Total Transportation System Solutions (T2S2)

INTRODUCTION

Evaluating and comparing the operation of different non-freeway roadway facilities is not as easy as it sounds.  There are numerous different ways to analyze the same non-freeway facility, all of them correct, and all yielding different results.  For example, under the Highway Capacity Manual, a multilane facility with signals could be evaluated under the signalized intersection method, the arterial method, or the multilane facility method, depending on the preference of the evaluator, with each method yielding a different result.

Because of the existence of numerous, correct, ways to evaluate the operational performance of the same facility, it is difficult, but not impossible, to determine exactly how a given facility is truly operating.  However, in order to maximize the benefits received from the investment of scarce and limited transportation dollars, most capital investment decisions involve a comparison of many different facilities, with the dollars being allocated to those facilities that will provide the greatest benefit.  Because of the numerous different methodologies available to analyze just one facility, the large number of permutations and combinations of these different methodologies when analyzing and comparing multiple facilities makes such comparisons daunting, if not impossible.  The difficulty in performing such an analysis is even greater when multiple jurisdictions, each with its own preferences in methodologies, are involved.  Thus, there is the need for a uniform system to evaluate roadway performance.

The purpose of this methodology, then, is to provide a tool to allow the operational comparison of numerous roadway facilities, for the purpose of effective distribution of transportation funds.  This methodology is an amalgamation of several different methodologies, most notably the Highway Capacity Manual methodologies and the 1988 Florida Department of Transportation methodologies, and is intended to provide a snapshot of how a facility is operating.  It is intended only to provide a general overview, and is not intended to be used to provide an analysis as detailed as any of the specific methodologies from which it was created.  Because the purpose of this methodology is only to provide a general snapshot, some factors affecting operational performance, such as the grade and the percentage of heavy vehicles, have not been included for the sake of simplicity.

DATA REQUIREMENTS

 The data required for this methodology is typically readily available and easily obtainable, thus making the methodology accessible even to the smallest organizations.  The input data required is as follows:  facility name, facility length, facility type, number of lanes, number of signals, narrowest lane width, minimum distance to any lateral obstruction along the facility, type of operation (i.e. one-way vs. two-way), the presence of medians, the presence of left-turn bays, average daily traffic, and the number of accidents on the facility per year.

Basic identification

This consists of the facility name and limits, including the length of the corridor in miles.  When determining the facility limits, the beginning and end should not be at a signalized intersection where possible, as a signal penalty is assigned and too many signals may cause artificially depressed results.

Facility type

As this methodology was intended for non-freeway facilities, the user is given the choice of four types of functionally classified non-freeway facilities:  collector, minor arterial, principal arterial, and parkway/expressway.  It should be noted that since, under ideal circumstances, the operational performance of local streets should not be of concern, evaluation of a local street is not given as an option.

Number of lanes

The number of lanes in the corridor is the minimum number of through lanes at any point along the facility.  As the operational potential of a facility is predicated by the minimum operational potential at any given point along the facility, then this minimum value will control the operational potential of the whole.  For example, if a facility consists of two multilane segments with a two-lane segment between them, the operational potential of the whole corridor is determined by the operational potential of the two-lane segment.  Because of this limitation, then, the multilane segments cannot meet their full operational potential.  In other words, the multilane segments will not operate at the maximum level of a multilane segment but rather at the maximum level of a two-lane segment.  By using the minimum number of through lanes at any point along the facility, then, the methodology provides a snapshot of the effect of a choke-point or bottleneck on a facility’s operation.  It should be noted that auxiliary lanes such as turn lanes and acceleration and deceleration lanes are not considered to be through lanes for the purposes of this methodology.

Number of signals

 The methodology assigns a penalty based on the number of signals per kilometer or mile -- the greater the number of signals, the greater the penalty.  Therefore, when possible, it is advisable to establish the facility termini at locations other than signalized intersections.  If it is not possible to have termini away from signalized intersections, then the number of signals in the corridor includes the signals at the termini if the facility being analyzed would have the right-of-way if the terminal intersection were unsignalized.

