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12-i Chapter 12âLRT Track in Mixed Traffic Table of Contents 12.1Â INTRODUCTION 12-1Â 12.2Â TRACK POSITION WITHIN LANES 12-1Â 12.2.1Â Vehicle Width 12-2Â 12.2.2Â Transverse Position within Lanes 12-2Â 12.2.3Â Adjacent Parking Lanes 12-2Â 12.3Â ON-STREET STATION/STOP PLATFORMS 12-2Â 12.3.1Â Alternative Configurations 12-3Â 12.3.2Â Motorist Perceptions 12-4Â 12.3.3Â Tracks Adjacent to Median Platforms 12-4Â 12.3.4Â Conclusions 12-5Â 12.4Â MINIMUM CURVE RADIUS 12-5Â 12.4.1Â Light Rail Vehicle Limitations 12-5Â 12.4.2Â Vehicles for Small Radius Turns 12-5Â 12.4.3Â Overhead Contact Wire Considerations 12-6Â 12.5Â TURNING MOVEMENTS 12-6Â 12.5.1Â Preferred Configuration 12-6Â 12.5.2Â LRT-Only Traffic Signals 12-6Â 12.6Â CLEARANCE ENVELOPE AND SWEPT PATH IN CURVES 12-7Â 12.6.1Â Difference from Rubber-Tired Traffic 12-7Â 12.6.2Â LRV Tail Swing 12-7Â 12.7Â STREET DRAINAGE, CROSS SLOPES, AND SUPERELEVATION 12-8Â 12.7.1Â Flangeway Drains 12-8Â 12.7.2Â Roadway Crown and Track Cross Slope 12-9Â 188.8.131.52Â Codes and Jurisdictional Issues 12-9Â 184.108.40.206Â Streets with Parabolic Crowns 12-9Â 220.127.116.11Â Special Trackwork Cross Slope 12-10Â 12.8Â PAVEMENT DESIGN FOR IN-STREET TRACKAGEâSEAM LOCATIONS 12-10Â 12.9Â SPECIAL TRACKWORK IN STREETS 12-11Â 12.9.1Â Switch Hardware 12-11Â 12.9.2Â Switch Location 12-11Â 18.104.22.168Â Hazard Issues 12-11Â 22.214.171.124Â Pedestrian Crosswalk Locations 12-12Â 126.96.36.199Â Advance Switch Positions 12-12Â List of Figures Figure 12.6.1 Custom signage for tight clearance zones 12-8Â Figure 12.9.1 Advance switch 12-13Â
12-1 CHAPTER 12âLRT TRACK IN MIXED TRAFFIC 12.1 INTRODUCTION While it is highly desirable that light rail transit lines have an exclusive or semi-exclusive guideway, there frequently are circumstances where the only practical or affordable alignment is to place the tracks within a public roadway in a mixed traffic environment. This is, of course, the sort of guideway configuration used by traditional streetcar lines since the 19th century. In the case of a modern streetcar project in an historic district, the project definition might actually stipulate mixed traffic operation because of some desired ambiance. Where these rail cars are operated in general traffic lanes, they are mixed with steerable, rubber-tired vehicles and should, to the greatest extent feasible, flow with traffic. They should not make any unusual or unexpected lateral movements, but rather function like the rubber- tired vehicles also using the street. However, there is a fundamental difference between rail cars and motor vehicles. Rail car operators can control the acceleration, deceleration, and speed of their vehicle so as to match the pace of traffic, but they cannot steer. Vehicle guidance is done by the tracks. This fundamental principle, which is well understood by the track designers, will not necessarily be so obvious to the civil/highway designers and the traffic engineers with whom they must interface. Collectively, the team must make certain that the goals mentioned above are achieved. Trackwork within streets creates significant discontinuities in the pavement surface by replacing longitudinal segments of conventional paving materials with steel rails plus adjacent open flangeways. To the maximum degree practical, these steel surfaces and openings should not create hazards to other users of the street, including pedestrians and rubber-tired vehicles of all types. Fitting trackage into locations where the street geometry imposes restrictions is challenging. Designers of the track, roadway, overhead contact system, and light rail vehicle all need to work closely with each other as well as with the project staff preparing the transit operations plan. These issues, which meld roadway, traffic, and track engineering, need to be resolved as early as possible in the design process. The designers in each discipline should work in concert as the conceptual design matures. 12.2 TRACK POSITION WITHIN LANES Typically, lane widths on urban streets are between 10 and 11 feet [3.0 and 3.3 meters] wide, occasionally narrower or wider. There are a number of factors that affect where, within these lanes, the LRT tracks might be placed. These include the actual lane width available, the dynamic width of the light rail vehicle, the presence or absence of station stops, and the presence of adjoining parking lanes. Occasionally, underground utility features that cannot be relocated may also constrain the position of the tracks.
