diff --git a/lib/get-spectrogram-data.js b/lib/get-spectrogram-data.js
index b3ee634..3a3d792 100644
--- a/lib/get-spectrogram-data.js
+++ b/lib/get-spectrogram-data.js
@@ -14,7 +14,7 @@ const MAX_TRIP_LENGTH = 120 // minutes
  *   marginal opportunities reachable in 120 minutes]
  */
 export default function getSpectrogramData ({origin, query, stopTreeCache, grid}) {
-  let output = []
+  const output = []
   for (let i = 0; i < origin.nMinutes; i++) output.push(new Uint32Array(MAX_TRIP_LENGTH))
 
   propagate({
@@ -35,7 +35,7 @@ export default function getSpectrogramData ({origin, query, stopTreeCache, grid}
       const val = grid.data[gridy * grid.width + gridx]
 
       for (let i = 0; i < travelTimesForDest.length; i++) {
-        let time = travelTimesForDest[i]
+        const time = travelTimesForDest[i]
         if (time === 255) continue // unreachable
 
         if (time < MAX_TRIP_LENGTH) {
diff --git a/lib/propagation.js b/lib/propagation.js
index 3a9ab74..fc5a90a 100644
--- a/lib/propagation.js
+++ b/lib/propagation.js
@@ -56,12 +56,12 @@ export default function propagate ({ query, stopTreeCache, origin, callback }) {
             // no need to check that travelTimeToPixel < 255 as travelTimesForDest[minute] is preinitialized to the nontransit time or 255
             if (travelTimesForDest[minute] > travelTimeToPixel) {
               travelTimesForDest[minute] = travelTimeToPixel
-              let inVehicle = inVehicleTravelTimesForDest[minute] = origin.data[offset + 1]
-              let wait = waitTimesForDest[minute] = origin.data[offset + 2]
+              const inVehicle = inVehicleTravelTimesForDest[minute] = origin.data[offset + 1]
+              const wait = waitTimesForDest[minute] = origin.data[offset + 2]
               // NB when we're talking about a particular trip, then walk + wait + inVehicle == total
               // However if you calculate summary statistics for each of these individually, that may
               // not be true. So we need to calculate walk here and explicitly calculate summary stats about it.
-              let walkTime = travelTimeToPixel - wait - inVehicle
+              const walkTime = travelTimeToPixel - wait - inVehicle
               walkTimesForDest[minute] = walkTime
             }
           }