A Comparative Study of Gap Acceptance at Priority Intersections

Gap acceptance and rejection are important parameters in determining the capacity of stop sign intersections. Gaps are either accepted or rejected depending on a number of factors including head ways, volume of traffic, drivers' characteristics etc. From the results obtained, it was observed that the critical gaps varied very slightly from weekdays to weekends, 3.8 sees for weekdays and 4 1 sees for weekends. It was also observed that the minimum acceptability gaps varied from driver to driver. INTRODUCTION A gap can be defined as the time interval between the passage of two successive vehicles on the major road at a priority intersection. Gap acceptance is the decision of a side-street (minor road) driver to use a gap created in a major road traffic to merge or maneuver safely with the major road traffic. A gap may be accepted if it is large enough or rejected if it is too small. A priority intersection is one which has a major road and a minor road crossing each other at the same level. A priority type of intersection is controlled either by the stop sign or the yield sign. At such an intersection the major road traffic has the right of way over lie minor street vehicles whenever conflicts occur The traffic flow is prioritized in the following order : i major road through traffic, ii major road right turning traffic, iv major street left turning traffic, v minor street through traffic, and vi minor street left turning traffic. Gap acceptance or rejection is basic to almost every traffic maneuver and it is fundamental to the description and understanding of traffic movement at priority or stop sign intersections. Blumenfield and Weirs (1) compared a simplified gap acceptance function based on the critical gap concept and other more realistic functions were within limits of normal experimental errors. Ramsey and Routledge (2) put forward a method that inferred the distribution of critical gaps for the population of drivers from the histogram of all offered gaps. Driver characteristics The following driver attitudes at stop sign intersections weir observed (3). For their study, 6 intersections were selected on the Following requirements: 1. The intersection should be located at least 0.25km away from any traffic signal on both major and minor roads, 2. The intersection should be controlled by either a stop – sign flasher on the minor roads, 3. The intersection should be similar with regards to geometry, and sight distance conditions, and 4. Speed limit for the major and minor roads should not be more than 55km/hr and 40km/hr respectively. Critical gap The critical gap is the time interval between two successive mar vehicles considered to be just adequate for a minor road vehicle' enter or cross the main road the size of the critical gap of a driver depends on his characteristics and his style of driving. It also depends on the design of the junction, the size and speed of the trailing vehicle creating the gap as well as the weather. Knowledge of the critical gap" of the drivers is very necessary for forecasting delay at such a: junction, Hewitt(4). Identified two problems, the first of which was that the number of drivers whose gap acceptance behavior at an intersection is observed would be reduced by th6 number of drivers who accepted the initial lag. A lag is defined as the time interval between the arrival of a side street vehicle at an intersection and the arrival of the next main street vehicle. Since drivers with short critical gaps are also likely to have short critical lags, they are likely to accept the initial lag and thus not be represented in the subset of drivers whose acceptance behavior is observed. This causes a large acceptance bias and an increase in the mean critical gap. The amount of bias will depend on the major road flow and the distribution of the size of the lag presented to each driver as he reaches the head of the queue. Hewitt (4) suggested the necessity of correcting this bias if data collected at one level of major road flow are to be used to forecast the operation of the intersection at a different level, The second problem with the measurement of the distribution of critical gaps arises due to the rejection of one or more gaps by some vehicles before one is accepted. Because of this, the proportion of gaps of a particular size accepted is not equal to the proportion of drivers who Driver sensitivity to gaps A gap acceptance phenomenon involves a stimulus and a response by the minor driver. Those phenomena were observed to involve stimulus and response and were bound by the relationship (5): Response = f (Sensitivity, Stimulus) Thus in the gap acceptance phenomenon, it is expected that the acceptance of an available gap by a driver depends hot only on the size of the gap but also on the drivers' sensitivity to such a gap. Two will accept a gap of that size when presented with it. Parameters that can be used as indirect measures of driver’s sensitivities are flow on the major road and duration of stopped delay experienced by the side street driver before the gap becomes available. This research was prompted by the need to obtain acceptable gaps for motorists at some priority intersections in Ilorin, Nigeria. It was therefore intended to: determine the average gap accepted to the minor street driver, determine the volume at which the critical gap occurs and predict drivers attitude when at the minor street of apriority intersection. MATERIALS AND METHODS The following sets of data were collected on four different priority intersections within Ilorin metropolis : time headway/gaps in the major road traffic, gaps accepted/rejected by the side street drivers, traffic volume on the major road. The four intersections made up of two 4 leg and two 3 – leg intersections were: Lajonrin Road/Offa Road intersection, Emirs Road/Opo Malu Road intersection, Gambari Road/SobiRoad intersection Amilegbe Road/Sobi Line Road intersection At all the intersections, the minor road drivers had a dear line of sight from within a distance of over 30m. The road intersect each other at approximately 90 degrees and observations were made during the dry weather when the visibility was very good. In collecting the data, it was assumed that traffic flow during the weekdays, Monday through Friday was similar and weekend traffic flow was also similar. Data were therefore collected on a weekday which was considered a representative of weekdays. For the weekends also, one of them was used to represent them. Gap acceptance study This was done manually by the use of stop watches to time individual vehicles as they pass the intersection from the major road. The gaps accepted or rejected by the major road vehicles were observed and noted also. The time headway of the vehicles on the major road was simultaneously recorded. The observation point was located at about 10m away from the intersection. The time on the stop watch at the observation point was recorded. The difference in time between the passages of two successive vehicles is the time headway or in this case the gap. RESULTS AND DISCUSSION Typical headway/gap data observed and collected are tabulated and shown for one of the intersections during a typical weekday and a typical weekend (Tables I IV). Table I. Cumulative number of rejected gaps for Emirs road/Opo Malu road intersection (Weekday). Gaps accepted(sacs) (GA) Frequency (F) Cumulative frequency of accepted gap(CFAG) GA F CFAG 2 2 0 12 3 65 3 7 2 13 1 68 4 4 9 14 2 69 5 11 13 15 1 71 6 13 24 16 2 72 7 8 37 18 1 74 8 8 45 19 1 75 9 2 53 20 1 76 10 6 55 21 . 2 77 11 4 61 25 2 79 Table II. Cumulative number of rejected gaps for Emirs road/Opo Malu road intersection (Weekday). Gaps rejected (sacs) Frequency Cumulative frequency of rejected gaps