Track Design Handbook for Light Rail Transit, Second Edition 12-2 12.2.1 Vehicle Width There is no standard streetcar or LRV body width, but it will always be less, usually by about 2 feet [0.61 meters], than the width of any lane in which a track is laid. This width difference offers some design flexibility in that the centerline of the track does not necessarily have to coincide with that of the traffic lane so long as the dynamic envelope of the rail car does not encroach into an adjacent travel lane. Moreover, there are good reasons to consider offsetting these two lines. 12.2.2 Transverse Position within Lanes Since the coefficient of friction between rubber and steel is lower than that of rubber on paving, especially when the roadway is wet, it is generally better for rubber-tired vehicles to travel with all tires on paving, rather than on steel rails. Offsetting the centerlines of the track and traffic lane facilitates this goal. The direction of the offset is immaterial; the intended effect is achieved either way. However, site-specific conditions (e.g., all stations are on the right side of the track) might indicate a preference for a particular juxtaposition of the two centerlines. 12.2.3 Adjacent Parking Lanes Situations in which a parking or curb loading lane is positioned alongside of the shared lane utilized by LRVs must be carefully considered. Typically, these lanes are no wider than 8 feet [2.44 m]âa dimension sufficient for an ordinary automobile, but not for a wide delivery truck. These trucks can have body widths of 8Â½ feet [2.59 meters], plus mirrors, and will thereby frequently overhang the line, even if the truck driver is diligent about getting the tires close to the curb. In such situations, if the position of the adjoining LRT track is biased toward the parking lane, it is likely that badly parked delivery trucks will foul not only the dynamic envelope of the LRV but perhaps even its static outline. In the latter case, if the truck driver cannot be found quickly, light rail service might be blocked for an extended period until the offending motor vehicle can be moved. In addition, since the LRV would be stopped in a general traffic lane, ordinary motor vehicle traffic could be impacted as well. Such situations may require that the parking lane width be larger than normal or that the track is biased away from the parking lane, or both. 12.3 ON-STREET STATION/STOP PLATFORMS If the design calls for the station platforms to be in the form of a sidewalk bulb-out, a new problem is created. Since ADAAG (and similar regulations in other countries) stipulates that the platform edge must fall within 3 inches [76 mm] of the threshold of the LRV doors, the platform edge could encroach a substantial distance into the traffic lane, where it would be a hazard to motorists. Obviously, this encroachment into the traffic lane is extremely undesirable. One well-known modern streetcar line has dozens of examples of this situation, and the leading edge of very nearly every platform is marred by impacts from rubber tires because some motorists realized too late that the platform encroached into their path. Note how this encroachment would exist even when the track is biased toward the platform side since the dynamic envelope of the LRV, which ordinarily should not be permitted to overhang the edge of the lane, is appreciably wider than the static envelope, which must match the platform. A
LRT Track in Mixed Traffic 12-3 compounding factor is that the typical platform edge (presuming a low-floor LRV) is 10 to 13 inches [0.25 to 0.33 m] above the top of rail and street surface, appreciably more than the maximum curb height endorsed by the AASHTO âGreen Book.â 12.3.1 Alternative Configurations Possible approaches to solving the problem of platform encroachment into traffic lanes include the following: â¢ Positioning the platform edge so that it is at, or very close to, the edge of the travel lane and biasing the track position in the traffic lane toward that edge. The drawback of this approach is that it places the static envelope of the rail car on the edge of the parking lane, where there could be a conflict with a badly parked vehicle, as noted above. â¢ As above, but increasing the width of the parking lane so as to prevent routine stationary vehicle conflicts. The drawback of this approach could be that motorists might erroneously presume that this extra space is part of the travel lane and collide with the platform anyway. Marking the outside edge of the parking/loading lane with an edge line might help mitigate this problem. Audible cues, such as rumble strip grooving of the pavement, might also assist in directing motorists back into the proper travel lane, although that detail might be disturbing to cyclists, particularly if the extra parking lane width becomes either an official or de facto bicycle lane. â¢ Transitioning the track from its normal position in the traffic lane toward the platform on the approach to the station and then back into the traffic lane on the departure end of a platform. A drawback of this technique is that it might introduce an LRT speed restriction