Las células bipolares

As mentioned in Section 2.9.3, it was found that due to the importance of the gap acceptance on merging process, there is a need for studying the minimum values of lead and lag gaps required for the lane changing process at the approaches to roadworks sections.

For the purpose of this study, three video recordings from three motorway sections (see Table 3.13) were used to investigate the gap acceptance. The selected sections were the M67

J2-J3 (roadwork section), the M61 J2-J3 (drop lane section, 2 lanes) and the M61 J2-J3 (drop lane section, 3 lanes). Drop lane sections were selected because of the similarity in drivers merging behaviour with that of roadwork sections and the unavailability of sufficient roadwork sections with a good vantage point to collect the data from. However, there are some differences between both sections which include the fact that speed limits might be different (for example, speed limits on motorway roadwork sections are likely to be reduced from 70 mph to 50 mph, whereas this reduction may not apply for drop lane sections).

Table 3.13: Gap acceptance field data details

Site No. 2 4 5

Site location M67 (J2 – J3) M61 (J2 – J3) M61 (J2 – J3)

Number of

lanes 2 2 3

Date Saturday 21/06/2014 Saturday 08/11/2014 Saturday 08/11/2014

Time 11:20 – 14:55 8:45 – 11:15 8:45 – 11:15

Duration 3.5 hours 2.5 hours 2.5 hours

Type of section

Roadwork section with offside lane closure

Drop lane section with offside lane drops

Drop lane section with offside lane drops

Speed limit 50 mph 70 mph 70 mph

Vehicles’ speeds and sizes of lead and lag gaps have been extracted by playing back the recorded video footages. Vehicles’ speeds were calculated by drawing two screen lines (datum lines) to cover a distance of about 100 m (i.e. 11 consecutive white road markings 9 m each). The time required for a vehicle to cross this distance is then measured using an event recorder. Simple calculations of distance over time were then used to convert the readings into speeds. It is worth mentioning here that the accuracy of speed measurement depends on the measured time which may be affected by human errors (i.e. time taken to manually press a button when vehicle passes the datum line). However, an attempt to check the accuracy of speed measurement was carried out by comparing some speed readings extracted from video recordings with those obtained from a radar speed meter for the same vehicles. In general, the paired results show good agreement between speed readings from video recordings and those from the radar speed meter.

Similarly, to calculate the sizes of lead and lag gaps, many screen lines were drawn on the PC monitor to make grids along the section under study. As the lane changing (subject) vehicle starts to deviate from its current lane, the lag gap is the time required for the following vehicle in the target (adjacent) lane to reach to the position of the subject vehicle at the instant of

deviation, whereas the lead gap is the time required for the subject vehicle to reach to the leading vehicle position at the instant of deviation (see Figure 2.5).

Figure 3.8 shows the relationship between relative speeds on the size of accepted lead and lag gaps for the M67, whereas Figure 3.9 shows the results of gap acceptance for the M61 (for both, 2 and 3 lane sections). Values of lead and lag gaps of less than or equal to 5 seconds were only considered in the analysis of the field data where other values (i.e. larger than 5 sec) were omitted from the analysis because they were considered so large. The dashed lines in the figures represent the minimum lead or lag gaps and suggest that the higher the speed differences, the higher the required lead/lag gaps.

Figure 3.8: Relationship between relative speeds on the size of accepted lead and lag gaps based on data from the M67 (Site No. 2)

0 1 2 3 4 5 -30 -20 -10 0 10 20 30 40 50 Lead gaps ( sec)

(Leader speed - lane changer speed) (km/hr)

Lead gaps Min. lead gaps

0 1 2 3 4 5 -30 -20 -10 0 10 20 30 40 50 La g gaps ( sec)

(Lane changer speed - follower speed) (km/hr)

Lag gaps Min. lag gaps

Figure 3.9: Relationship between relative speeds on the size of accepted lead and lag gaps based on data from the M61 (Site No. 4 and 5)

The minimum observed lead and lag gaps for the M67 were about 0.2 and 0.4 seconds, respectively, whereas the minimum lead and lag gaps for the M61 (both sections with 2 and 3 lanes) were about 0.4 and 0.6 seconds, respectively. The difference in the results between the M67 and the M61 could be attributed to the differences in traffic flow conditions between those sites. The flow rates for the M67 site was around 1000 veh/hr, whereas the flow rates for the M61 site were around 300 veh/hr for the 2-lane section (i.e. site no. 4) and 500 veh/hr for the 3-lane section (i.e. site no. 5). However, for the purpose of this study, the results presented in Figure 3.8 from the M67 (Site No. 2) have been used in the development of the new micro- simulation model (see Section 6.6), since the results presented in Figure 3.9 are limited to low flow conditions only.

0 1 2 3 4 5 -30 -20 -10 0 10 20 30 40 50 Lead gap s (se c)

(Leader speed - lane changer speed) (km/hr)

Lead gaps Min. lead gaps

0 1 2 3 4 5 -30 -20 -10 0 10 20 30 40 50 Lag g ap s (se c)

(Lane changer speed - follower speed) (km/hr)

Lag gaps Min. lag gaps