Post on 20-Dec-2015
transcript
Unsignalized Intersections
CTC-340
Hmwk
• At end of powerpoint
• STOP & YIELD controlled
• Include TWSC, AWSC and Roundabouts
• All models are based on a gap acceptance model
Gap Acceptance
• Gap – distance between back of veh and front of next veh – not headway– Each gap can allow at least 1 veh to move– Vehicle using gap is based on a rank order– Figure next slide– Rank 1 – 2,3,5,6,15,16– Rank 2 – 1,4,13,14,9,12– Rank 3 - 8,11– Rank 4 – 7,10– Why this ranking?
Rank order
Conflicting Volume
– Each movement must content with a different group of conflicting flows
– Figure next slide– Look at footnotes
• RT from major street do not conflict but some are counted in conflicting volume
• 2 stage gap acceptance – median or TWLTL present – cars can cross 1 direction of traffic at a time
Conflicting Volume
– Each movement must content with a different group of conflicting flows
– Critical volume cmx = cpxpvippj– cpx= potential movement capacity
– pvi= probability that impeding veh movement j will not block flow (impedance factor)
– ppj= probability that impeding ped movement j will not block flow (impedance factor)
Critical Gap
• Minimum average acceptable gap that allows entry for 1 turning movement– Any gap smaller than critical gap is not used
• Follow up time – minimum average acceptable time for a second queued vehicle to use a gap large enough to admit 2+ vehicles
Critical Gap
– Critical Gap • tcx = tcb + tcHVPHV + tcGG – tcT – t3LT
– Follow up time• tfx = tfb +tfHVPHV
• tcb = base critical gap, T23.2
• tcHV= critical gap adjustment for HV
• PHV= percent HV
• tcG= critical gap adjustment for grade
• tcT = critical gap adjustment for 2 stage gap acceptance
• t3LT = critical gap adjustment for intersection geo
• tfb = base follow up time, T 23.2
Potential Capacity
• Assumes that all available gaps are used by subject movement– No higher priority movements will be at
intersection– Assumes movement operates in exclusive
lane• cpx = vcx[(e^-(vcx*tcx/3600))/(1-e^-(vcx*tfx/3600))]
• vcx = conflicting flow for movement x
Impedance Effects
– Effects due to higher ranked movements using a gap• Reduces the available gaps for the subject movement• Figure next page
– First find movement capacity• cmx = cpxpvippj
• cmx = movement capacity
• cpx = potential capacity
• pvi = probability that movement i is not blocking subject flow
• ppj = probability that pedestrian movement j is not blocking subject flow
Impedance Effects
– Effects due to higher ranked movements using a gap• Reduces the available gaps for the subject movement
Impedance Effects
• pvi = 1 – vi/cmi
• vi = demand flow for impeding movement i
• cmi = movement capacity for impeding movement i
• The lower the v/c ratio for the impeding movement – the more likely that the subject flow will not be impeded
• Rank 4 movements are impeded by many movements- may end up double counting impedance factor
• p” = Pv1*Pv4*PvTH
Impedance Effects
• p’ = 0.65p” – (p”/(p”+3) + 0.6SQRT(p”)• p’” = unadjusted impedance factor• p’ = adjusted impedance factor
– Need to modify Major St LT when in shared lane • P*v1/4 = 1 – ((1-Pv1/4)/(1-(vmTH/smTH + vmRT/smRT)))
– Ped impedance factor• ppj = 1 – (vj(w/Sp)/3600)
• vj = ped flow rate
• w = lane width
• Sp= ped speed fps
Shared Lane Cap
• Movement capacities assume exclusive lanes for each movement– When movements operate out of a shared
lane• cSH = vy/vy/cmy)
• Capacity = total flow rate/ cSH
Upstream Signals
• Gap acceptance assume random arrivals for all vehicles– If signalized intersections within ¼ mile – not
true• Each platoon gives a different conflicting flow to
the downstream intersection• Very complex
2 stage gap acceptance
• Occurs at divided highways or TWLTL
• Increases capacity for minor street movements due to ability to cross 1 traffic stream at a time.
• Limiting factors are the # of vehicles that can be in the median at the same time
Flared Lanes
• Lane operates between exclusive lane and shared lane – Need to know average queue length of RT
traffic– If max queue length <= # of flared spaces –
operates like a separate lane – If max queue > # of flared spaces then
capacity is a constrained
Delay
• What is it
• Control delay – includes time stopped in queue + time to decel + accel
• Geometric delay – delay due to decel/accel to get thru intersection
• HCM uses control delay as its MOE– dx = 3600/cmx +900T((vx/cmx-1)+SQRT((vx/cmx-
1)^2 + (3600/cmx)(vx/cmx)/450T)) + 5
Delay
• Delay is given for approach lane groups– Each exclusive lane or each shared lane– Major St LT – Major street thru assumed to have no delay
• Depends on whether LT has an exclusive lane• Usually very small if it does occur
Queue Length
– Q95x = 3600/cmx +900T((vx/cmx-1)+ SQRT((vx/cmx-1)^2 + (3600/cmx)(vx/cmx)/150T)) *(cmx/3600)
– 95th percentile queue– Gives a sense of congestion at intersection– Higher queue means lower LOS
AWSC
• Based on FIFO queue– Looks at probability of intersection in a certain
condition – Determines the probability of each condition
occurring given volumes and assesses the impact
– Each approach affects the others– Iterative process
Roundabouts
– Roundabouts must be Yield controlled and have a splitter island
Example 1
A
B
35’
40’
Spd lmt = 35 mph for A45mph for B
Example 2
Homework
• Prob 23-2 Determine the potential capacities for movements 1,7,8,9
• Prob 23-3 Determine the movement capacities for movements 1,7,8,9
• Prob 23-4 Determine the shared lane capacities for movements 7,8