SOME FACTORS INFLUENCING THE REGULARITY OF SHORT HEADWAY URBAN BUS OPERATION*

Transcription

1 NZOR volume 4 number 2 July 1976 SOME FACTORS INFLUENCING THE REGULARITY OF SHORT HEADWAY URBAN BUS OPERATION* W.J, Frith M inistry of Transport W ellington S u m m a r y T h e i m p o r t a n c e o f r e g u l a r i t y i n the r u n n i n g o f s h o r t h e a d w a y b u s s e r v i c e s is d i s c u s s e d, a s is t he p r o p a g a t i o n m e c h a n i s m o f h e a d w a y d i s t u r b a n c e s w h i c h p r o d u c e i r r e g u l a r i t y. S u r v e y s c a r r i e d o u t in N e w c a s t l e U p o n T y n e to i n v e s t i g a t e f a c t o r s a f f e c t i n g r e g u l a r i t y a r e d e s c r i b e d, a n d the f a c t o r s m o s t p o s i t i v e l y i d e n t i f i e d a r e l i s t e d. T h e r e l a t i v e i m p o r t a n c e o f t h e s e f a c t o r s is d i s c u s s e d, a n d c o n s i d e r a t i o n is g i v e n to the a p p l i c a b i l i t y o f the s u r v e y m e t h o d a n d c o n c l u s i o n s to o t h e r b u s s e r v i c e s, p a r t i c u l a r l y t h o s e in N e w Z e a l a n d. T h e m a i n c o n c l u s i o n s a r e : 1. U n s a t i s f a c t o r y t e r m i n a l h e a d w a y s w e r e i m p o r t a n t c o n t r i b u t o r s to i r r e g u l a r i t y. T h i s w a s a t t r i b u t e d m a i n l y to u n r e a l i s t i c t i m e t a b l e a l l o w a n c e s i n d u c i n g d r i v e r s to l e a v e e a r l y, a n d to s c h e d u l e d b u s e s not r u n n i n g. 2. T h e v a r i a n c e s o f t o t a l s t o p - t i m e a t b u s s t o p s a n d t o t a l r u n - t i m e b e t w e e n b u s s t o p s w e r e r o u g h l y the s a m e, s u g g e s t i n g t h a t m e a s u r e s to c u r b the v a r i a n c e o f e i t h e r q u a n t i t y w o u l d h a v e s i m i l a r e f f e c t s o n r e g u l a r i t y. Glossary Boarding determined stops: are stops where the time spent at the stop is determined by the boarding passengers without interference from those alighting. Dead-time: refers to that part of the stop-time during which no passengers are boarding or alighting. Journey time: refers to the total time spend by a bus on a route section. Marginal boarding time per passenger: refers to the average time taken for a passenger on board. * Manuscript submitted on April 9, 1976; revised on May 21, 1976.

2 144 Run-time: refers to time spent on the route between bus-stops. Stop- time: refers to time spent stationary at a bus-stop. 1. Introduction The operation of any urban bus route is affected by many variables. A major objective of research should be to identify and weight these variables and find ways to control them so that the route runs more regularly. Route Regularity Surveys of bus users [6] have usually shown that regularity is a bus service attribute highly ranked by bus users. London Transport users ranked service attributes in the following order of importance (assuming fares are held constant): (a) (b) (c) (d) (e) (f) Regularity Frequency Shelters More routes Comfort of vehicle Speed Bus users want a bus to appear at a stop when it is advertised to appear, and they want to arrive at their destinations reasonably close to their expected times. There exist quantitative measures of bus regularity [2] which are based on a macroscopic view of route operation; but these were not used in the study described here, where regularity was approached from the more microscopic level of the interactions between individual buses. The mechanism producing bus route irregularities Once the equality of headways in a bus service is disturbed, the disturbance is likely to propagate until buses come together to form bunches. The mechanism by which this occurs has been described in (4] and [5].

3 145 Basically what happens is that, if one bus is behind time, more passengers will collect and hoard it along the route than would had it been on time. It is thus further delayed. Similarly, the following bus, if initially on time, has fewer passengers to pick up and covers ground more quickly. The result is that the buses come together in a bunch. A similar effect can occur when alighting. A crucial factor can he the event which causes the initial headway disturbance. Depending on the characteristics of the service, this can come about from: (a) (b) (c) (d) scheduled buses not running; buses leaving term ini at the wrong time; congestion or some other factor causing large differences in the run times of successive buses between stops ; unusually long boarding and alighting times. Other factors which tend to be important, not because they cause an initial headway disturbance, but because they have a continual effect on regularity along the route, are: (e) (f) (g) (h) (ij bad routing strategy; between-driver variation in boarding speed and alighting speed, and running speed, and variability of these differences with different driver pairs; the use of different types of bus and fare-collecting equipment on the same route; variation in the number of passengers desiring to board and alight at each stop; the slower the boarding speed the greater this effect; non-optimal bus stop placement. In order to study the effects of these factors, surveys were conducted on the running of irregular short headwav urban bus routes in Newcastle Upon Tyne I 31. Figure 1 is a time distance diagram of the trajectories of morning peak buses on a section of one of the routes surveyed and illustrates the extent of the problem on this route.

