and (b) reach driveways on the opposite side of the street. If U-turn traffic would otherwise proceed downstream to an intersection with a median opening, then this treatment should reduce VMT. The removal of left turns from a downstream intersection will increase the capacity of that downstream intersection. There are only three conflict points, which is considerably lower than at an intersection. Disadvantages Turning traffic must cross one direction of travel and enter the roadway in the other direction of travel. Suitable gaps could be limited during periods of heavy traffic. During periods of heavy traffic, there could be major delays. The need to make a right-hand exit in order to make a U-turn may be confusing to drivers. The introduction of a bi-directional (although segregated) opening may increase accidents; however, this may be offset by a reduction in accidents at downstream intersections. U-turn vehicles entering the through lanes may delay full-speed through traffic. Figure 35. Advantages and disadvantages of median opening type 7b--midblock jughandle to the right for U-turn maneuvers. TABLE 14 Factors that influence the operational and safety performance of median openings Category Factor Roadway environment degree of urbanization (urban/suburban/rural) operating speed access density Operational demands major-road volume minor-road volume left-turn volume U-turn volume Physical constraints sight distance right-of-way width building setback Roadway geometrics number of lanes median width Nearby features proximity to driveways proximity to traffic signal proximity to adjacent median openings proximity to unsignalized intersections