Minimum lane width

The operational potential of a roadway facility is affected by the lane width in that the narrower the lanes, the lower the potential.  Analogous to the limitation on operating potential caused by the minimum number of through lanes at any point along the facility, the operating potential of the entire facility is also limited by the narrowest through lane width at any point along the facility.  The methodology’s use of the narrowest lane width provides insight into the lost potential of the entire facility.

Lateral distance to nearest obstruction

Obstructions near the road affect driver behavior and rob a facility of operating potential.  As with the number of through lanes and lane width, the minimum lateral distance to an obstruction at any point along a facility will limit the maximum operating potential of the entire roadway.  The methodology provides insight into the effect of the closest lateral obstruction on the operational performance of the entire facility.  It should be noted that, due to the psychological effects of curb inlets and barrier curbs on motorists, both curb inlets and non-mountable barrier curbs are considered to be lateral obstructions for the purposes of this methodology.

One-way operation

It is common knowledge that one-way operation increases operating potential by eliminating conflicting turning movements.  Ideally, a facility that has both one-way and two-way operation should be analyzed as two separate facilities.  However, if such a separation is not possible or practical, then the entire facility should be analyzed as a two-way facility, the type of facility with the least operating potential.

Median

By reducing potential conflicts, medians, both flush and raised, increase operating potential.  A raised median of any width will provide this benefit, as well as a flush median, either paved or turf, at least 3.05 meters (10 feet) in width.  Note that if the raised median has a non-mountable curb, and that curb is less than 1.83 meters (6 feet) from the marked driving surface, then the lateral distance penalty applies.

Left Turn Bays

As with medians, left turn bays increase operating potential by eliminating potential traffic conflicts.  The methodology considers a left turn bay as any marked or delineated left turn storage area.  It should be noted that flush paved medians do not, in and of themselves, constitute a left turn bay.

METHODOLOGY COMPUTATIONS

The ultimate output of the methodology is a number between 0 and 100 called the Total Quality Rating (T.Q.R.), with 0 being the worst and 100 the best.  The T.Q.R. is the average of the Accident Quality Rating (A.Q.R.) and the Volume/Capacity Quality Rating (V.Q.R.), as both accident frequency and congestion are important indicators of perceived roadway quality.  If motorists perceive a roadway to be functioning poorly, it will not be used.  The concept behind the methodology is to maximize use by improving its perceived and actual functionality, in other words, its total quality, with the T.Q.R., A.Q.R., and V.Q.R. being effective tools to evaluate different strategies for maximizing operation potential.

FIGURE 1.  Methodology spreadsheet

Accident Quality Rating (A.Q.R.)

Like the T.Q.R., The A.Q.R. is a number between 0 and 100, with 0 being the worst.  The A.Q.R. is calculated by subtracting the Accident Rate per million vehicle miles from 100, and then rounding the result to the nearest integer.  If the accident rate is greater than 100, the A.Q.R. is the minimum value, zero.  The calculation of the A.Q.R. is shown on the sample spreadsheet in Figure 1.

Volume/Capacity Quality Rating (V.Q.R.)

The V.Q.R., like the T.Q.R. and the A.Q.R., is also a number between 0 and 100, with 0 being the worst.  The V.Q.R. is calculated by subtracting 100 times the adjusted Volume-to-Capacity Ratio from 100, and then rounding the result to the nearest integer.  If the adjusted Volume-to-Capacity Ratio is greater than one, then the V.Q.R. is the minimum value, zero.  The calculation of the V.Q.R. is a multi-step process shown on the sample spreadsheet in Figure 1.

Step One:  Determine unadjusted capacity

The unadjusted capacity values are given for each type of facility and are very general “rule-of-thumb” values assigned as follows:

                        Expressway/Parkway -- 8000 vehicles per day per lane (vpdpl)

                        Principal Arterial -- 7000 vpdpl

                        Minor Arterial -- 6000 vpdpl

                        Collector -- 5000 vpdpl

The unadjusted capacity for a given facility is then calculated as the number of through lanes multiplied by the assigned value per lane by facility type.