at each station platform, regardless of whether there are any passengers to board or alight at that location. The transitional zones where the track is biased toward the platform would also become areas of possible conflict with parked vehicles as described above. Since it is typically necessary to keep the track tangent for a full LRV carbody module length ahead and beyond the platform, the length of the parking zone affected could be substantial and create problems in dense urban neighborhoods where mixed traffic operation is more likely and parking is usually at a premium. Arguably, the lateral movement of the rail vehicle might also confuse a motorist in a parallel traffic lane although this should be no more of a factor than a similar lane shift by a large truck or bus. â¢ Constructing a gauntlet track at each platform, so only those LRVs that need to handle passengers at that location need shift toward the platform. The problem with this approach is that it introduces significant additional complexity and cost into the track structure as well as additional steel into the pavement surface. It also presupposes that the LRV operator is diligent and can visually confirm the presence of a potential passenger on the platform sufficiently far in advance to throw the switch and make the lateral movementâsomething that seems rather unlikely. A âpassenger stop requestâ button at the platform, similar to an elevator request button, could provide the LRV operator with advance notice, but that would add further complexity and cost. It might
Track Design Handbook for Light Rail Transit, Second Edition 12-4 also be subject to misuse. Another issue with a passenger stop request button is where it should be placed so it can be readily found by the visually impaired. â¢ Allowing the platform encroachment into the traffic lane, but providing signage, pavement markings, rumble strips, and other cues telling the motorists that they need to shift toward the other side of the lane. The problem with this is that the path of steerable vehicles is, ultimately, at the discretion of their drivers. Delineating an irregular and unnatural path, such as a reverse curve, within an otherwise straight roadway lane does not guarantee that every motorist will steer that path. (On the other hand, if a reverse curve is designed into a track alignment, as discussed above, it can be guaranteed that every rail car will follow it.) Accordingly, permitting any platform encroachment into the general traffic lane is discouraged. â¢ In combination with any of the above options, designers might consider placement of either active or passive warning devices on the leading edge of the platform. This was common on legacy streetcar lines, where both the tracks and a âsafety islandâ loading platform might be one or two lanes away from the curbline. These warnings would typically consist of yellow blinking lights and illuminated signage directing motorists to the right or left of the platform. Such measures were not 100% effective at preventing motorists from colliding with the island, and a review of city newspaper articles from the trolley era will find more than a few photographs of the failures. 12.3.2 Motorist Perceptions One issue that compounds the platform interface problem is that motorists tend to associate any longitudinal line in the street as a visual cue for where the traffic lanes are located. In the case of lanes which include streetcar tracks, not only the rails, but also any pavement joints paralleling those rails may be interpreted by motorists as indications of where their vehicles should be positioned. Differences in roadway surface color or texture (such as between asphalt and concrete pavement or between concrete and brick pavement) will also often be interpreted as the location of the edge of a lane even when formal pavement markings indicate otherwise. On a dark rainy night, the shiny rails may well be the most prominent visual cue, and a motorist who is instinctively following their alignment might be unintentionally directed too close to the platform edge. Since there is nothing the track designer can do to make the rails invisible, it is highly desirable that pavement joints and other informal lane delineations in the vicinity of the shared LRT lane do not conflict with the formal pavement markings. This will often entail extending the track area paving material laterally beyond what is required for structural purposes. See Article 12.8.1 for additional discussion on this point. 12.3.3 Tracks Adjacent to Median Platforms If the street has a median that is also used for the station platforms, the problem is much easier to resolve. There is no abutting parking lane, and, the rail car, with its precise guidance, can run with its left side adjacent to an open median without danger of striking it. However, if elsewhere along the line the median area is used for a left-turn lane for general traffic, careful consideration must be given to potential clearance conflicts between the rail car, with its trajectory offset to the left very close to the edge of the through lane, and any vehicle in the left-turn lane.
