- #1
ra_forever8
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Comparing Traffic Flow Models: Which One is More Realistic?
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SUMMARY
This discussion focuses on comparing two traffic flow models: Model (i) defined as \(u = u_{SL}(1 - \frac{\rho^n}{\rho_{max}^n})\) and Model (ii) defined as \(u = u_{SL} \ln(\frac{\rho_{max}}{\rho})\). The key takeaway is that both models represent the relationship between traffic velocity \(u\) and density \(\rho\), with Model (i) showing a linear decrease in speed as density increases, while Model (ii) demonstrates a logarithmic relationship. Participants express confusion about the mathematical derivations and seek clarification on the implications of these models in real-world scenarios.
PREREQUISITES- Understanding of traffic flow theory and basic principles of density and velocity relationships.
- Familiarity with mathematical modeling, particularly partial differential equations.
- Knowledge of conservation of mass principles in traffic dynamics.
- Basic grasp of logarithmic functions and their applications in modeling.
- Explore the derivation of partial differential equations in traffic flow modeling.
- Study the implications of different traffic flow models on congestion management.
- Investigate real-world applications of traffic flow models in urban planning.
- Learn about the limitations and assumptions inherent in traffic flow modeling techniques.
Traffic engineers, urban planners, researchers in transportation systems, and anyone involved in optimizing traffic flow and congestion management strategies.
- #2
CaptainBlack
- 801
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grandy said:
For part (a) you need to observe that the flow rate \(f(x,t)\) in vehicles per unit time is \(u(x,t) \rho(x,t)\).
Now you need to show that for (i) and (ii) that \(u(x,t)\le u_{sl}\), then as \(\rho(x,t) \le \rho_{max}\) you will have shown that the flow rate:
\[f(x,t)\le u_{sl}\rho_{max}\]
- #3
CaptainBlack
- 801
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grandy said:
The next step is to write down the partial differential equation satisfied by the traffic density. This is derivable from a conservation of mass (or vehicle numbers) argument that you will have seen innumerable times.
CB
- #4
ra_forever8
- 106
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CaptainBlack said:
Thank you very much captainBlack. I am very humble with your reply but same time i m lost with your answers provided. Would please go through little deeply and clarify it nicely please. Thnx
- #5
CaptainBlack
- 801
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grandy said:
Consider a road element between \(x\) and \(x+\Delta x\) the traffic flow into the element at \(x\) per unit time is \(u(\rho(x,t))\rho(x,t)\) and out at \(x+\Delta x\) is \(u(\rho(x+\Delta x,t))\rho(x+\Delta x,t)\) Therefore the rate of change of car numbers in the element is:
\[\frac{\partial N}{\partial t}=u(\rho(x,t))\rho(x,t)-u(\rho(x+\Delta x,t))\rho(x+\Delta x,t)\]
and so the rate of change of density in the element is:
\[\frac{1}{\Delta x}\frac{\partial N}{\partial t}=\frac{u(\rho(x,t))\rho(x,t)-u(\rho(x+\Delta x,t))\rho(x+\Delta x,t)}{\Delta x}\]
Now take the limit as \(\Delta x \to 0 \) to get:
\[\frac{\partial \rho}{\partial t}=\frac{\partial}{\partial x}u(\rho)\rho\]
CB
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- #6
CaptainBlack
- 801
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CaptainBlack said:
In small time interval \(\Delta t\) all the vehicles less than a distance \(u(x,t)\Delta t\) down stream of \(x\) will pass \(x\). The number of vehicles in this stretch of road is \(u(x,t)\rho(x,t)\Delta t\), so \(u(x,t)\rho(x,t)\Delta t\) vehicles pass \(x\) in \(\Delta t\) so the vehicle flow rate at \(x\) is \(u(x,t)\rho(x,t)\) vehicles per unit time.
CB
- #7
ra_forever8
- 106
- 0
Thank you very much for your lovely answer. Please explain me what do you mean by Triangle sign x represent. And also in next line below what does Triangle sign t mean?
- #8
ra_forever8
- 106
- 0
Would please help me with the last part of this question as well please. At maximum speed limit of 40 m.p.h ...
- #9
ra_forever8
- 106
- 0
Traffic flow Modelling
On a stretch of single-lane road with no entrances or exits the traffic density ρ(x,t) is a continuous function of distance x and time t, for all t > 0, and the traffic velocity ) u( ρ) is a function of density alone.
Two alternative models are proposed to represent u:
i)u = u_(SL)*(1- ρ^n/ρ^n_max ), where n is a positive constant
ii) u = u_(SL)* In (ρ_max / ρ)
Where u_SL represents the maximum speed limit on the road and p_max represents maximum density of traffic possible on the road(meaning bumper-to-bumper traffic)
Compare the realism of the 2 models for u above. You should consider in particular the variations of velocity with density for each model, and the velocities for high and low densities in each case. State which model you prefer, giving reasons.
=>
I did for case i) which is u = u_(SL)*(1- ρ^n/ρ^n_max ),
u(ρ) = u_(SL)*(1- ρ^n/ρ^n_max ), for 0<ρ<ρ_max
Since ρ>= 0, cannot exceed u_SL
when ρ= ρ_max , u (ρ_max)= u_SL(1- ρ_max/ρ_max) =0
when ρ=0, u(0)= u_SL(1-0/ρ_max)= u_SL
Also, du/dρ= (- u_SL/ρ_max ) <0, so drivers reduce speed as density increase
Can anyone please help me for case ii) and state which model to choose?
On a stretch of single-lane road with no entrances or exits the traffic density ρ(x,t) is a continuous function of distance x and time t, for all t > 0, and the traffic velocity ) u( ρ) is a function of density alone.
Two alternative models are proposed to represent u:
i)u = u_(SL)*(1- ρ^n/ρ^n_max ), where n is a positive constant
ii) u = u_(SL)* In (ρ_max / ρ)
Where u_SL represents the maximum speed limit on the road and p_max represents maximum density of traffic possible on the road(meaning bumper-to-bumper traffic)
Compare the realism of the 2 models for u above. You should consider in particular the variations of velocity with density for each model, and the velocities for high and low densities in each case. State which model you prefer, giving reasons.
=>
I did for case i) which is u = u_(SL)*(1- ρ^n/ρ^n_max ),
u(ρ) = u_(SL)*(1- ρ^n/ρ^n_max ), for 0<ρ<ρ_max
Since ρ>= 0, cannot exceed u_SL
when ρ= ρ_max , u (ρ_max)= u_SL(1- ρ_max/ρ_max) =0
when ρ=0, u(0)= u_SL(1-0/ρ_max)= u_SL
Also, du/dρ= (- u_SL/ρ_max ) <0, so drivers reduce speed as density increase
Can anyone please help me for case ii) and state which model to choose?
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