Abstract. Mobility systems in urbanized territories have been featured out in Travel Demand Models by state variables of land-use occupation, trip generation, trip distribution, modal split and network assignment, with emphasis on causal relationships between the variables and on spatial detail for each kind of variables. The article is aimed to provide notional averages, say ratios, for each kind of variables, and to state the causal relationships between the variables as simple analytical formulas between the ratios. This is achieved by going along the classical four steps of travel demand modeling, in a theoretical way for an idealized territory satisfying three postulates of homogeneity: namely, at block level, at link level and of indefinite spatial extension. The said formulas constitute rules of thumb linking the mobility ratios of spatial density of human occupation, trip emission rates, average trip lengths, modal shares, generalized trip cost per length unit, together with traffic variables of speed, flow rate and vehicular density at the link level. The model is stated in eight steps, namely (i) territorial composition, (ii) trip generation, (iii) trip lengths and traffic formation, (iv) quality of service, (v) trip distribution using a gravity model, (vi) modal split by multinomial logit, (vii) traffic laws, (viii) traffic equilibrium. It is followed by a Discussion of the model outreach and limitations. Areas of further research include traffic laws, impact assessment and economic analysis.

Keywords. Spatial homogeneity; State laws; Four-step travel demand model; Traffic equilibrium

Highlights

• Idealized territory with geometric regularities
• Mobility ratios as spatial averages in idealized territory
• Local balance of generated and carried traffic
• Gravity distribution yields simple formula of average axial trip length
• Monomodal traffic equilibrium as a single equation in axial speed only

 

Abstract. Urban streets are laid out as longitudinal stretches of space and their width is divided between sidewalks for pedestrians and lanes for vehicle running or parking. The specific width assigned to each function determines its flowing capacity. Considering the respective spatial footprints of modal vehicles, urban planners have shown that pedestrians and bikes use public space more efficiently than cars. However, traffic engineers and economists continue to refer to modal trip flow rates as the basic indicators of a street’s social utility. The article has a twofold objective: firstly, to unify the respective measurements used by urban planners and economists and, secondly, to investigate the urban conditions under which street capacity is a scarce resource. The first objective is achieved by relating vehicle spatial footprint to vehicular spatial concentration, and vehicle time-area footprint (TAF) to vehicular flow rate. At the link level the demand-supply ratios of TAF and trip flow rate are fairly equivalent. The second objective is addressed at the city level by modeling urban conditions both on the demand and supply sides of people mobility: this includes, on the demand side, population density, trip generation rate and average trip length per travel mode and per longitudinal axiality versus, on the supply side, the lateral, “way density” of longitudinal routes and their respective widths and lanes. The model is applied to five cities encompassing the range of population density in urban France, by summarizing their respective mobility parameters and street network geometry. It comes out that urban conditions with population density below 2,000 persons/km² have some slack street space, so that scarcity is not an issue, whereas densities above 3,000 p/km² combined to typical street geometry require transit services on the ground and also underground beyond 8,000 p/km².

Keywords: Time-space consumption; Time-area footprint; Supply-demand ratio; Traffic variables; Way capacity; Urban planning

Highlights:

• Comparing the respective occupation concepts and indicators used by urban planners and traffic engineers
• Multilevel model of street space occupation, from the individual and vehicle level to link level and up to city level
• Time-area footprint of travel modes per functional unit of person.km or veh.km
• Urban conditions depicted in terms of population density, mobility behavioral parameters and “lateral way density” of link capacity and network geometry
• Application to 5 cities in France, encompassing a wide range of population density