LRT Track in Mixed Traffic 12-5 12.3.4 Conclusions The appropriate relationship of station platforms, lanes, and track location/alignment will not be the same on every project. Different solutions might be more appropriate for heavily trafficked boulevards versus quiet side streets, although there is something to be said for a uniform approach within any particular city. Track designers must work closely with the projectâs traffic engineers and station architects to determine the optimal solution for their project. Very often, the municipal traffic engineer will have veto power over this portion of the design, so it is advisable to include that agency in the discussions at an early date. 12.4 MINIMUM CURVE RADIUS A likely location for an in-street LRT operation is a built-up area such as a central business district (CBD) where the street geometry is well established and lined with buildings and other substantial structures. In zones of this type, there is usually very limited opportunity to alter street widths or increase the radius of corner curb lines to accommodate the track alignment design. The tracks therefore need to be fit into the available space. 12.4.1 Light Rail Vehicle Limitations In accomplishing this task, a major factor that comes into play is minimum track curve radius. It is possible to design tracks with relatively short radii. Curves with a 35-foot [10.67-meter] centerline radius were once common on streetcar networks, and some still exist. However, extremely sharp curves are generally undesirable for various reasons as cited in Chapter 4. In addition, the universe of available rolling stock becomes larger as the minimum curve radius increases. A common minimum turning radius for a majority, but not all, of current LRV models is 82 feet [25 meters]. Track curves of that radius are compatible with roadway intersection geometry where streets are wide, such as in the case of many European boulevards, but few business districts in the United States routinely have streets of sufficient width. Use of 82-foot [25-meter] radius curves in situations where the streets are not of generous width has been done, but it typically results in a rail car trajectory that is not very compatible with either normal traffic patterns or efficient rail operations and is not recommended unless the rail line can be situated in exclusive lanes, such as in a transit mall, where ordinary traffic need not be a design factor. If that is not possible, it might be necessary to either change the routing to avoid making turns at constrained intersections or to procure light rail vehicles that can negotiate tighter turns. 12.4.2 Vehicles for Small Radius Turns Several legacy streetcar systems still utilize very constrained track alignments that were fitted into existing street geometry a century or more ago. When it became necessary to procure new rolling stock for those routes, because the track geometry could not be redesigned to accommodate the minimum turning radius of many contemporary LRV models, the new cars had to be designed to accommodate the track geometry. Toronto has a well-established network of streetcar trackage with hundreds of curves with radii well below 82 feet [25 meters], dozens of which have a radius of about 36 feet [11 meters]. In 2008, when rail car manufacturers were
Track Design Handbook for Light Rail Transit, Second Edition 12-6 invited to propose on an order for a 204-vehicle fleet of new, 100% low-floor streetcars, designs compatible with the existing track geometry were forthcoming. Less recently, but still within the modern light rail era, a fleet of 112 rigid-body, high-floor LRVs was manufactured for operation on Philadelphiaâs streetcar lines, where the minimum curve centerline radius is 35.59 feet [10.85 meters]. The new car fleet was designed to accommodate these parameters. The legacy light rail systems in San Francisco and Boston have similar curve radius constraints. See Chapter 2, Article 2.4.1 for additional discussion on the topic of minimum vehicle curving limitations. 12.4.3 Overhead Contact Wire Considerations The overhead contact wire must be kept within a relatively narrow width above the rails so that the vehicle current collector can properly follow it. When the track is in a sharp curve, it is often necessary for the OCS to include supplemental âpull-offsâ between the vertical support points so as to keep the wires over the track, In extremely sharp curves of appreciable length, additional poles sometimes must be added. These extra pull-off wires and poles can add to the visual impact of the OCS, which may be at odds with the projectâs urban design goals. Hence, the introduction of additional curves in the track to facilitate traffic engineering goals may indirectly be at odds with other project objectives. The track designer may be in a central position to mediate such issues, even though the topic has relatively little to do with the track. 