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5 147 1C % time in minutes UC % 60 a<5b refer to buses 2 U Distance measured in bus stops 7 - e. 9-1C< U. 15' C-

6 Survey Method The surveys were done by placing single observers on all the buses running on a route during a typical weekday morning peak period. Each observer was equipped with a stopwatch accurate to 1/100 of a minute, and a survey sheet on which to take observations. The observers noted the times at which buses stopped at each bus stop together with the number of boarders and alighters and also the times of arrival and departure at other known delay points. The survey sheets listed bus stops and known delay points, and space was provided for comments about anything unusual which might occur. The buses used were two-door one-man operated I.eyland doubledeckers. The only route control measures used on the routes surveyed were adjustments made by isolated inspectors stationed at bus stops. These adjustments were infrequent and appeared to be largely ineffective. 3. Survey Results The results of the survey indicated that all the factors mentioned above were to some extent affecting regularity. The factors most positively identified were: Driver Behaviour Drivers often did not leave termini at the scheduled times. This can be seen from Figure 1, where the zero point is a terminus, by comparing the actual leaving time with the scheduled leaving time. In addition, the following results were obtained for a sample of fifteen morning peak buses scheduled to leave their terminus at a six minute frequency. "On time" is defined as leaving within half a minute of the scheduled time. Apart from general driver indiscipline and lack of control by the operator, the main reason why drivers left early appeared to be the unrealistic nature of the journey-

7 149 No. of buses on time No. of buses leaving early No. of buses leaving late Mean deviation from schedule Minutes o of scheduled headway time allowance. ('Hie average joumcy-timc on the route segment in question was 52 minutes while the timetable allowance was S1 minutes). Differences between drivers in run-speed and boarding rate also appeared to exist. Figure 2. shows two successive buses travelling with diminishing headway on a route section. Run-speed On the above section the following bus (a) had run-times consistently shorter than the leading bus (b). This contributed 1.8 minutes to the reduction in headway over the section, and could be attributed to differences between drivers. Boarding Rate Stopping events may be analysed by linear regression methods to give estimates of average dead-time and marginal boarding time per passenger. the model is of the form: where T = a x + b For boarding-determined stops b = dead-time a = marginal boarding time per passenger, T = stop time, and x = number of boarders. Discussion of the assumptions and limitations of this model may be found in [ 1] and [3]. A sample of boarding- determined stopping events from the above route was analysed by this method and the following regression equation was obtained:

8 150 T = x minutes, with S(a) = and S(b) = Using the average value of dead-time per stop from the above equation, the following table was obtained for boarding determined stops along the section of route in Figure 2: Bus No. of stops Total stop-time (min) Total number boarding Total dead-time (min) Total boarding time (min) Marginal boarding times per passenger (min) a b This gives a difference in marginal boarding time of minutes (=1.4 seconds) between the two buses, which can be attributed to differences between the drivers. The average dead time per stop for the route from the regression was used rather than assigning different dead times to each driver because the sample of stopping events for each driver was too small for the regression method to be reliable ( 3). could question whether it is valid to assume this, while at the same time arguing that drivers differ in other respects. However, even when the dead-times for both buses are independently varied by one standard deviation on each side of the regression derived average, the difference in marginal boarding time always stays between min, and min. Stop-time One There were instances of large stop-time differences between successive buses, due to the leading bus loading more passengers than the following bus. This problem could well have been made smaller if buses had left the terminus regularly, so avoiding headway inequalities at the start of the run.

