Effect of Rainfall on Traffic Stream Characteristics during Peak and Non-Peak Periods

This paper examined the effect of rainfall on traffic stream behaviour during peak and non-peak periods on a basic highway section. Data on this section which is located on the J5 was collected for four months during which 99 rainfall events occurred. The traffic consisted of 75.80% cars, 10.23% motorcycles, 3.51% trucks and 10.46% of other vehicles. Traffic was observed for both rain and no-rain conditions and the data was analysed to see the effect of the rain. The results showed decreases in the speed as the rain intensity increased. Similarly, the traffic flow rates decreased as the rain intensity increased. This trend was observed for both peak and non-peak periods and for both directions. It is concluded that the effect of rain during peak period could have more serious consequences on the traffic flow than during non-peak periods because of the higher flow rates and the constrained nature of the flow. Consequently, capacity degradations up to 30% during peak periods would require resources to be employed to manage the traffic. Keywords— Traffic Stream; Peak Period; Non-Peak Period; Rainfall Intensity; Highway Capacity; Speed-Density; Flow-Density.


I. INTRODUCTION
The macroscopic traffic flow parameters of speed, volume and density are continually being employed in the qualitative and quantitative assessment of Freeways and Urban road networks. These parameters are now regularly monitored on freeway systems where loop detectors have been installed with a view to ensuring the sustainability and efficiency of traffic flows.
The increasing urbanisation, vehicle population growth and congestions within the urban conurbations, in addition to limited urban space to add more lanes to existing infrastructure have made technical improvement in traffic management more attractive. Traffic demand from the various urban land uses eventually converge on the major arterial highways and freeways. During certain times of the day, the demand reaches its maximum value and traffic flow instabilities develop. At certain points on the network, the demand overwhelms the traffic carrying ability of the road. The unfolding scenario at these locations includes queue build up, excessive travel times over short distances, fuel consumption and economic loss. These conditions are also common at bottleneck locations such as on-off ramps, intersections, points of geometric lane changes etc.
It is known that rainfall exacerbates these conditions, inducing speed drops and causing further flow contractions, thus making mobility, and comfort of motorists difficult and safety, a major concern. Alhassan and Ben-Edigbe [1,2] , Aggarwal et al., [3] and Billot et al.
[4] among others have indicated that traffic flow parameters are degraded by rainfall. Earlier evidence about the effect of rainfall on traffic flow parameters have been given by Tanner [5] who observed reduction in traffic volume between two weeks in August of 1949 and 1950 and compared the weekend flows between these periods. There were 1.3% and 3.1% differences between the weekday and weekend traffic flows. Other studies [6-9], have similarly pointed to various contractions of flow during rainfall including reductions in speed and increases in small headways.
The problems of highway capacity reductions as a result of rainfall have been reported also by [10] who found traffic demand reductions by 2.9% during weekdays and by 7.9% on Saturday and 5.2% on Sunday. Chung et al. [11] have reported capacity reduction in light rain to be 4.7% and 14% in heavy rain while Alhassan and Ben-Edigbe [12,13] have  con  inte  furt  The  be m  in a  T  buil  flow  incl  how  pea  qua  evo  wea  T  on  hou  pea  with  org  II  con   Joh  this  exc  fou  inst  pas  stat  use  abs  rain  traf  hea  whi  sep  obs  for  whi  8am  peri  mot  rem  view  effe  faci  amb  obs  Thu   T  rain  peri  Pon  stre   nfirmed reduc  ensity variabi  ther give the s  ese are require  made on these  adverse weath  This presuppos  ld-up and  l evidence for dverse weather mary of the tr ions are show 4 for non-pea in both dire es. In the Po % between no n no-rain and n no-rain and re at about hal tion, the capa 4.82% for med n this directio the higher fl ai direction ca Moreover, the e an the mornin s a higher capa Models between the Pontian-Skud nd Medium Rain ndition model is r the existenc r is clearly dis raffic flow sta wn in tables 3 ak period flow ections decre ontian directio o-rain and lig medium rain d heavy rain lf of the poste acity reduction dium rain. No on for this stu lows during t arries the hig evening peak ng peak and it acity loss.