12.5 TURNING MOVEMENTS 12.5.1 Preferred Configuration At any given location, the track alignment should mimic the path that a steerable vehicle would follow in the same circumstances. For example, when a rail car executes a right turn, that maneuver should begin in the rightmost travel lane, and when a rail car executes a left turn, it should begin either in the leftmost lane or from an exclusive left-turn lane, if there is one. Turning movements must not âcut acrossâ adjacent lanes carrying through traffic unless the conflicting movements can be time-separated by signalization. 12.5.2 LRT-Only Traffic Signals It is important to understand that unless the rail car is originating its turn from an exclusive lane, temporal separation cannot be achieved by means of special traffic signal phases. If the LRV is in a mixed traffic lane, even if it is given a signal indication distinctly different from that of standard highway traffic signals, the necessary physical separation of the different movements would not exist. A motor vehicle traveling in a shared lane but stopped by a red signal would physically block the movement of any rail car behind it, even during the phase of the signal cycle when a special âproceedâ signal is displayed for the rail car. Similarly, a rail car halted by its âstopâ signal would block the path of a motor vehicle behind it, even if that vehicle had a green traffic signal. An additional concern is that inattentive motorists in either the same lane as the streetcar or an adjoining lane, upon seeing the rail car begin its movement, may well presume that they may also proceed even if a conventional aspect traffic signal clearly indicates otherwise. This could create conflicts with not only the rail vehicleâs path but perhaps also with any other vehicular traffic that is simultaneously executing a movement that is in accord with the traffic signals.
LRT Track in Mixed Traffic 12-7 12.6 CLEARANCE ENVELOPE AND SWEPT PATH IN CURVES As explained in Chapters 2 and 3, on a curved alignment the dynamic envelope of a rail car is widened. The outside corners at the ends of the body move farther from the track centerline on the outside of the curve while the center of the body (or each carbody segment, if there are more than one) moves farther from the centerline on the inside of the curve. 12.6.1 Difference from Rubber-Tired Traffic For vehicles of any given length, the dynamic envelope of a rail car sweeps over a narrower overall area than does a steerable vehicle of similar length. This is because, unlike a truck, bus, or automobile, the rear wheels of a rail car follow exactly the same path as the front wheels. This minimizes the width of the swept area. However, this characteristic also has a negative consequence, which is discussed below. 12.6.2 LRV Tail Swing When a rail car enters a curve from tangent track, the portion of the car body behind the rear axle will swing away from the track centerline in the direction opposite from that of the turn. If the rail car turns toward the right, the left rear corner will swing toward the left. It is imperative that this âtail swingâ does not encroach into an adjacent travel lane. This usually can be avoided by designing a long spiral at the entrance to the curve, possibly including a short section of skewed tangent alignment, so that the swing does not reach its full extent until the rear axle has moved laterally a sufficient distance in the direction of the turn to keep the swing within the marked traffic lane. In certain situations this might require some adjustment of lane widths, which is another example of the need for the track and roadway designers to work in concert. The potential negative impact of tail swing can also be mitigated by adopting a vehicle body design with tapered ends. This can significantly reduce the width of the dynamic envelope on turns. For this, the cooperation of the vehicle designers is obviously required. As noted above, tail swing should never encroach into an adjacent travel lane. However, there could be situations in which a tail swing into a narrow curb loading or parking lane would be difficult to avoid. This would almost always occur at the end of the parking/loading lane, in the immediate vicinity of an intersection. This is a zone where stopping in the curb lane is normally prohibited for a distance from the cross street that is designated by ordinance or statute. In practice these âcorner clearancesâ do not receive a high degree of respect from motorists, even where prominent signage and pavement markings are used. Some motorists rationalize that they will not be penalized for parking âjust a little bitâ into the zone, or stopping in it âbriefly.â These individuals need to understand that, even in the absence of a traffic citation, their vehicle may be vulnerable to damage from the tail swing of a turning rail car. The edge of the dynamic envelope, if it protrudes into the curb lane, should be marked with a line defining the area swept by the tail of the turning rail car. This might be supplemented by specialized
