accessibilty() always returns an integer

[c-voulgaris]

Brief description of the problem:

I love the ability to specify a non-step decay function in the accessibility function, but we lose some of the benefit when the resulting accessibility values are apparently rounded to the integer.

In the example below, I think I've specified a decay function that should decrease linearly from 1 to zero as travel time increases from 0 to 20 minutes. Since opportunities are never fully-weighted in this case, it seems like the accessibility values should vary continuously and return very few, if any, whole numbers. But with the rounding that's apparently happening, the results are mostly the same as what I'd get with a step function with a cutoff at 10 minutes.

I realize this is almost definitely an r5/Conveyal issue rather than an r5r issue, so maybe there's nothing to be done (other than maybe calculating travel times with r5, but applying the decay function outside of r5).

Reproducible example here

library(r5r)
library(dplyr)

# build transport network
data_path <- system.file("extdata/poa", package = "r5r")
r5r_core <- setup_r5(data_path = data_path, temp_dir = TRUE, verbose = FALSE)

# load origin/destination points
points <- read.csv(file.path(data_path, "poa_hexgrid.csv"))

access_linear_decay <- accessibility(r5r_core, 
                        origins = points, 
                        destinations = points,
                        opportunities_colname = "schools",
                        decay_function = "linear",
                        cutoffs = 10,
                        decay_value = 20, 
                        verbose=FALSE) %>%
  rename(linear_decay = accessibility)

access_step <- accessibility(r5r_core, 
                        origins = points, 
                        destinations = points,
                        opportunities_colname = "schools",
                        decay_function = "step",
                        cutoffs = 10, 
                        verbose=FALSE) %>%
  rename(step_decay = accessibility)

comparison <- full_join(access_linear_decay, access_step) %>%
  mutate(diff = step_decay - linear_decay)

head(comparison, 20)

table(comparison$diff)

[mvpsaraiva]

Hi @c-voulgaris. Sorry for the late reply.

Thanks for pointing this out. Indeed, r5 does all calculations using real numbers, but converts the results to integers before passing them along to r5r. I guess the rationale from Conveyal is that, in practice, you can't access a fraction of a school or job. I'll open an issue in r5's repository to check if they can change this behaviour in future versions.

Meanwhile, you'll need to work around this by applying the decay function outside r5r, as you suggested. Unfortunately, this is much more resource intensive, as you need to load the full travel time matrix in memory. Another 'hackish' solution that would allow you to use the accessibility() function is to multiply all opportunity counts of the inputs by 1000, for example, and then divide the accessibility results by 1000. This should produce accessibility results with precision of up to 3 decimal places.

[mvpsaraiva]

I've found a solution for this issue, which is implemented now in the dev branch. It will be on CRAN when version 1.0 is released.

[rafapereirabr]

Thank you @mvpsaraiva .

@c-voulgaris , the accessibility estimates from accessibility() are now of returned as doubles / class numeric, except when using a step decay function. This is already available in the dev version and will be included in the next release of r5r on CRAN. Closing this issue for now.

[c-voulgaris]

Thanks!!