Step Two:  Adjustments

After the unadjusted capacity is determined, applicable adjustments are then applied.  The adjustments are as follows:

Lane width penalty.  A reduction of 16.4 percent per meter (5 percent per foot) is applied for every meter (foot) of a lane less than 3.6 meters (12 feet) in width.  For example, a 3.35 meter (11 foot) lane would have its unadjusted capacity reduced by 5 percent.  This value, 16.4 percent per meter of width less than 3.6 meters (5 percent per foot of width less than 12 feet), is a linear approximation of the lane width penalties found in the Highway Capacity Manual.

Lateral Clearance Penalty.  A reduction of 5.48 percent per meter less than 1.8 meters (1.67 percent per foot less than 6 feet) is applied for lateral obstructions closer than six feet to the traveled way.  An obstruction 1.2 meters (4 feet) from the traveled way would result in a reduction of 3.33 percent of unadjusted capacity.  As with the lane width penalty, this penalty is also a linear approximation of the penalty given in the Highway Capacity Manual.

One-way operation.  A 20 percent bonus is given for one-way operation.  This value was derived from the Florida Level of Service standards.  It should be noted that this bonus is applicable only if the facility is one-way in its entirety between the two termini.  For example, if a facility is two kilometers long, and operates as a one-way for 1.9 of those two kilometers, it will not receive the 20 percent bonus, because it is not one-way throughout its entirety.  Just as with the number of lanes, lane width, and lateral clearance, the minimum operational standard, in this case the two-way operation, controls, regardless of the magnitude of that standard.  Thus, if you had a facility that was 100 kilometers long, and 99 kilometers were one-way, it would be evaluated as a two-way even though the two-lane portion represents only one percent of its entire length, because the two-lane section, as small as it is, would regulate the operation potential of the entire 100 kilometer segment.  Again, this methodology allows the user to understand the effect of one single chokepoint, in this case the one kilometer of two-way operation, on the entire facility.

Medians.  The unadjusted capacity of a facility is reduced five percent if it does not have a raised median of any width or a flush median, either turf or paved, less than 3.05 meters (10 feet) in width.  Like the one-way operation bonus, this penalty was derived from the Florida Level of Service Standards.

While raised medians and flush medians serve basically the same function, to separate opposing vehicles and prevent head-on collisions, narrow flush medians do not provide adequate recovery room for errant motorists to effectively achieve this goal, which is the rational for assigning the median penalty to facilities with a narrow flush median.  Realistically, a flush median of less than 3.05 meters will not provide any more protection from a head-on collision than the traditional double solid yellow line, as there is not enough room or time for the driver of an errant vehicle to recognize that the vehicle is crossing into oncoming traffic and to take the appropriate evasive action.  On the other hand, a narrow raised median, such as concrete barrier, is still effective at preventing head-on collisions.  In addition, raised medians, no matter how narrow prevent perpendicular movements, thus improving traffic flow and eliminating another possible type of collision. By reducing the number of collisions, raised medians reduce the accident rate.  The net result is that with a lowering of the accident rate, the A.Q.R., and, consequently, the T.Q.R., increase.

Because of the direct beneficial effect of raised medians, no matter how narrow, on both capacity (by eliminating perpendicular movements) and the accident rate (and, with it, the A.Q.R. and the T.Q.R.), there is no penalty for the use of any raised median, no matter how narrow, as contrasted to the penalty for the use of a narrow flush median.

Left Turn Bays.  The unadjusted capacity of a facility is reduced fifteen percent if it does not have left turn bays.  Like the one-way bonus and the median penalty, theLeftTurnBay penalty is also derived from the Florida Level of Service Standards.

The rationale behind this penalty is straightforward – left turn bays eliminate the delay caused by stopped vehicles wanting to turn left, as stopped vehicles rob a facility of capacity.  The more that vehicles are stopped, the fewer the number of vehicles that can go through and the longer the inconvenience to other motorists.  In addition to the improvements to the capacity realized by left turn bays, the accident rate, and consequently the A.Q.R., is also improved due to the reduction in rear-end collisions.

