Uber - storage design

the suggested solution:
Geohash:(DriverID, TS) <<< need to be updated every 4 sec for TS, logN lookup update
DriverID:(Latitude, longitude) << need to be updated every 4 sec for locatiopn

somehow I feel this makes more sense?
Geohash:(DriverID) << Redis Sorted Set, no need to update.
DriverID:(Latitude, longitude, TS) << this need to be updated every 4 sec

does this make sense? or what did i miss?

Geohash table 没办法 expire 超过30秒的数据,Driver ID list 不断增加。