Track Design Handbook for Light Rail Transit, Second Edition 12-8 signage, such as that shown in Figure 12.6.1, to give motorists who are considering stopping in a curb lane a clear indication of the potential consequences of parking within this zone. A secondary benefit of physically marking the edge of a dynamic envelope on curves is to provide the rail car operator with a clear indication when a stationary motor vehicle has encroached into the swept path of the LRV. (This is generally unnecessary for experienced streetcar operators, who usually develop a good eye for detecting whether or not they can get past a potential encroachment.) On a few LRT projects a different paving material has been used to delineate the edges of the trackway, usually as an artistic statement rather than a traffic engineering control. This practice can tend to deemphasize the standard pavement markings and should be avoided. This is discussed further in Article 12.8 of this chapter. Figure 12.6.1 Custom signage for tight clearance zones 12.7 STREET DRAINAGE, CROSS SLOPES, AND SUPERELEVATION Where tracks are embedded in roadway paving, attention must be given to drainage of storm water. In urban roadway design, this is usually addressed by using a pavement cross section that slopes away from the center of the street and placing gutters and inlets at the curb with connections to a storm drain system to collect the gutter water. 12.7.1 Flangeway Drains When embedded rails are included in the roadway, some storm water will be captured in the flangeways and flow longitudinally along the rails rather than transversely into the gutters. Special provisions, such as track drains, may be needed to deal with this runoff. The frequency of these flangeway drains can vary depending on circumstances. In all cases, they should be located at the low points of sag vertical curves and immediately upstream of any embedded special trackwork. LRT lines in frostbelt climates must carefully consider the probability of water freezing in the flangeways on streets with flat grades, thereby risking a derailment.
LRT Track in Mixed Traffic 12-9 12.7.2 Roadway Crown and Track Cross Slope The roadway crown, which is an essential element of the storm water drainage system of any street or roadway, often means that any tracks embedded in a mixed traffic lane will have something other than zero cross slope between the rails. Typically, the roadway is crowned on a 2% cross slope, resulting in about 1 1/8 inch [29 mm] of cross slope in the track. In tangent track, up to about 1 Â½ inches (38 mm) of cross slope can be allowed without making the passengers feel uncomfortable. This includes both tracks in curves and those on a tangent alignment. Where the alignment is curved, the roadway design (per the AASHTO Green Book or other adopted standard) will typically dictate the superelevation of the tracks, not the formulae used for tracks in exclusive right-of-way. Sometimes this means that the value of Eu may be higher than the desirable maximum. 188.8.131.52 Codes and Jurisdictional Issues If the street maintains a normal crown through both tangents and curves, there will be situations in which the outside track of a double-track line will actually have unavoidable reverse superelevation, exacerbating the natural effect of centrifugal force rather than compensating for it. While this negative superelevation may at first glance appear to be at odds with 49CFR213, paragraph 213.57, it is consistent with paragraph 213.63. Moreover, embedded track in the street used only by light rail vehicles is not currently under the jurisdiction of the U.S. Federal Railway Administration. Hence, deliberate inclusion of negative cross slope in a track used only by LRVs is not a violation of 49CFR213 or any other code. In particular, it should be noted that since embedded track is a rigid trackform, as-constructed cross slope is extremely unlikely to ever change. The same cannot be said about ballasted track since once a negative cross slope condition develops in ballasted track, it is highly likely to get worse. It was that prospect that led to the adoption of paragraph 213.57 in the FRA Track Safety Standards. It should also be noted that the impact of a slight negative cross slope on passengers is no different than it would be with transit buses operating in the same lane as the track. In curved track, any adverse cross slope on the outer track of a curve must be factored in as a reduction in the allowable Eu for purposes of determining the allowable speed on the curve. However, it must be included in determination of the minimum spiral length. 184.108.40.206 Streets with Parabolic Crowns The aforementioned cross slope usually does not exceed an acceptable gradient of 2% unless the street crown is a parabolic curve. Although that design is no longer common, where it exists, the cross slope in the inner lanes of the street could be very close to flat while those in the curb lanes could be considerably steeper than 2%. If the track is proposed to be in the curb lane of a street with a parabolic crown, the steeper cross slope at that location may be more than can be acceptable, thereby requiring some reconstruction of that part of the street, with associated changes in drainage patterns, etc. However, if the existing curb heights are substandard because of decades of street resurfacing, the LRT project may be forced into substantial reconstruction of not only the lane proposed to be
