Im Making a Horse Racing world with customly made datapack, it's my first time making a datapack, and ive made this:
'
#` position calculating
execute as @e[tag=racer,limit=1] store result score @s ponsX run data get entity @s Pos[0] 1000000
accel amount
execute as @e[tag=racer,limit=1] unless score @s posX = @s posXPrev run scoreboard players add @s accel 3
accel limit
execute as @e[tag=racer,scores={accel=54..},limit=1] run scoreboard players set @s accel 50
position reset
execute as @e[tag=racer,limit=1] run scoreboard players operation @s posXPrev = @s posX
accel remove
execute as @e[limit=1,tag=racer,scores={accel=1..}] if score @s posX = @s posXPrev run scoreboard players remove @s accel 1

Effects!

execute as @e[limit=1,tag=racer,scores={accel=10..30}] run effect give @s speed 1 0 true
execute as @e[limit=1,tag=racer,scores={accel=31..49}] run effect give @s speed 1 1 true
execute as @e[limit=1,tag=racer,scores={accel=50..}] run effect give @s speed 1 2 true
execute as @e[limit=1,tag=racer,scores={accel=50..}] at @s run particle minecraft:white_smoke ~ ~ ~ 0 0 0 0.1 1 force
'`
As i've said, its my first time, i need it multiplayer friendly.
The mobs tagged with racer, are horse like mobs (Horse, zombie horse, skeleton horse, mule, and donkey)
The system works only for one player at a time
I made it execute in tick.json