9 151 The main reasons Lor the unsatisfactory departure head ways were: (a) Buses leaving early by varying amounts (see "Driver Behaviour" above), combined with an occasional late bus. (b) Scheduled buses not running. Congest ion Congestion contributed to irregularity by affecting the between-bus -stop run-times of successive buses to different extents, thus producing sometimes large variations. This is readily apparent from Figure 1. Journey-time Allowances Journey-time allowances were inadequate and adversely affected the leaving times of buses from termini. This inadequacy was thought likely to lead to late departures from termini on later route sections, and could have adversely affected the general performance of drivers by placing them under avoidable stress. Siting of Bus-stops Bus-stop siting was found to be poor, leading to unevenness of passenger generation at different stops. On one route surveyed, most of the passengers generated were concentrated at a small number of stops, and many other stops were used by only a small number of passengers. This helps to induce irregularity by producing a '.ltuation where' (a) it is unprcdicable whether.1 bus will have to stop at a bus stop or not; (b ) a few stops generate extreme Iv large numbers of passengers and thus can de.av the leading bus of a potentially bunching pair enough to sometimes substantially accelerate bunching. Routing Strategies On one of the routes alternate buses travelled further

10 152 from the terminus than the others. These buses thus served more destinations than the others, and tended to attract more passengers. The resultant stop-time differences between successive buses were observed to add impetus to the bunching process. 4. Weighting of Factors Although they were all seen to deleterious 1v affect route performance, because of the large amount of interaction it was impossible to formally weight the factors involved. However, once the headways of a bus service are seriously disturbed very little can be done to stop the situation getting worse, so the bad terminal headway distribution was a very important factor. This by implication brought into question the adequacy of both the journey-time allowance, and the control exercised by route inspectors on the bus drivers. It would have been very interesting to investigate the effects of congestion and the variation of stop-time without this initial disturbance; but this was not possible in this survey as none of the factors were able to be controlled. Similar studies, where one or more factors are controlled while others are allowed to vary, could be of use in establishing the quantitative weighting of factors. 5. Significance and Application of Survey Finding The most significant result apart from those cited above in Section 3 was that although the time spent at stops and the run-time between stops were very different (run-time being much larger), the variances of the two quantities were remarkably similar. For instance, on one section of route the following table was obtained using results from 7 buses: Mean S.D. Total Run-time (min) Total Stop-time (min)

11 153 This means that changes in run-time and changes in stoptime contributed to a similar extent to the total variance and thus to the total amount of irregularity. Reducing the variability of total stop time and, likewise, total run-time are in this case about equally important methods of reducing i rregu1ari ty. The surveys suggested that variations in stop-time and run-time may be attributed, in the main, to the following factors: 1. Stop-time variation: (a) unsatisfactory terminus headways; (b) between driver variation in boarding rates of passengers; (c) congestion affecting different buses to different extents, thus making some buses pick up more passengers than others; (d) large stop-times (the larger the absolute stop-time becomes the greater the potential variance of the above factors). 2. Run-time variation: (a) congestion; (b) differences in driving speed between drivers. Thus it would appear that the following areas are those where efforts to improve regularity could be most effective: (a) (b) (c) (d) (e) Congestion control (affects both stop times and run -1 imes) ; Control of terminus headways (affects stop times); The setting of realistic journey-time allowances (affects both stop-times and run-times by affecting terminus headways and the general performance of drivers); Increasing boarding speed (affects stop-times); The prevention of existing headway disturbances from propagating by physical control strategies

12 154 (affects both stop-times and run-times'). On a more radical level, a complete resurvey of the route and the placement of its bus-stops could be attempted. 6. Application to New Zealand As New Zealand has many irregular short -headway urban bus services similar to those studied in Newcastle Upon Tyne, it is probable that the factors identified in Newcastle arc also operative here, but not necessarily with the same weighting. Similar surveys should be encouraged here to establish the importance of each factor with a view to taking appropriate counter-measures. Acknowledgements The work on which this paper is based was done by the author in 1975 as part of an M.Sc. course in Transport Engineering at the University of Newcastle Upon Tyne. Acknowledgement is due to members of the Transport Operations Research Group at this University for their help, and to the New Zealand Ministry of Transport for allowing the author to undertake this period of study. REFERENCES [ 1 ] Chapman, R.A., "Bus Boarding Times - A Review of Studies and Suggestions for Interpretation," Working Paper No. 8, Transport Operations Research Group, University of Newcastle Upon Tyne, [2 ] Chapman, R.A., "Bus Reliability," Working Paper No. 18, Transport Operations Research Group, University of Newcastle Upon Tyne, [ 3 ] Frith, W.J., "Some factors influencing urban bus operation," Unpublished M.Sc. dissertation, University of Newcastle Upon Tyne, [ 4 Newell, G.F. and R.B. Potts, "Maintaining a Bus Schedule", Proceedings Second Conference Australian Road Research Board, Vol~] 2 CH i 5 Potts, R.B. and E.A. Tamlin, "Pairing of Buses," Australian Road Research, Vol. 2 (2), 1964 [6 I Research Projects Ltd. (1966), The Importance of Service Features to passengers and the effects on traffic, A report commissioned by the Management Services Department, London Transport Board.