Track Design Handbook for Light Rail Transit, Second Edition 12-10 occupied, but also the adjacent curb and sidewalk on one side and the through traffic lane on the opposite side. This can become particularly problematic in urban areas where buildings and doorway thresholds directly abut the rear edge of the sidewalk, making it difficult or perhaps even impossible to raise the grade of the sidewalk. 220.127.116.11 Special Trackwork Cross Slope Even when the straight and curved track conforms to the normal crown of the street, it may be necessary to have a zero cross slope in segments where special trackwork is installed. In such cases, the track will need to warp from the ordinary cross level condition to zero cross slope over some distance. That distance will be dictated by the allowable twist for the design LRV. 12.8 PAVEMENT DESIGN FOR IN-STREET TRACKAGEâSEAM LOCATIONS For a variety of reasons, concrete is commonly used as a paving material for embedded track in lanes shared with general traffic and at crossings. The use of concrete paving in the track area of a street otherwise paved with another material can have traffic impacts. Construction of embedded track is expensive and disruptive to the community. Because of those factors, it is not unusual for projects to limit the width of the track slab to the maximum degree physically possible. On some streetcar projects, the track slab has been less than 8 feet [2.4 meters] wideâfar less than that of the traffic lane containing the track. However, there are several good reasons why track slabs should be appreciably wider than the absolute minimum necessary to construct the track. The seam of the two paving materials of disparate appearance (e.g., dark asphalt adjacent to a light gray concrete) is very likely to be wrongly interpreted by some motorists as an intended traffic marking such as a lane line, edge line, or stop line. This possibility should be taken into account when designing the details of the trackway paving. If a track is located in a travel lane next to a curb lane used for parking or loading, the seam could be interpreted as a clearance edge line. In this situation, a true edge line would serve two purposes. One would be to define the edge of the travel lane for the moving traffic. The other would be to define the outer limit of the curb lane within which vehicles may be safely parked or stopped for loading. If, for construction reasons, the minimum width of the track structure is such that it would produce a seam that would be inside the dynamic envelope of the rail cars, the resulting appearance could provide false assurance to motorists stopping or parking in the curb lane that their vehicle will be clear of the path of the LRV provided it does not span the seam. To address this, even where not required for structural reasons, the concrete paving used in the track lane should, at a minimum, extend for 6 to 9 inches [0.15 to 0.23 meters] beyond the outside edge of the dynamic envelope and preferably all of the way to the edge of the actual travel lane. Similar problems could arise between two travel lanes. If the track slab is narrower than the lane in which it is located, the motorist could be presented with up to six longitudinal visual cues as to where the lane is located:
LRT Track in Mixed Traffic 12-11 â¢ The two edges of the actual lane, presumably delineated by pavement markings such as traffic paint. â¢ The two edges of the track slab. â¢ The two rails. Such visual clutter presents the motorist with too many choices about where to position his/her vehicleâ66% of which are misleading. To simplify things, it is highly desirable that the edges of the track slab match the location of the standard traffic markings separating the shared lane from the adjacent lanes. If concrete paving extends beyond the area between the two rails it should occupy the entire lane width. This reduces the visual cues concerning lane location by 33%, making it more likely that motorists will follow a predictable path. Ideally, the paving material in a lane shared by rail cars and general vehicular traffic should be the same as that used in the adjacent lanes. This allows the standard markings to be the predominant guidance for motorists. 