I could not find anyone using the same module as I am and since im a beginner myself i thought i would post my progress here and maybe it will help someone in the same position as me.
Im trying to make a "SpeedSign" that shows passing cars their current speed. Commercial products pricing usually start at about $1500-2000 and thats not an option for recreational use 
Anyway, i made some code to read the radars serial output and display the raw data. I fount the protocol in a Chinese pdfdocument and used goolge translate to translate it to english. http://h.hlktech.com/download/HLK-LD2451-24G/1/LD2451%20%E4%B8%B2%E5%8F%A3%E9%80%9A%E4%BF%A1%E5%8D%8F%E8%AE%AE%20V1.03.pdf
And here is my experimental code to read the data from the radar module. Perhaps someone could make it nicer and more efficient or have a better way to find the start of the dataframe.
/*
*
* Frame header: 4 bytes - 0xF4 0xF3 0xF2 0xF1
* Data length in frame: 2 bytes
* In-frame data
* Target quantity: 1 byte
* Alarm information: 1 byte (target aproacing = 01)
* Angle in degrees = reported value - 0x80: 1 byte
* Distance in meters: 1 byte
* Speed direction (01 = approach, 00 = away): 1 byte
* Speed (in km/h, max 120): 1 byte
* Signal to noise ratio: 0-255: 1 byte
*
* In-frame data is sent for each target (max 5)
*
* End frame: 4 bytes - 0xF8 0xF7 0xF6 0xF5
*
*/
#include<SoftwareSerial.h>
SoftwareSerial Serial1(2,3);
int j=0;
void setup()
{
Serial.begin(115200);
delay(100);
Serial.println("RadarTest");
Serial1.begin(115200);
Serial.println("Started dataread");
}
void loop()
{
byte response[] = {0xF4, 0xF3, 0xF2, 0xF1 }; //Expected response from radar.
if(Serial1.available()) {
int temp = Serial1.read();
if(temp!=response[j]) { //Check if read data matches with expected response
Serial.print("Read fail Data = "); //Debug //We did not get the response we expected in the order that we expected
Serial.println(temp, HEX); //Debug
Serial.print("J = "); //Debug
Serial.println(j); //Debug
j=0; //Start over, looking for correct byte 0
return(0);
}
else {
j++;
Serial.print("Read ok, Data = "); //Debug
Serial.println(temp, HEX); //Debug
Serial.print("J = "); //Debug
Serial.println(j); //Debug
}
if(j>3) { //Now we have the 4 message startbytes.
readData(); //Read dataframe
j=0; //Start over looking for the 4 startbytes.
}
}
}
void readData() {
int messageSize=0, i=0;
byte dataSize[2];
if(Serial1.available()) {
Serial1.readBytes(dataSize, 2); //The next 2 bytes tells the size in bytes of the dataframe thata follows
}
Serial.print("Messagesize 1:"); //Debug
Serial.print(dataSize[0]); //Debug
Serial.print(" Messagesize 2:"); //Debug
Serial.println(dataSize[1]); //Debug
messageSize = word(dataSize[1], dataSize[0]);
Serial.print("messageSize = "); //Debug
Serial.println(messageSize); //Debug
messageSize = messageSize+4; //Add the 4 "end of message" bytes to the total length
byte data[messageSize];
if(Serial1.available()) {
Serial1.readBytes(data, messageSize); //Read all the data and the 4 endbytes
}
while((messagesize)>0) { //Display all the data in hex format
Serial.print((String)"Data["+i+"] = ");
Serial.println(data[i], HEX);
i++;
messagesize--;
}
}