Signals.  A penalty of 4.5 percent is assigned for each signal per 1.6 kilometers (1 mile).  This value is a linear approximation of the tables showing the effect of signals on capacity given in the Florida Level of Service Standards.  Being a linear approximation of several tables, this penalty is not an accurate reflection of theFlorida tables on facilities with low or high signal frequencies, where it tends to under penalize and over penalize, respectively, and should not be used in lieu of theFlorida tables.  However, because it provides consistent and uniform results, it is useful in comparing and contrasting different facilities, as any differences in results when compared to the Florida tables are consistently applied, and thus, for the purposes of this methodology, are moot.

Step Three:  Determine adjusted capacity and V/C Ratio

To determine the adjusted capacity, the adjustments enumerated in Step Two above are added and subtracted from the unadjusted capacity calculated in Step One.  The resulting value is the adjusted capacity.

To compute the V/C Ratio used in the calculation of the V.Q.R., the existing traffic volume of the facility is divided by the adjusted capacity.  A V/C Ratio in excess of one is treated as a V/C ratio of one for the purposes of computing the V.Q.R.

CONCLUSION

This methodology provides a quick and easy method to gain some insight into the operation and performance of not only a single non-freeway facility, but multiple facilities.  By providing a consistent and uniform methodology for analyzing facilities, comparisons can be made, which will allow for the more efficient allocation of scarce transportation dollars.  Because of its relative simplicity, this methodology can be utilized by jurisdictions of all sizes with staffs of varying degrees of technical expertise.  It should be noted, however, that this methodology is merely one tool that can be used in evaluating roadway facilities, and is not intended to supersede or replace any other evaluation methodologies.  Although the adjustment factors may not yield the same results as the methodologies form which they are derived, the adjustment factors will yield, in and of themselves, consistent results, and thus are valuable and useful in comparing and contrasting different facilities.

This methodology provides a way to gain some insight into the effect of choke-points and bottlenecks on the operation of an entire corridor.  By using the minimum values with respect to lane widths, number of lanes, lateral obstruction clearance, and type of operation, this methodology yields a realistic snapshot of the diminished performance of an entire corridor caused by the presence of one bottleneck, regardless of its magnitude.  This is a realistic portrait, as one small bottleneck, such as pavement narrowing or fewer lanes, can have a significant and devastating effect on overall performance of the facility.

By incorporating signal frequency, this methodology provides the user with an insight into the potentially devastating effects of traffic signals on performance.  Field observations have shown that traffic signals are a significant cause of delay and traffic congestion; yet, many methodologies do not take them into consideration.  Thus, one can have a facility that, theoretically, should provide superior service but yet does not, as many operation performance methodologies consider only the approaches to a signal without ever considering the effect of the signal itself.  There are numerous locales that experience significant increases in cut-through traffic on local streets in the vicinity of signals.  With this methodology, by taking into account the lost capacity created by signals, such locations are easier to identify with the identification of low performing collectors, arterials, expressways, and parkways.  Although this methodology cannot and was not intended to yield the detailed analysis available with specialized signal analysis methodologies, it does help identify facilities that need further study.

Finally, by yielding one number, this methodology provides a way for non-technical individuals, such as political leaders and average citizens, the ability to make their own inferences as to how a facility is performing, where problems in the transportation network are occurring, and what strategies should be implemented to improve performance, and what the effects of those strategies will be.  This methodology, as a planning methodology, allows the analyst to experiment with numerous “what-if” strategies without having to spend monies to test these strategies in the field.  The net result, then, is that scarce financial resources will only be spent on strategies that will be the most effective.

For a zip file with methodology instructions and Excel spreadsheets, click here.

CITE AS:

Schrader, M.H. (2002)  A Q&D method for evaluating roadway performance.  Total Transportation System Solutions.

FIGURE 1.  Methodology spreadsheet.