12.9 SPECIAL TRACKWORK IN STREETS Ideally, switches should not be located in a mixed traffic lane. There are several reasons for this, including some having to do with costs. Perhaps the most important reason is that a switch in a mixed traffic lane needs to be inspected and maintained on a regular basis. Those activities expose the maintenance employees not only to the hazards associated with working along any active railway track but also to the dangers inherent in working in a public street. For that reason alone, the track alignment should seek to locate switches in some form of exclusive right-of-way. That is, however, not always possible. When such is the case, the track switches for rail car operations in trafficked lanes must be compatible with the roadway environment. Two elements need to be considered. 12.9.1 Switch Hardware The switch itself must be one that is designed for use in a paved street, not a railroad switch adapted for an unintended use. Flexive tongue switches of European design that are compatible with street environments are commercially available from multiple sources, including some North American manufacturers. These designs take into account that the points will be driven over by rubber-tired vehicles. The gaps between the point and the rail, which are inherent and unavoidable elements of every track switch, are kept to a minimum so that the tire widths of motor vehicles are sufficient to span them. 12.9.2 Switch Location Moreover, the primary challenge for the track designer is not the selection of the proper hardware, but how these switches are deployed in a street setting. Both vehicular and pedestrian movements need to be considered, and one can affect the other. 18.104.22.168 Hazard Issues Track switches of any type, even those designed for use in trafficked roadways, are incompatible with areas where people walk. The gaps between the movable points and other rails are
Track Design Handbook for Light Rail Transit, Second Edition 12-12 potential hazards. The risk that the âholeâ on the walking surface might cause pedestrians to trip is obvious. In addition, the housing of certain types of switches has a metal surface that is much wider than a single rail. Such surfaces can be slippery when wet. However, there is an even more serious concern. Customarily, there is a power mechanism that moves the points from one position to the other, closing the gap on one side of each point and opening one on the other. If the foot of a person (or an animal, such as a guide dog) is in that gap when it the switch is being remotely thrown by the power switch mechanism, or even on top of the switch blade, severe injury would likely result. For this reason, switch points should never be located either in or close to a marked crosswalk or any other legitimate pedestrian path. In the event that a preliminary design indicates that the ideal location for the points of a switch would be in or close to a pedestrian crossing, one or the other should be repositioned. 22.214.171.124 Pedestrian Crosswalk Locations There is not much flexibility in pedestrian crossing design. Best practice calls for designing a pedestrian crossing so it follows the path along which people would most naturally walk in the absence of a physical constraint. If a formal crosswalk is shifted too far from a natural walking route, a significant proportion of pedestrians will ignore it and instead take the natural path. Furthermore, crosswalk markings are not visible when covered with snow and can also be less noticeable during hours of darkness, especially if the road surface is wet and reflecting street lighting. For these reasons, it is best that pedestrians have no reason to ever encounter track switches and other special trackwork components, such as the frogs where two rails intersect. 126.96.36.199 Advance Switch Positions In addressing this concern, there is more flexibility in track design. Should the ideal location for the switch point be in a crosswalk or other legitimate pedestrian path it might be possible to reposition it a few feet upstation or downstation simply by altering the curve radius within reasonable limits. An alternative might be to install the switch points a safe distance ahead of the crosswalk with the frog beyond it, connecting the two with a section of gauntlet track as shown in the photo presented as Figure 12.9.1. In that photo, a double tongue flexive switch has been installed nearly 130 feet [40 meters] in advance of the frog of the turnout. This position achieved three goals: â¢ The switch is not in a pedestrian crosswalk, which can be seen at the top of the photo. â¢ The switch is not in a section of curved track, which would require a custom fabrication. â¢ The switch is not in the middle of a street intersection, where inspection and maintenance would be hazardous.
LRT Track in Mixed Traffic 12-13 Figure 12.9.1 Advance switch